value - nullable decimal c#




Convert string to nullable type(int, double, etc…) (11)

Another thing to keep in mind is that the string itself might be null.

public static Nullable<T> ToNullable<T>(this string s) where T: struct
{
    Nullable<T> result = new Nullable<T>();
    try
    {
        if (!string.IsNullOrEmpty(s) && s.Trim().Length > 0)
        {
            TypeConverter conv = TypeDescriptor.GetConverter(typeof(T));
            result = (T)conv.ConvertFrom(s);
        }
    }
    catch { } 
    return result;
}

I am attempting to do some data conversion. Unfortunately, much of the data is in strings, where it should be int's or double, etc...

So what I've got is something like:

double? amount = Convert.ToDouble(strAmount);

The problem with this approach is if strAmount is empty, if it's empty I want it to amount to be null, so when I add it into the database the column will be null. So I ended up writing this:

double? amount = null;
if(strAmount.Trim().Length>0)
{
    amount = Convert.ToDouble(strAmount);
}

Now this works fine, but I now have five lines of code instead of one. This makes things a little more difficult to read, especially when I have a large amount of columns to convert.

I thought I'd use an extension to the string class and generic's to pass in the type, this is because it could be a double, or an int, or a long. So I tried this:

    public static class GenericExtension
    {
        public static Nullable<T> ConvertToNullable<T>(this string s, T type) where T: struct
        {
            if (s.Trim().Length > 0)
            {
                return (Nullable<T>)s;
            }
            return null;
        }
    }

But I get the error: Cannot convert type 'string' to 'T?'

Is there a way around this? I am not very familiar with creating methods using generics.


Another variation. This one

  • Does not swallow exceptions
  • Throws a NotSupportedException if the type can not be converted from string. For instance, a custom struct without a type converter.
  • Otherwise returns a (T?)null if the string fails to parse. No need to check for null or whitespace.
using System.ComponentModel;

public static Nullable<T> ToNullable<T>(this string s) where T : struct
{
    var ret = new Nullable<T>();
    var conv = TypeDescriptor.GetConverter(typeof(T));

    if (!conv.CanConvertFrom(typeof(string)))
    {
        throw new NotSupportedException();
    }

    if (conv.IsValid(s))
    {
        ret = (T)conv.ConvertFrom(s);
    }

    return ret;
}

Here's something based on accepted answer. I removed the try/catch to make sure all the exceptions are not swallowed and not dealt with. Also made sure that the return variable (in accepted answer) is never initialized twice for nothing.

public static Nullable<T> ToNullable<T>(this string s) where T: struct
{
    if (!string.IsNullOrWhiteSpace(s))
    {
        TypeConverter conv = TypeDescriptor.GetConverter(typeof(T));

        return (T)conv.ConvertFrom(s);
    }

    return default(Nullable<T>);
}

I like Joel's answer, but I've modified it slightly as I'm not a fan of eating exceptions.

    /// <summary>
    /// Converts a string to the specified nullable type.
    /// </summary>
    /// <typeparam name="T">The type to convert to</typeparam>
    /// <param name="s">The string to convert</param>
    /// <returns>The nullable output</returns>
    public static T? ToNullable<T>(this string s) where T : struct
    {
        if (string.IsNullOrWhiteSpace(s))
            return null;

        TypeConverter conv = TypeDescriptor.GetConverter(typeof (T));
        return (T) conv.ConvertFrom(s);
    }

    /// <summary>
    /// Attempts to convert a string to the specified nullable primative.
    /// </summary>
    /// <typeparam name="T">The primitive type to convert to</typeparam>
    /// <param name="data">The string to convert</param>
    /// <param name="output">The nullable output</param>
    /// <returns>
    /// True if conversion is successfull, false otherwise.  Null and whitespace will
    /// be converted to null and return true.
    /// </returns>
    public static bool TryParseNullable<T>(this string data, out T? output) where T : struct
    {
        try
        {
            output = data.ToNullable<T>();
            return true;
        }
        catch
        {
            output = null;
            return false;
        }
    }

Let's add one more similar solution to the stack. This one also parses enums, and it looks nice. Very safe.

