c# - object解決 - unity nullreferenceexception: object reference not set to an instance of an object




什么是NullReferenceException,以及如何解决它? (20)

NullReference异常 - Visual Basic

Visual BasicNullReference ExceptionC#中NullReference Exception没有区别。 毕竟,他们都报告了它们都使用的.NET Framework中定义的相同异常。 Visual Basic特有的原因很少见(可能只有一种)。

这个答案将使用Visual Basic术语,语法和上下文。 所使用的示例来自大量过去的Stack Overflow问题。 这是通过使用帖子中常见的情况来最大化相关性。 还为可能需要它的人提供了更多解释。 类似于你的例子可能在这里列出。

注意:

  1. 这是基于概念的:没有代码可以粘贴到您的项目中。 它旨在帮助您了解导致NullReferenceException (NRE)的原因,如何找到它,如何解决该问题以及如何避免它。 NRE可能会有很多种方式,所以这不太可能是你唯一的遭遇。
  2. 这些例子(来自Stack Overflow的帖子)并不总是展示最好的方式来做一些事情。
  3. 通常,使用最简单的补救措施。

基本含义

“对象未设置为对象实例”消息表示您正在尝试使用尚未初始化的对象。 这归结为其中之一:

  • 你的代码声明了一个对象变量,但它没有初始化它(创建一个实例或' 实例化 '它)
  • 你的代码假设会初始化一个对象,但没有
  • 可能地,其他代码过早地使仍在使用的对象无效

寻找原因

由于问题是一个Nothing的对象引用,答案是要检查它们以找出哪一个。 然后确定它未被初始化的原因。 将鼠标放在各种变量上,Visual Studio(VS)将显示它们的值 - 罪魁祸首将是Nothing

您还应该从相关代码中删除任何Try / Catch块,尤其是那些Catch块中没有任何内容的代码块。 这会导致您的代码在尝试使用Nothing的对象时崩溃。 这是你想要的,因为它可以确定问题的确切位置 ,并允许你识别造成它的对象。

Catch中的MsgBox显示Error while...将没有多大帮助。 此方法也会导致Stack Overflow问题非常糟糕 ,因为您无法描述实际的异常,所涉及的对象或者代码行。

你也可以使用Locals WindowDebug - > Windows - > Locals )来检查你的对象。

一旦你知道问题出在什么地方,通常很容易解决问题,并且比发布新问题更快。

也可以看看:

示例和补救措施

类对象/创建实例

Dim reg As CashRegister
...
TextBox1.Text = reg.Amount         ' NRE

问题是Dim不会创建一个CashRegister 对象 ; 它只声明一个名为该类型的reg的变量。 声明一个对象变量并创建一个实例是两件不同的事情。

补救

New运算符通常可用于在声明它时创建实例:

Dim reg As New CashRegister        ' [New] creates instance, invokes the constructor

' Longer, more explicit form:
Dim reg As CashRegister = New CashRegister

以后只适合于创建实例:

Private reg As CashRegister         ' Declare
  ...
reg = New CashRegister()            ' Create instance

注意: 不要在过程中再次使用Dim ,包括构造函数( Sub New ):

Private reg As CashRegister
'...

Public Sub New()
   '...
   Dim reg As New CashRegister
End Sub

这将创建一个局部变量reg ,它只存在于该上下文(sub)中。 您将在其他地方使用的模块级别为Scopereg变量保持为Nothing

New运算符的缺失是NullReference Exceptions的第一个原因,在所检查的Stack Overflow问题中看到。

Visual Basic尝试使用New重复清除该过程:使用New Operator创建一个对象并调用Sub New - 构造函数 - 您的对象可以在其中执行任何其他初始化。

要明确, Dim (或Private )只声明一个变量及其Type 。 变量的范围 - 无论它存在于整个模块/类还是程序的本地 - 由其声明的位置决定。 Private | Friend | Public Private | Friend | Public定义访问级别,而不是范围

有关更多信息,请参阅:

数组

数组也必须实例化:

Private arr as String()

这个数组只是被声明的,没有被创建。 有几种方法可以初始化数组:

Private arr as String() = New String(10){}
' or
Private arr() As String = New String(10){}

' For a local array (in a procedure) and using 'Option Infer':
Dim arr = New String(10) {}

注意:从VS 2010开始,当使用文字和Option Infer初始化本地数组时, As <Type>New元素是可选的:

Dim myDbl As Double() = {1.5, 2, 9.9, 18, 3.14}
Dim myDbl = New Double() {1.5, 2, 9.9, 18, 3.14}
Dim myDbl() = {1.5, 2, 9.9, 18, 3.14}

数据类型和数组大小是从正在分配的数据中推断出来的。 类/模块级声明仍然需要As <Type> with Option Strict

Private myDoubles As Double() = {1.5, 2, 9.9, 18, 3.14}

示例:类对象数组

Dim arrFoo(5) As Foo

For i As Integer = 0 To arrFoo.Count - 1
   arrFoo(i).Bar = i * 10       ' Exception
Next

该数组已创建,但其中的Foo对象尚未创建。

补救

For i As Integer = 0 To arrFoo.Count - 1
    arrFoo(i) = New Foo()         ' Create Foo instance
    arrFoo(i).Bar = i * 10
Next

使用List(Of T)将会使得没有有效对象的元素变得非常困难:

Dim FooList As New List(Of Foo)     ' List created, but it is empty
Dim f As Foo                        ' Temporary variable for the loop

For i As Integer = 0 To 5
    f = New Foo()                    ' Foo instance created
    f.Bar =  i * 10
    FooList.Add(f)                   ' Foo object added to list
Next

有关更多信息,请参阅:

列表和集合

.NET集合(其中有许多种类 - 列表,词典等)也必须实例化或创建。

Private myList As List(Of String)
..
myList.Add("ziggy")           ' NullReference

由于相同的原因,您会得到相同的异常 - myList仅被声明,但没有创建实例。 补救措施是一样的:

myList = New List(Of String)

' Or create an instance when declared:
Private myList As New List(Of String)

常见的疏忽是使用集合Type

Public Class Foo
    Private barList As List(Of Bar)

    Friend Function BarCount As Integer
        Return barList.Count
    End Function

    Friend Sub AddItem(newBar As Bar)
        If barList.Contains(newBar) = False Then
            barList.Add(newBar)
        End If
    End Function

任一过程都会导致NRE,因为barList只是声明的,没有实例化。 创建Foo的实例也不会创建内部barList的实例。 它可能是在构造函数中执行此操作的意图:

