c# - lambda教學 - lambda expression java




從lambda表達式中檢索屬性名稱 (12)

通過lambda表達式傳入時是否有更好的方法來獲取屬性名稱? 這是我目前擁有的。

例如。

GetSortingInfo<User>(u => u.UserId);

只有當屬性是一個字符串時,它才會將它作為一個元素表達式進行處理。 因為不是所有的屬性都是字符串,我不得不使用對象,但它會返回一個一元表達式。

public static RouteValueDictionary GetInfo<T>(this HtmlHelper html, 
    Expression<Func<T, object>> action) where T : class
{
    var expression = GetMemberInfo(action);
    string name = expression.Member.Name;

    return GetInfo(html, name);
}

private static MemberExpression GetMemberInfo(Expression method)
{
    LambdaExpression lambda = method as LambdaExpression;
    if (lambda == null)
        throw new ArgumentNullException("method");

    MemberExpression memberExpr = null;

    if (lambda.Body.NodeType == ExpressionType.Convert)
    {
        memberExpr = 
            ((UnaryExpression)lambda.Body).Operand as MemberExpression;
    }
    else if (lambda.Body.NodeType == ExpressionType.MemberAccess)
    {
        memberExpr = lambda.Body as MemberExpression;
    }

    if (memberExpr == null)
        throw new ArgumentException("method");

    return memberExpr;
}

Array.Length有一個邊界案例。 雖然“長度”作為屬性公開,但不能將其用於以前提出的任何解決方案中。

using Contract = System.Diagnostics.Contracts.Contract;
using Exprs = System.Linq.Expressions;

static string PropertyNameFromMemberExpr(Exprs.MemberExpression expr)
{
    return expr.Member.Name;
}

static string PropertyNameFromUnaryExpr(Exprs.UnaryExpression expr)
{
    if (expr.NodeType == Exprs.ExpressionType.ArrayLength)
        return "Length";

    var mem_expr = expr.Operand as Exprs.MemberExpression;

    return PropertyNameFromMemberExpr(mem_expr);
}

static string PropertyNameFromLambdaExpr(Exprs.LambdaExpression expr)
{
         if (expr.Body is Exprs.MemberExpression)   return PropertyNameFromMemberExpr(expr.Body as Exprs.MemberExpression);
    else if (expr.Body is Exprs.UnaryExpression)    return PropertyNameFromUnaryExpr(expr.Body as Exprs.UnaryExpression);

    throw new NotSupportedException();
}

public static string PropertyNameFromExpr<TProp>(Exprs.Expression<Func<TProp>> expr)
{
    Contract.Requires<ArgumentNullException>(expr != null);
    Contract.Requires<ArgumentException>(expr.Body is Exprs.MemberExpression || expr.Body is Exprs.UnaryExpression);

    return PropertyNameFromLambdaExpr(expr);
}

public static string PropertyNameFromExpr<T, TProp>(Exprs.Expression<Func<T, TProp>> expr)
{
    Contract.Requires<ArgumentNullException>(expr != null);
    Contract.Requires<ArgumentException>(expr.Body is Exprs.MemberExpression || expr.Body is Exprs.UnaryExpression);

    return PropertyNameFromLambdaExpr(expr);
}

現在示例用法:

int[] someArray = new int[1];
Console.WriteLine(PropertyNameFromExpr( () => someArray.Length ));

如果PropertyNameFromUnaryExpr未檢查ArrayLengthArrayLength “someArray”打印到控制台(編譯器似乎生成直接訪問背景長度字段 ,作為優化,即使在調試中,因此也是特例)。


以下是Cameron提出的方法更新。 第一個參數不是必需的。

public PropertyInfo GetPropertyInfo<TSource, TProperty>(
    Expression<Func<TSource, TProperty>> propertyLambda)
{
    Type type = typeof(TSource);

    MemberExpression member = propertyLambda.Body as MemberExpression;
    if (member == null)
        throw new ArgumentException(string.Format(
            "Expression '{0}' refers to a method, not a property.",
            propertyLambda.ToString()));

    PropertyInfo propInfo = member.Member as PropertyInfo;
    if (propInfo == null)
        throw new ArgumentException(string.Format(
            "Expression '{0}' refers to a field, not a property.",
            propertyLambda.ToString()));

    if (type != propInfo.ReflectedType &&
        !type.IsSubclassOf(propInfo.ReflectedType))
        throw new ArgumentException(string.Format(
            "Expresion '{0}' refers to a property that is not from type {1}.",
            propertyLambda.ToString(),
            type));

    return propInfo;
}

您可以執行以下操作:

var propertyInfo = GetPropertyInfo<SomeType>(u => u.UserID);
var propertyInfo = GetPropertyInfo((SomeType u) => u.UserID);

擴展方法:

public static PropertyInfo GetPropertyInfo<TSource, TProperty>(this TSource source,
    Expression<Func<TSource, TProperty>> propertyLambda) where TSource : class
{
    return GetPropertyInfo(propertyLambda);
}

public static string NameOfProperty<TSource, TProperty>(this TSource source,
    Expression<Func<TSource, TProperty>> propertyLambda) where TSource : class
{
    PropertyInfo prodInfo = GetPropertyInfo(propertyLambda);
    return prodInfo.Name;
}