/// <summary>
    /// <para>More convenient than using T.TryParse(string, out T). 
    /// Works with primitive types, structs, and enums.
    /// Tries to parse the string to an instance of the type specified.
    /// If the input cannot be parsed, null will be returned.
    /// </para>
    /// <para>
    /// If the value of the caller is null, null will be returned.
    /// So if you have "string s = null;" and then you try "s.ToNullable...",
    /// null will be returned. No null exception will be thrown. 
    /// </para>
    /// <author>Contributed by Taylor Love (Pangamma)</author>
    /// </summary>
    /// <typeparam name="T"></typeparam>
    /// <param name="p_self"></param>
    /// <returns></returns>
    public static T? ToNullable<T>(this string p_self) where T : struct
    {
        if (!string.IsNullOrEmpty(p_self))
        {
            var converter = System.ComponentModel.TypeDescriptor.GetConverter(typeof(T));
            if (converter.IsValid(p_self)) return (T)converter.ConvertFromString(p_self);
            if (typeof(T).IsEnum) { T t; if (Enum.TryParse<T>(p_self, out t)) return t;}
        }

        return null;
    }

https://github.com/Pangamma/PangammaUtilities-CSharp/blob/master/PangammaUtilities/Extensions/ToNullableStringExtension.cs


My example for anonimous types:

private object ConvertNullable(object value, Type nullableType)
{
    Type resultType = typeof(Nullable<>).MakeGenericType(nullableType.GetGenericArguments());
    return Activator.CreateInstance(resultType, Convert.ChangeType(value, nullableType.GetGenericArguments()[0]));
}

...

Type anonimousType = typeof(Nullable<int>);
object nullableInt1 = ConvertNullable("5", anonimousType);
// or evident Type
Nullable<int> nullableInt2 = (Nullable<int>)ConvertNullable("5", typeof(Nullable<int>));

There is a generic solution (for any type). Usability is good, but implementation should be improved: http://cleansharp.de/wordpress/2011/05/generischer-typeconverter/

This allows you to write very clean code like this:

string value = null;
int? x = value.ConvertOrDefault<int?>();

and also:

object obj = 1;  

string value = null;
int x = 5;
if (value.TryConvert(out x))
    Console.WriteLine("TryConvert example: " + x); 

bool boolean = "false".ConvertOrDefault<bool>();
bool? nullableBoolean = "".ConvertOrDefault<bool?>();
int integer = obj.ConvertOrDefault<int>();
int negativeInteger = "-12123".ConvertOrDefault<int>();
int? nullableInteger = value.ConvertOrDefault<int?>();
MyEnum enumValue = "SecondValue".ConvertOrDefault<MyEnum>();

MyObjectBase myObject = new MyObjectClassA();
MyObjectClassA myObjectClassA = myObject.ConvertOrDefault<MyObjectClassA>();

There is no way around this. Nullable, as well as your method, is constrained to using only value types as it's argument. String is a reference type and hence is incompatible with this declaration.


You can use the following with objects, unfortunately this does not work with strings though.

double? amount = (double?)someObject;

I use it for wrapping a session variable in a property (on a base page).. so my actual usage is (in my base page):

public int? OrganisationID
{
    get { return (int?)Session[Constants.Session_Key_OrganisationID]; }
    set { Session[Constants.Session_Key_OrganisationID] = value; }
}

I'm able to check for null in page logic:

if (base.OrganisationID == null)
    // do stuff

You could try using the below extension method:

public static T? GetValueOrNull<T>(this string valueAsString)
    where T : struct 
{
    if (string.IsNullOrEmpty(valueAsString))
        return null;
    return (T) Convert.ChangeType(valueAsString, typeof(T));
}

This way you can do this:

double? amount = strAmount.GetValueOrNull<double>();
int? amount = strAmount.GetValueOrNull<int>();
decimal? amount = strAmount.GetValueOrNull<decimal>();

public static class GenericExtension
{
    public static T? ConvertToNullable<T>(this String s) where T : struct 
    {
        try
        {
            return (T?)TypeDescriptor.GetConverter(typeof(T)).ConvertFrom(s);
        }
        catch (Exception)
        {
            return null;
        }
    }
}




type-conversion