Public Sub New         ' Constructor
    ' Stuff to do when a new Foo is created...
    barList = New List(Of Bar)
End Sub

和以前一样,这是不正确的:

Public Sub New()
    ' Creates another barList local to this procedure
     Dim barList As New List(Of Bar)
End Sub

有关更多信息,请参见List(Of T)

数据提供者对象

使用数据库为NullReference提供了很多机会,因为可以同时使用许多对象( CommandConnectionTransactionDatasetDataTableDataRows ....)。 注意:您使用的是哪种数据提供程序无关紧要 - MySQL,SQL Server,OleDB等 - 概念相同。

例1

Dim da As OleDbDataAdapter
Dim ds As DataSet
Dim MaxRows As Integer

con.Open()
Dim sql = "SELECT * FROM tblfoobar_List"
da = New OleDbDataAdapter(sql, con)
da.Fill(ds, "foobar")
con.Close()

MaxRows = ds.Tables("foobar").Rows.Count      ' Error

与之前一样, ds数据集对象已声明,但实例从未创建。 DataAdapter将填充现有的DataSet ,而不是创建一个。 在这种情况下,由于ds是局部变量, 因此IDE会警告您可能会发生这种情况:

当被声明为模块/类级别变量时,与con似乎一样,编译器无法知道对象是否由上游过程创建。 不要忽视警告。

补救

Dim ds As New DataSet

例2

ds = New DataSet
da = New OleDBDataAdapter(sql, con)
da.Fill(ds, "Employees")

txtID.Text = ds.Tables("Employee").Rows(0).Item(1)
txtID.Name = ds.Tables("Employee").Rows(0).Item(2)

错字是这里的一个问题: Employees vs Employee 。 没有创建名为“Employee”的DataTable ,因此尝试访问它的NullReferenceException结果。 另一个潜在的问题是假定当SQL包含WHERE子句时,将会出现不可能出现的Items

补救

由于这使用一个表,所以使用Tables(0)将避免拼写错误。 检查Rows.Count还可以帮助:

If ds.Tables(0).Rows.Count > 0 Then
    txtID.Text = ds.Tables(0).Rows(0).Item(1)
    txtID.Name = ds.Tables(0).Rows(0).Item(2)
End If

Fill是一个函数,返回受影响的Rows数,也可以进行测试:

If da.Fill(ds, "Employees") > 0 Then...

例3

Dim da As New OleDb.OleDbDataAdapter("SELECT TICKET.TICKET_NO,
        TICKET.CUSTOMER_ID, ... FROM TICKET_RESERVATION AS TICKET INNER JOIN
        FLIGHT_DETAILS AS FLIGHT ... WHERE [TICKET.TICKET_NO]= ...", con)
Dim ds As New DataSet
da.Fill(ds)

If ds.Tables("TICKET_RESERVATION").Rows.Count > 0 Then

DataAdapter将提供如上例所示的TableNames ,但它不会从SQL或数据库表中解析名称。 因此, ds.Tables("TICKET_RESERVATION")引用一个不存在的表。

补救措施是一样的,请参考表格索引:

If ds.Tables(0).Rows.Count > 0 Then

另请参阅DataTable类

对象路径/嵌套

If myFoo.Bar.Items IsNot Nothing Then
   ...

代码只是测试ItemsmyFooBar可能是Nothing。 补救措施是一次测试一个对象的整个链或路径:

If (myFoo IsNot Nothing) AndAlso
    (myFoo.Bar IsNot Nothing) AndAlso
    (myFoo.Bar.Items IsNot Nothing) Then
    ....

AndAlso也很重要。 一旦遇到第一个False条件,后续测试将不会执行。 这允许代码一次安全地“钻取”到一个“级别”的对象中,只有在(和if) myFoo被确定为有效之后才评估myFoo.Bar 。 编码复杂对象时,对象链或路径可能会变得很长:

myBase.myNodes(3).Layer.SubLayer.Foo.Files.Add("somefilename")

无法引用null对象的任何'下游'。 这也适用于控制:

myWebBrowser.Document.GetElementById("formfld1").InnerText = "some value"

在这里, myWebBrowserDocument可能是Nothing,或者formfld1元素可能不存在。

UI控件

Dim cmd5 As New SqlCommand("select Cartons, Pieces, Foobar " _
     & "FROM Invoice where invoice_no = '" & _
     Me.ComboBox5.SelectedItem.ToString.Trim & "' And category = '" & _
     Me.ListBox1.SelectedItem.ToString.Trim & "' And item_name = '" & _
     Me.ComboBox2.SelectedValue.ToString.Trim & "' And expiry_date = '" & _
     Me.expiry.Text & "'", con)

除此之外,此代码不会预期用户可能没有在一个或多个UI控件中选择某些内容。 ListBox1.SelectedItem可能是Nothing ,因此ListBox1.SelectedItem.ToString将导致NRE。

补救

在使用它之前验证数据(也使用Option Strict和SQL参数):

Dim expiry As DateTime         ' for text date validation
If (ComboBox5.SelectedItems.Count > 0) AndAlso
    (ListBox1.SelectedItems.Count > 0) AndAlso
    (ComboBox2.SelectedItems.Count > 0) AndAlso
    (DateTime.TryParse(expiry.Text, expiry) Then

    '... do stuff
Else
    MessageBox.Show(...error message...)
End If

或者,您可以使用(ComboBox5.SelectedItem IsNot Nothing) AndAlso...

Visual Basic窗体

Public Class Form1

    Private NameBoxes = New TextBox(5) {Controls("TextBox1"), _
                   Controls("TextBox2"), Controls("TextBox3"), _
                   Controls("TextBox4"), Controls("TextBox5"), _
                   Controls("TextBox6")}

    ' same thing in a different format:
    Private boxList As New List(Of TextBox) From {TextBox1, TextBox2, TextBox3 ...}

    ' Immediate NRE:
    Private somevar As String = Me.Controls("TextBox1").Text

这是获得NRE的一种相当常见的方式。 在C#中,根据编码的方式,IDE将报告Controls在当前上下文中不存在,或者“不能引用非静态成员”。 所以,在某种程度上,这是一个纯VB的情况。 它也很复杂,因为它可能导致级联失败。

数组和集合不能通过这种方式进行初始化。 此初始化代码将构造函数创建FormControls 之前运行。 结果是:

  • 列表和集合将简单地为空
  • 该数组将包含Nothing的五个元素
  • somevar赋值将导致立即NRE,因为Nothing没有.Text属性

稍后引用数组元素将导致NRE。 如果您在Form_Load执行此操作,由于存在奇怪的错误,IDE在发生异常时可能不会报告异常。 当您的代码尝试使用该数组时,将在以后弹出该异常。 这篇“无声的例外” 在这篇文章中有详细介绍 。 就我们的目的而言,关键在于,当创建表单( Sub NewForm Load事件)时发生灾难性事件时,异常可能未被报告,代码退出过程并仅显示表单。

由于Sub NewForm Load事件中没有其他代码会在NRE之后运行,因此很多其他事情可能会保持未初始化状态。

Sub Form_Load(..._
   '...
   Dim name As String = NameBoxes(2).Text        ' NRE
   ' ...
   ' More code (which will likely not be executed)
   ' ...
End Sub

请注意,这适用于任何以及所有控制和组件引用,因为它们是非法的:

Public Class Form1

    Private myFiles() As String = Me.OpenFileDialog1.FileName & ...
    Private dbcon As String = OpenFileDialog1.FileName & ";Jet Oledb..."
    Private studentName As String = TextBox13.Text

部分补救措施

很奇怪VB没有提供警告,但补救措施是在表单级别声明容器,但在控件确实存在时,在表单加载事件处理程序中初始化它们。 只要您的代码在InitializeComponent调用之后,就可以在Sub New完成:

' Module level declaration
Private NameBoxes as TextBox()
Private studentName As String

' Form Load, Form Shown or Sub New:
'
' Using the OP's approach (illegal using OPTION STRICT)
NameBoxes = New TextBox() {Me.Controls("TextBox1"), Me.Controls("TestBox2"), ...)
studentName = TextBox32.Text           ' For simple control references

数组代码可能还没有走出困境。 Me.Controls找不到任何位于容器控件(如GroupBoxPanel )中的Me.Controls ; 他们将在该Panel或GroupBox的Controls集合中。 当控件名称拼写错误时( "TeStBox2" ),也不会返回控件。 在这种情况下, Nothing将再次存储在这些数组元素中,并且当您尝试引用它时会导致NRE。

这些应该很容易找到,现在你知道你在找什么:

“Button2”驻留在Panel

补救

而不是使用表单的Controls集合按名称的间接引用,请使用控件引用:

' Declaration
Private NameBoxes As TextBox()

' Initialization -  simple and easy to read, hard to botch:
NameBoxes = New TextBox() {TextBox1, TextBox2, ...)

' Initialize a List
NamesList = New List(Of TextBox)({TextBox1, TextBox2, TextBox3...})
' or
NamesList = New List(Of TextBox)
NamesList.AddRange({TextBox1, TextBox2, TextBox3...})

函数返回Nothing

Private bars As New List(Of Bars)        ' Declared and created

Public Function BarList() As List(Of Bars)
    bars.Clear
    If someCondition Then
        For n As Integer = 0 to someValue
            bars.Add(GetBar(n))
        Next n
    Else
        Exit Function
    End If

    Return bars
End Function

这种情况下,IDE会警告您' 并非所有路径都返回值并且可能导致NullReferenceException '。 您可以通过将Exit Function替换为Return Nothing来取消警告,但这并不能解决问题。 当someCondition = False时,任何试图使用return的都会导致NRE:

bList = myFoo.BarList()
For Each b As Bar in bList      ' EXCEPTION
      ...

补救

使用Return bList替换Exit Function中的Exit Function 。 返回 List与返回Nothing 。 如果返回的对象有可能是Nothing ,请在使用之前进行测试:

 bList = myFoo.BarList()
 If bList IsNot Nothing Then...

很难实施Try / Catch

严重实施的Try / Catch可以隐藏问题所在并导致新问题:

Dim dr As SqlDataReader
Try
    Dim lnk As LinkButton = TryCast(sender, LinkButton)
    Dim gr As GridViewRow = DirectCast(lnk.NamingContainer, GridViewRow)
    Dim eid As String = GridView1.DataKeys(gr.RowIndex).Value.ToString()
    ViewState("username") = eid
    sqlQry = "select FirstName, Surname, DepartmentName, ExtensionName, jobTitle,
             Pager, mailaddress, from employees1 where username='" & eid & "'"
    If connection.State <> ConnectionState.Open Then
        connection.Open()
    End If
    command = New SqlCommand(sqlQry, connection)

    'More code fooing and barring

    dr = command.ExecuteReader()
    If dr.Read() Then
        lblFirstName.Text = Convert.ToString(dr("FirstName"))
        ...
    End If
    mpe.Show()
Catch

Finally
    command.Dispose()
    dr.Close()             ' <-- NRE
    connection.Close()
End Try

这是一个没有按预期创建的对象,但也演示了一个空Catch的计数器实用性。

There is an extra comma in the SQL (after 'mailaddress') which results in an exception at .ExecuteReader . After the Catch does nothing, Finally tries to perform clean up, but since you cannot Close a null DataReader object, a brand new NullReferenceException results.

An empty Catch block is the devil's playground. This OP was baffled why he was getting an NRE in the Finally block. In other situations, an empty Catch may result in something else much further downstream going haywire and cause you to spend time looking at the wrong things in the wrong place for the problem. (The "silent exception" described above provides the same entertainment value.)

Remedy

Don't use empty Try/Catch blocks - let the code crash so you can a) identify the cause b) identify the location and c) apply a proper remedy. Try/Catch blocks are not intended to hide exceptions from the person uniquely qualified to fix them - the developer.

DBNull is not the same as Nothing

For Each row As DataGridViewRow In dgvPlanning.Rows
    If Not IsDBNull(row.Cells(0).Value) Then
        ...

The IsDBNull function is used to test if a value equals System.DBNull : From MSDN:

The System.DBNull value indicates that the Object represents missing or non-existent data. DBNull is not the same as Nothing, which indicates that a variable has not yet been initialized.

Remedy

If row.Cells(0) IsNot Nothing Then ...

As before, you can test for Nothing, then for a specific value:

If (row.Cells(0) IsNot Nothing) AndAlso (IsDBNull(row.Cells(0).Value) = False) Then

例2

Dim getFoo = (From f In dbContext.FooBars
               Where f.something = something
               Select f).FirstOrDefault

If Not IsDBNull(getFoo) Then
    If IsDBNull(getFoo.user_id) Then
        txtFirst.Text = getFoo.first_name
    Else
       ...

FirstOrDefault returns the first item or the default value, which is Nothing for reference types and never DBNull :

If getFoo IsNot Nothing Then...

控制

Dim chk As CheckBox

chk = CType(Me.Controls(chkName), CheckBox)
If chk.Checked Then
    Return chk
End If

If a CheckBox with chkName can't be found (or exists in a GroupBox ), then chk will be Nothing and be attempting to reference any property will result in an exception.

Remedy

If (chk IsNot Nothing) AndAlso (chk.Checked) Then ...

The DataGridView

The DGV has a few quirks seen periodically:

dgvBooks.DataSource = loan.Books
dgvBooks.Columns("ISBN").Visible = True       ' NullReferenceException
dgvBooks.Columns("Title").DefaultCellStyle.Format = "C"
dgvBooks.Columns("Author").DefaultCellStyle.Format = "C"
dgvBooks.Columns("Price").DefaultCellStyle.Format = "C"

If dgvBooks has AutoGenerateColumns = True , it will create the columns, but it does not name them, so the above code fails when it references them by name.

Remedy

Name the columns manually, or reference by index:

dgvBooks.Columns(0).Visible = True

Example 2 — Beware of the NewRow

xlWorkSheet = xlWorkBook.Sheets("sheet1")

For i = 0 To myDGV.RowCount - 1
    For j = 0 To myDGV.ColumnCount - 1
        For k As Integer = 1 To myDGV.Columns.Count
            xlWorkSheet.Cells(1, k) = myDGV.Columns(k - 1).HeaderText
            xlWorkSheet.Cells(i + 2, j + 1) = myDGV(j, i).Value.ToString()
        Next
    Next
Next

When your DataGridView has AllowUserToAddRows as True (the default), the Cells in the blank/new row at the bottom will all contain Nothing . Most attempts to use the contents (for example, ToString ) will result in an NRE.

Remedy

Use a For/Each loop and test the IsNewRow property to determine if it is that last row. This works whether AllowUserToAddRows is true or not:

For Each r As DataGridViewRow in myDGV.Rows
    If r.IsNewRow = False Then
         ' ok to use this row

If you do use a For n loop, modify the row count or use Exit For when IsNewRow is true.

My.Settings (StringCollection)

Under certain circumstances, trying to use an item from My.Settings which is a StringCollection can result in a NullReference the first time you use it. The solution is the same, but not as obvious. 考虑:

My.Settings.FooBars.Add("ziggy")         ' foobars is a string collection

Since VB is managing Settings for you, it is reasonable to expect it to initialize the collection. It will, but only if you have previously added an initial entry to the collection (in the Settings editor). Since the collection is (apparently) initialized when an item is added, it remains Nothing when there are no items in the Settings editor to add.

Remedy

Initialize the settings collection in the form's Load event handler, if/when needed:

If My.Settings.FooBars Is Nothing Then
    My.Settings.FooBars = New System.Collections.Specialized.StringCollection
End If

Typically, the Settings collection will only need to be initialized the first time the application runs. An alternate remedy is to add an initial value to your collection in Project -> Settings | FooBars , save the project, then remove the fake value.

关键点

You probably forgot the New operator.

要么

Something you assumed would perform flawlessly to return an initialized object to your code, did not.

Don't ignore compiler warnings (ever) and use Option Strict On (always).

MSDN NullReference Exception

我有一些代码,当它执行时,它会抛出一个NullReferenceException ,并说:

你调用的对象是空的。

这是什么意思,我能做些什么来解决这个错误?


原因是什么?

底线

您试图使用null (或VB.NET中的Nothing )。 这意味着你要么将它设置为null ,要么你根本没有设置它。

像其他任何东西一样, null传递。 如果方法“A”中为null ,则可能是该方法“B”将null传递方法“A”。

本文的其余部分会更详细地介绍许多程序员经常犯的错误,这可能会导致NullReferenceException

进一步来说

NullReferenceException的运行时总是意味着同样的事情:您试图使用引用,并且引用未初始化(或者初始化后,但不再初始化)。

这意味着引用为null ,并且您无法通过null引用访问成员(例如方法)。 最简单的情况:

string foo = null;
foo.ToUpper();

这将在第二行中抛出NullReferenceException ,因为您无法在指向nullstring引用上调用实例方法ToUpper()

调试

你如何找到NullReferenceException的来源? 除了查看异常本身,这些异常本身将会发生在它发生的位置,Visual Studio中调试的一般规则适用于:放置战略断点并检查变量 ,方法是将鼠标悬停在其名称上,打开(快速)观察窗口或使用各种调试面板,如本地和汽车。

如果您想查找引用的位置或未设置,请右键单击其名称并选择“查找所有引用”。 然后,您可以在每个找到的位置放置一个断点,并使用附加的调试器运行程序。 每当调试器断开这样一个断点时,您需要确定您是否期望引用是非空的,检查该变量,并在您希望的时候验证它是否指向一个实例。

通过这种方式遵循程序流程,您可以找到实例不应该为null的位置,以及为什么它没有正确设置。

例子

可引发异常的一些常见情况:

通用

ref1.ref2.ref3.member

如果ref1或ref2或ref3为null,那么您将得到一个NullReferenceException 。 如果你想解决这个问题,那么通过将表达式重写为更简单的等价形式来找出哪一个是空的:

var r1 = ref1;
var r2 = r1.ref2;
var r3 = r2.ref3;
r3.member

特别是,在HttpContext.Current.User.Identity.NameHttpContext.Current可能为null,或者User属性可能为null,或者Identity属性可能为null。

间接

public class Person {
    public int Age { get; set; }
}
public class Book {
    public Person Author { get; set; }
}
public class Example {
    public void Foo() {
        Book b1 = new Book();
        int authorAge = b1.Author.Age; // You never initialized the Author property.
                                       // there is no Person to get an Age from.
    }
}

如果你想避免child(Person)空引用,你可以在父(Book)对象的构造函数中初始化它。

嵌套对象初始化器

这同样适用于嵌套对象初始值设定项:

Book b1 = new Book { Author = { Age = 45 } };

这意味着

Book b1 = new Book();
b1.Author.Age = 45;

虽然使用了new关键字,但它仅创建Book的新实例,但不创建Person的新实例,所以Author的属性仍然为null

嵌套集合初始化器

public class Person {
    public ICollection<Book> Books { get; set; }
}
public class Book {
    public string Title { get; set; }
}

嵌套集合初始化器的行为相同:

Person p1 = new Person {
    Books = {
        new Book { Title = "Title1" },
        new Book { Title = "Title2" },
    }
};

这意味着

Person p1 = new Person();
p1.Books.Add(new Book { Title = "Title1" });
p1.Books.Add(new Book { Title = "Title2" });

new Person只创建new Person的实例,但Books集合仍然为null 。 集合初始化器语法不会为p1.Books创建集合,它只能转换为p1.Books.Add(...)语句。

排列

int[] numbers = null;
int n = numbers[0]; // numbers is null. There is no array to index.

数组元素

Person[] people = new Person[5];
people[0].Age = 20 // people[0] is null. The array was allocated but not
                   // initialized. There is no Person to set the Age for.

铁血阵列

long[][] array = new long[1][];
array[0][0] = 3; // is null because only the first dimension is yet initialized.
                 // Use array[0] = new long[2]; first.

集合/列表/字典

Dictionary<string, int> agesForNames = null;
int age = agesForNames["Bob"]; // agesForNames is null.
                               // There is no Dictionary to perform the lookup.

范围变量(间接/延迟)

public class Person {
    public string Name { get; set; }
}
var people = new List<Person>();
people.Add(null);
var names = from p in people select p.Name;
string firstName = names.First(); // Exception is thrown here, but actually occurs
                                  // on the line above.  "p" is null because the
                                  // first element we added to the list is null.

活动

public class Demo
{
    public event EventHandler StateChanged;

    protected virtual void OnStateChanged(EventArgs e)
    {        
        StateChanged(this, e); // Exception is thrown here 
                               // if no event handlers have been attached
                               // to StateChanged event
    }
}

错误的命名约定:

如果您将字段命名为与本地人不同,则可能意识到您从未初始化该字段。

public class Form1 {
    private Customer customer;

    private void Form1_Load(object sender, EventArgs e) {
        Customer customer = new Customer();
        customer.Name = "John";
    }

    private void Button_Click(object sender, EventArgs e) {
        MessageBox.Show(customer.Name);
    }
}

这可以通过遵循约定为字段添加下划线来解决:

private Customer _customer;

ASP.NET页面生命周期:

public partial class Issues_Edit : System.Web.UI.Page
{
    protected TestIssue myIssue;

    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            // Only called on first load, not when button clicked
            myIssue = new TestIssue(); 
        }
    }

    protected void SaveButton_Click(object sender, EventArgs e)
    {
        myIssue.Entry = "NullReferenceException here!";
    }
}

ASP.NET会话值

// if the "FirstName" session value has not yet been set,
// then this line will throw a NullReferenceException
string firstName = Session["FirstName"].ToString();

ASP.NET MVC空视图模型

如果在ASP.NET MVC视图中引用@Model的属性时发生异常,则需要了解在return视图时, Model会在操作方法中设置。 当您从控制器返回一个空模型(或模型属性)时,当视图访问它时会发生异常:

// Controller
public class Restaurant:Controller
{
    public ActionResult Search()
    {
         return View();  // Forgot the provide a Model here.
    }
}

// Razor view 
@foreach (var restaurantSearch in Model.RestaurantSearch)  // Throws.
{
}

<p>@Model.somePropertyName</p> <!-- Also throws -->

WPF控件创建顺序和事件

WPF控件是在调用InitializeComponent中按照它们出现在可视化树中的顺序创建的。 在InitializeComponent引用后期创建的控件的过程中触发事件处理程序等的早期创建控件时,会引发NullReferenceException

例如 :

<Grid>
    <!-- Combobox declared first -->
    <ComboBox Name="comboBox1" 
              Margin="10"
              SelectedIndex="0" 
              SelectionChanged="comboBox1_SelectionChanged">
        <ComboBoxItem Content="Item 1" />
        <ComboBoxItem Content="Item 2" />
        <ComboBoxItem Content="Item 3" />
    </ComboBox>

    <!-- Label declared later -->
    <Label Name="label1" 
           Content="Label"
           Margin="10" />
</Grid>

这里comboBox1是在label1之前创建的。 如果comboBox1_SelectionChanged试图引用`label1,它还不会被创建。

private void comboBox1_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
    label1.Content = comboBox1.SelectedIndex.ToString(); // NullReference here!!
}

更改XAML中声明的顺序(即,在comboBox1之前列出label1 ,忽略设计原理问题,至少可以解决NullReferenceException

as投射

var myThing = someObject as Thing;

这不会抛出InvalidCastException,但在转换失败时(以及someObject本身为null)时返回null。 所以要注意这一点。

LINQ FirstOrDefault()和SingleOrDefault()

没有任何内容时,Plain First()Single()异常。 在这种情况下,“OrDefault”版本返回null。 所以要注意这一点。

的foreach

当您尝试迭代空集合时, foreach会抛出。 通常由返回集合的方法导致意外的null结果。

 List<int> list = null;    
 foreach(var v in list) { } // exception

更现实的例子 - 从XML文档中选择节点。 如果找不到节点,将会抛出,但初始调试显示所有属性都有效:

 foreach (var node in myData.MyXml.DocumentNode.SelectNodes("//Data"))

避免的方法

显式检查null并忽略空值。

如果您希望引用有时为空,那么可以在访问实例成员之前检查它为null

void PrintName(Person p) {
    if (p != null) {
        Console.WriteLine(p.Name);
    }
}

显式检查null并提供默认值。

方法调用您期望返回一个实例可以返回null ,例如当找不到的对象时。 在这种情况下,您可以选择返回默认值:

string GetCategory(Book b) {
    if (b == null)
        return "Unknown";
    return b.Category;
}

明确检查方法调用中的null并抛出一个自定义异常。

你也可以抛出一个自定义的异常,只是为了在调用代码中捕获它:

string GetCategory(string bookTitle) {
    var book = library.FindBook(bookTitle);  // This may return null
    if (book == null)
        throw new BookNotFoundException(bookTitle);  // Your custom exception
    return book.Category;
}

如果值不应为null ,则使用Debug.Assert以在发生异常之前捕获问题。

当你在开发过程中知道某个方法可能会,但从不应该返回null ,可以使用Debug.Assert()在发生时尽快中断:

string GetTitle(int knownBookID) {
    // You know this should never return null.
    var book = library.GetBook(knownBookID);  

    // Exception will occur on the next line instead of at the end of this method.
    Debug.Assert(book != null, "Library didn't return a book for known book ID.");

    // Some other code

    return book.Title; // Will never throw NullReferenceException in Debug mode.
}

虽然此检查不会在您的发行版本中结束 ,但在发布模式下运行时book == null时,会导致它再次抛出NullReferenceException

对于可为null的值类型,使用GetValueOrDefault()可以在它们为null时提供默认值。

DateTime? appointment = null;
Console.WriteLine(appointment.GetValueOrDefault(DateTime.Now));
// Will display the default value provided (DateTime.Now), because appointment is null.

appointment = new DateTime(2022, 10, 20);
Console.WriteLine(appointment.GetValueOrDefault(DateTime.Now));
// Will display the appointment date, not the default

使用空合并运算符: ?? [C#]或If() [VB]。

遇到null时提供默认值的简写:

IService CreateService(ILogger log, Int32? frobPowerLevel)
{
    var serviceImpl = new MyService(log ?? NullLog.Instance);

    // Note that the above "GetValueOrDefault()" can also be rewritten to use
    // the coalesce operator:
    serviceImpl.FrobPowerLevel = frobPowerLevel ?? 5;
}

使用null条件运算符: ?.?[x]用于数组(可用于C#6和VB.NET 14):

这有时也被称为安全导航或埃尔维斯(在它的形状之后)运营商。 如果运算符左侧的表达式为空,则不会评估右侧,而是返回null。 这意味着这样的情况:

var title = person.Title.ToUpper();

如果这个人没有标题,这会抛出一个异常,因为它试图在一个空值属性上调用ToUpper

在C#5及以下版本中,可以通过以下方式来保护它:

var title = person.Title == null ? null : person.Title.ToUpper();

现在title变量将是null,而不是引发异常。 C#6为此引入了更短的语法:

var title = person.Title?.ToUpper();

这将导致title变量为null ,并且如果person.Titlenull则不会调用ToUpper

当然,您仍然需要检查null的title ,或者将空条件运算符与空合并运算符( ?? )一起使用以提供默认值:

// regular null check
int titleLength = 0;
if (title != null)
    titleLength = title.Length; // If title is null, this would throw NullReferenceException

// combining the `?` and the `??` operator
int titleLength = title?.Length ?? 0;

同样,对于数组,您可以使用?[i] ,如下所示:

int[] myIntArray=null;
var i=5;
int? elem = myIntArray?[i];
if (!elem.HasValue) Console.WriteLine("No value");

这将执行以下操作:如果myIntArray为null,则表达式返回null,您可以安全地检查它。 如果它包含一个数组,它将执行相同的操作: elem = myIntArray[i]; 并返回第i 元素。


TL;DR: Try using Html.Partial instead of Renderpage

I was getting Object reference not set to an instance of an object when I tried to render a View within a View by sending it a Model, like this:

@{
    MyEntity M = new MyEntity();
}
@RenderPage("_MyOtherView.cshtml", M); // error in _MyOtherView, the Model was Null

Debugging showed the model was Null inside MyOtherView. Until I changed it to:

@{
    MyEntity M = new MyEntity();
}
@Html.Partial("_MyOtherView.cshtml", M);

And it worked.

Furthermore, the reason I didn't have Html.Partial to begin with was because Visual Studio sometimes throws error-looking squiggly lines under Html.Partial if it's inside a differently constructed foreach loop, even though it's not really an error:

@inherits System.Web.Mvc.WebViewPage
@{
    ViewBag.Title = "Entity Index";
    List<MyEntity> MyEntities = new List<MyEntity>();
    MyEntities.Add(new MyEntity());
    MyEntities.Add(new MyEntity());
    MyEntities.Add(new MyEntity());
}
<div>
    @{
        foreach(var M in MyEntities)
        {
            // Squiggly lines below. Hovering says: cannot convert method group 'partial' to non-delegate type Object, did you intend to envoke the Method?
            @Html.Partial("MyOtherView.cshtml");
        }
    }
</div>

But I was able to run the application with no problems with this "error". I was able to get rid of the error by changing the structure of the foreach loop to look like this:

@foreach(var M in MyEntities){
    ...
}

Although I have a feeling it was because Visual Studio was misreading the ampersands and brackets.


你能为这个做什么?

There is a lot of good answers here explaining what a null reference is and how to debug it. But there is very little on how to prevent the issue or at least make it easier to catch.

Check arguments

For example, methods can check the different arguments to see if they are null and throw an ArgumentNullException , an exception obviously created for this exact purpose.

The constructor for the ArgumentNullException even takes the name of the parameter and a message as arguments so you can tell the developer exactly what the problem is.

public void DoSomething(MyObject obj) {
    if(obj == null) 
    {
        throw new ArgumentNullException("obj", "Need a reference to obj.");
    }
}

Use Tools

There are also several libraries that can help. "Resharper" for example can provide you with warnings while you are writing code, especially if you use their attribute: NotNullAttribute

There's "Microsoft Code Contracts" where you use syntax like Contract.Requires(obj != null) which gives you runtime and compile checking: Introducing Code Contracts .

There's also "PostSharp" which will allow you to just use attributes like this:

public void DoSometing([NotNull] obj)

By doing that and making PostSharp part of your build process obj will be checked for null at runtime. See: PostSharp null check

Plain Code Solution

Or you can always code your own approach using plain old code. For example here is a struct that you can use to catch null references. It's modeled after the same concept as Nullable<T> :

[System.Diagnostics.DebuggerNonUserCode]
public struct NotNull<T> where T: class
{
    private T _value;

    public T Value
    {
        get
        {
            if (_value == null)
            {
                throw new Exception("null value not allowed");
            }

            return _value;
        }
        set
        {
            if (value == null)
            {
                throw new Exception("null value not allowed.");
            }

            _value = value;
        }
    }

    public static implicit operator T(NotNull<T> notNullValue)
    {
        return notNullValue.Value;
    }

    public static implicit operator NotNull<T>(T value)
    {
        return new NotNull<T> { Value = value };
    }
}

You would use very similar to the same way you would use Nullable<T> , except with the goal of accomplishing exactly the opposite - to not allow null . 这里有些例子:

NotNull<Person> person = null; // throws exception
NotNull<Person> person = new Person(); // OK
NotNull<Person> person = GetPerson(); // throws exception if GetPerson() returns null

NotNull<T> is implicitly cast to and from T so you can use it just about anywhere you need it. For example, you can pass a Person object to a method that takes a NotNull<Person> :

Person person = new Person { Name = "John" };
WriteName(person);

public static void WriteName(NotNull<Person> person)
{
    Console.WriteLine(person.Value.Name);
}

As you can see above as with nullable you would access the underlying value through the Value property. Alternatively, you can use an explicit or implicit cast, you can see an example with the return value below:

Person person = GetPerson();

public static NotNull<Person> GetPerson()
{
    return new Person { Name = "John" };
}

Or you can even use it when the method just returns T (in this case Person ) by doing a cast. For example, the following code would just like the code above:

Person person = (NotNull<Person>)GetPerson();

public static Person GetPerson()
{
    return new Person { Name = "John" };
}

Combine with Extension

Combine NotNull<T> with an extension method and you can cover even more situations. Here is an example of what the extension method can look like:

[System.Diagnostics.DebuggerNonUserCode]
public static class NotNullExtension
{
    public static T NotNull<T>(this T @this) where T: class
    {
        if (@this == null)
        {
            throw new Exception("null value not allowed");
        }

        return @this;
    }
}

And here is an example of how it could be used:

var person = GetPerson().NotNull();

GitHub上

For your reference I made the code above available on GitHub, you can find it at:

https://github.com/luisperezphd/NotNull

Related Language Feature

C# 6.0 introduced the "null-conditional operator" that helps with this a little. With this feature, you can reference nested objects and if any one of them is null the whole expression returns null .

This reduces the number of null checks you have to do in some cases. The syntax is to put a question mark before each dot. Take the following code for example:

var address = country?.State?.County?.City;

Imagine that country is an object of type Country that has a property called State and so on. If country , State , County , or City is null then address will be null . Therefore you only have to check whether address is null`.

It's a great feature, but it gives you less information. It doesn't make it obvious which of the 4 is null.

Built-in like Nullable?

C# has a nice shorthand for Nullable<T> , you can make something nullable by putting a question mark after the type like so int?

It would be nice if C# had something like the NotNull<T> struct above and had a similar shorthand, maybe the exclamation point (!) so that you could write something like: public void WriteName(Person! person) .


Adding a case when the class name for entity used in entity framework is same as class name for a web form code-behind file.

Suppose you have a web form Contact.aspx whose codebehind class is Contact and you have an entity name Contact.

Then following code will throw a NullReferenceException when you call context.SaveChanges()

Contact contact = new Contact { Name = "Abhinav"};
var context = new DataContext();
context.Contacts.Add(contact);
context.SaveChanges(); // NullReferenceException at this line

For the sake of completeness DataContext class

public class DataContext : DbContext 
{
    public DbSet<Contact> Contacts {get; set;}
}

and Contact entity class. Sometimes entity classes are partial classes so that you can extend them in other files too.

public partial class Contact 
{
    public string Name {get; set;}
}

The error occurs when both the entity and codebehind class are in same namespace. To fix this, rename the entity class or the codebehind class for Contact.aspx.

Reason I am still not sure about the reason. But whenever any of the entity class will extend System.Web.UI.Page this error occurs.

For discussion have a look at NullReferenceException in DbContext.saveChanges()


An example of this exception being thrown is: When you are trying to check something, that is null.

例如:

string testString = null; //Because it doesn't have a value (i.e. it's null; "Length" cannot do what it needs to do)

if (testString.Length == 0) // Throws a nullreferenceexception
{
    //Do something
} 

The .NET runtime will throw a NullReferenceException when you attempt to perform an action on something which hasn't been instantiated ie the code above.

In comparison to an ArgumentNullException which is typically thrown as a defensive measure if a method expects that what is being passed to it is not null.

More information is in C# NullReferenceException and Null Parameter .


Another general case where one might receive this exception involves mocking classes during unit testing. Regardless of the mocking framework being used, you must ensure that all appropriate levels of the class hierarchy are properly mocked. In particular, all properties of HttpContext which are referenced by the code under test must be mocked.

See " NullReferenceException thrown when testing custom AuthorizationAttribute " for a somewhat verbose example.


Another scenario is when you cast a null object into a value type . For example, the code below:

object o = null;
DateTime d = (DateTime)o;

It will throw a NullReferenceException on the cast. It seems quite obvious in the above sample, but this can happen in more "late-binding" intricate scenarios where the null object has been returned from some code you don't own, and the cast is for example generated by some automatic system.

One example of this is this simple ASP.NET binding fragment with the Calendar control:

<asp:Calendar runat="server" SelectedDate="<%#Bind("Something")%>" />

Here, SelectedDate is in fact a property - of DateTime type - of the Calendar Web Control type, and the binding could perfectly return something null. The implicit ASP.NET Generator will create a piece of code that will be equivalent to the cast code above. And this will raise a NullReferenceException that is quite difficult to spot, because it lies in ASP.NET generated code which compiles fine...


I have a different perspective to answering this. This sort of answers "what else can I do to avoid it? "

When working across different layers , for example in an MVC application, a controller needs services to call business operations. In such scenarios Dependency Injection Container can be used to initialize the services to avoid the NullReferenceException . So that means you don't need to worry about checking for null and just call the services from the controller as though they will always to available (and initialized) as either a singleton or a prototype.

public class MyController
{
    private ServiceA serviceA;
    private ServiceB serviceB;

    public MyController(ServiceA serviceA, ServiceB serviceB)
    {
        this.serviceA = serviceA;
        this.serviceB = serviceB;
    }

    public void MyMethod()
    {
        // We don't need to check null because the dependency injection container 
        // injects it, provided you took care of bootstrapping it.
        var someObject = serviceA.DoThis();
    }
}

If one is getting this message during saving or compiling the build, just close all the files and then open any file to compile and save.

For me the reason was that I had rename the file and old file was still open.


If you have not initialized a reference type, and you want to set or read one of its properties, it will throw a NullReferenceException .

例:

Person p = null;
p.Name = "Harry"; // NullReferenceException occurs here.

You can simply avoid this by checking if the variable is not null:

Person p = null;
if (p!=null)
{
    p.Name = "Harry"; // Not going to run to this point
}

To fully understand why a NullReferenceException is thrown, it is important to know the difference between value types and reference types .

So, if you're dealing with value types , NullReferenceExceptions can not occur. Though you need to keep alert when dealing with reference types !

Only reference types, as the name is suggesting, can hold references or point literally to nothing (or 'null'). Whereas value types always contain a value.

Reference types (these ones must be checked):

  • 动态
  • 目的

Value types (you can simply ignore these ones):

  • Numeric types
  • Integral types
  • Floating-point types
  • 十进制
  • 布尔
  • User defined structs

Interestingly, none of the answers on this page mention the two edge cases, hope no one minds if I add them:

Edge case #1: concurrent access to a Dictionary

Generic dictionaries in .NET are not thread-safe and they sometimes might throw a NullReference or even (more frequent) a KeyNotFoundException when you try to access a key from two concurrent threads. The exception is quite misleading in this case.

Edge case #2: unsafe code

If a NullReferenceException is thrown by unsafe code, you might look at your pointer variables, and check them for IntPtr.Zero or something. Which is the same thing ("null pointer exception"), but in unsafe code, variables are often cast to value-types/arrays, etc., and you bang your head against the wall, wondering how a value-type can throw this exception.

(Another reason for non-using unsafe code unless you need it, by the way)


It means you are trying to manipulate something which has reference but not yet initialized
The first thing to do here is check every instance created.

Use breakpoints , watches , inspect your varibale values.
Follow stack trace and search for exact row and column which is creating problem


It means your code used an object reference variable that was set to null (ie it did not reference an actual object instance).

To prevent the error, objects that could be null should be tested for null before being used.

if (myvar != null)
{
    // Go ahead and use myvar
    myvar.property = ...
}
else
{
    // Whoops! myvar is null and cannot be used without first
    // assigning it to an instance reference
    // Attempting to use myvar here will result in NullReferenceException
}

NullReferenceException or Object reference not set to an instance of an object occurs when an object of the class you are trying to use is not instantiated. 例如:

Assume that you have a class named Student.

public class Student
{
    private string FirstName;
    private string LastName;
    public string GetFullName()
    {
        return FirstName + LastName;
    }
}

Now, consider another class where you are trying to retrieve the student's full name.

public class StudentInfo
{      
    public string GetStudentName()
    {
        Student s;
        string fullname = s.GetFullName();
        return fullname;
    }        
}

As seen in the above code, the statement Student s - only declares the variable of type Student, note that the Student class is not instantiated at this point. Hence, when the statement s.GetFullName() gets executed, it will throw the NullReferenceException.


On the matter of "what should I do about it" , there can be many answers.

A more "formal" way of preventing such error conditions while developing is applying design by contract in your code. This means you need to set class invariants , and/or even function/method preconditions and postconditions on your system, while developing.

In short, class invariants ensure that there will be some constraints in your class that will not get violated in normal use (and therefore, the class will not get in an inconsistent state). Preconditions mean that data given as input to a function/method must follow some constraints set and never violate them, and postconditions mean that a function/method output must follow the set constraints again without ever violating them. Contract conditions should never be violated during execution of a bug-free program, therefore design by contract is checked in practice in debug mode, while being disabled in releases , to maximize the developed system performance.

This way, you can avoid NullReferenceException cases that are results of violation of the constraints set. For example, if you use an object property X in a class and later try to invoke one of its methods and X has a null value, then this will lead to NullReferenceException :

public X { get; set; }

public void InvokeX()
{
    X.DoSomething(); // if X value is null, you will get a NullReferenceException
}

But if you set "property X must never have a null value" as method precondition, then you can prevent the scenario described before:

//Using code contracts:
[ContractInvariantMethod]
protected void ObjectInvariant () 
{
    Contract.Invariant ( X != null );
    //...
}

For this cause, Code Contracts project exists for .NET applications.

Alternatively, design by contract can be applied using assertions .

UPDATE: It is worth mentioning that the term was coined by Bertrand Meyer in connection with his design of the Eiffel programming language .


The error line "Object reference not set to an instance of an object. " states that you have not assigned instance object to a object reference and still you are accessing properies/methods of that object.

for example: let say you have a class called myClass and it contains one property prop1.

public Class myClass
{
   public int prop1 {get;set;}
}

Now you are accessing this prop1 in some other class just like below:

public class Demo
{
     public void testMethod()
     {
        myClass ref = null;
        ref.prop1 = 1;  //This line throws error
     }
}

above line throws error because reference of class myClass is declared but not instantiated or an instance of object is not assigned to referecne of that class.

To fix this you have to instantiate (assign object to reference of that class).

public class Demo
{
     public void testMethod()
     {
        myClass ref = null;
        ref = new myClass();
        ref.prop1 = 1;  
     }
}

To use methods and member of an object you first have to create that object. If you didn't create it (variable that should hold the object is not initialized), but you try to use it's methods or variables you'll get that error.

Sometime you may just forgot to do initialization.

Edited: new can't return null, but fire's exception when failed. Long time ago it was the case in some languages, but not any more. Thanks @John Saunders for pointing that out.


While what causes a NullReferenceExceptions and approaches to avoid/fix such an exception have been addressed in other answers, what many programmers haven't learned yet is how to independently debug such exceptions during development.

In Visual Studio this is usually easy thanks to the Visual Studio Debugger .

First, make sure that the correct error is going to be caught - see How do I allow breaking on 'System.NullReferenceException' in VS2010? Note 1

Then either Start with Debugging (F5) or Attach [the VS Debugger] to Running Process . On occasion it may be useful to use Debugger.Break , which will prompt to launch the debugger.

Now, when the NullReferenceException is thrown (or unhandled) the debugger will stop (remember the rule set above?) on the line on which the exception occurred. Sometimes the error will be easy to spot.

For instance, in the following line the only code that can cause the exception is if myString evaluates to null. This can be verified by looking at the Watch Window or running expressions in the Immediate Window .

var x = myString.Trim();

In more advanced cases, such as the following, you'll need to use one of the techniques above (Watch or Immediate Windows) to inspect the expressions to determine if str1 was null or if str2 was null.

var x = str1.Trim() + str2.Trim();

Once where the exception is throw has been located, it's usually trivial to reason backwards to find out where the null value was [incorrectly] introduced --

Take the time required to understand the cause of the exception. Inspect for null expressions. Inspect the previous expressions which could have resulted in such null expressions. Add breakpoints and step through the program as appropriate. Use the debugger.

1 If Break on Throws is too aggressive and the debugger stops on an NPE in the .NET or 3rd-party library, Break on User-Unhandled can be used to limit the exceptions caught. Additionally, VS2012 introduces Just My Code which I recommend enabling as well.

If you are debugging with Just My Code enabled, the behavior is slightly different. With Just My Code enabled, the debugger ignores first-chance common language runtime (CLR) exceptions that are thrown outside of My Code and do not pass through My Code


You are using the object that contains the null value reference. So it's giving a null exception. In the example the string value is null and when checking its length, the exception occurred.

例:

string value = null;
if (value.Length == 0) // <-- Causes exception
{
    Console.WriteLine(value); // <-- Never reached
}

The exception error is:

Unhandled Exception:

System.NullReferenceException: Object reference not set to an instance of an object. at Program.Main()





nullreferenceexception