您可以:

SomeType someInstance = null;
string propName = someInstance.NameOfProperty(i => i.Length);
PropertyInfo propInfo = someInstance.GetPropertyInfo(i => i.Length);

從.NET 4.0開始,您可以使用ExpressionVisitor來查找屬性:

class ExprVisitor : ExpressionVisitor {
    public bool IsFound { get; private set; }
    public string MemberName { get; private set; }
    public Type MemberType { get; private set; }
    protected override Expression VisitMember(MemberExpression node) {
        if (!IsFound && node.Member.MemberType == MemberTypes.Property) {
            IsFound = true;
            MemberName = node.Member.Name;
            MemberType = node.Type;
        }
        return base.VisitMember(node);
    }
}

以下是你如何使用這個訪客:

var visitor = new ExprVisitor();
visitor.Visit(expr);
if (visitor.IsFound) {
    Console.WriteLine("First property in the expression tree: Name={0}, Type={1}", visitor.MemberName, visitor.MemberType.FullName);
} else {
    Console.WriteLine("No properties found.");
}

我使用C#6之前項目的擴展方法,以及針對C#6目標的nameof

public static class MiscExtentions
{
    public static string NameOf<TModel, TProperty>(this object @object, Expression<Func<TModel, TProperty>> propertyExpression)
    {
        var expression = propertyExpression.Body as MemberExpression;
        if (expression == null)
        {
            throw new ArgumentException("Expression is not a property.");
        }

        return expression.Member.Name;
    }
}

我稱之為:

public class MyClass 
{
    public int Property1 { get; set; }
    public string Property2 { get; set; }
    public int[] Property3 { get; set; }
    public Subclass Property4 { get; set; }
    public Subclass[] Property5 { get; set; }
}

public class Subclass
{
    public int PropertyA { get; set; }
    public string PropertyB { get; set; }
}

// result is Property1
this.NameOf((MyClass o) => o.Property1);
// result is Property2
this.NameOf((MyClass o) => o.Property2);
// result is Property3
this.NameOf((MyClass o) => o.Property3);
// result is Property4
this.NameOf((MyClass o) => o.Property4);
// result is PropertyB
this.NameOf((MyClass o) => o.Property4.PropertyB);
// result is Property5
this.NameOf((MyClass o) => o.Property5);

它適用於字段和屬性。


我已經完成了類似於以下方法的INotifyPropertyChanged實現。 這裡的屬性存儲在下面顯示的基類的字典中。 當然,使用繼承並不總是可取的,但對於視圖模型,我認為它是可以接受的,並且在視圖模型類中提供非常乾淨的屬性引用。

public class PhotoDetailsViewModel
    : PropertyChangedNotifierBase<PhotoDetailsViewModel>
{
    public bool IsLoading
    {
        get { return GetValue(x => x.IsLoading); }
        set { SetPropertyValue(x => x.IsLoading, value); }
    }

    public string PendingOperation
    {
        get { return GetValue(x => x.PendingOperation); }
        set { SetPropertyValue(x => x.PendingOperation, value); }
    }

    public PhotoViewModel Photo
    {
        get { return GetValue(x => x.Photo); }
        set { SetPropertyValue(x => x.Photo, value); }
    }
}

下面顯示了更複雜的基類。 它處理從lambda表達式到屬性名稱的翻譯。 請注意,由於僅使用名稱,因此屬性實際上是偽屬性。 但它對於視圖模型以及對視圖模型屬性的引用將是透明的。

public class PropertyChangedNotifierBase<T> : INotifyPropertyChanged
{
    readonly Dictionary<string, object> _properties = new Dictionary<string, object>();

    protected U GetValue<U>(Expression<Func<T, U>> property)
    {
        var propertyName = GetPropertyName(property);

        return GetValue<U>(propertyName);
    }

    private U GetValue<U>(string propertyName)
    {
        object value;

        if (!_properties.TryGetValue(propertyName, out value))
        {
            return default(U);
        }

        return (U)value;
    }

    protected void SetPropertyValue<U>(Expression<Func<T, U>> property, U value)
    {
        var propertyName = GetPropertyName(property);

        var oldValue = GetValue<U>(propertyName);

        if (Object.ReferenceEquals(oldValue, value))
        {
            return;
        }
        _properties[propertyName] = value;

        RaisePropertyChangedEvent(propertyName);
    }

    protected void RaisePropertyChangedEvent<U>(Expression<Func<T, U>> property)
    {
        var name = GetPropertyName(property);
        RaisePropertyChangedEvent(name);
    }

    protected void RaisePropertyChangedEvent(string propertyName)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }
    }

    private static string GetPropertyName<U>(Expression<Func<T, U>> property)
    {
        if (property == null)
        {
            throw new NullReferenceException("property");
        }

        var lambda = property as LambdaExpression;

        var memberAssignment = (MemberExpression) lambda.Body;
        return memberAssignment.Member.Name;
    }

    public event PropertyChangedEventHandler PropertyChanged;
}

我更新了@ Cameron的答案,以包含對Convert類型的lambda表達式進行的一些安全檢查:

PropertyInfo GetPropertyName<TSource, TProperty>(
Expression<Func<TSource, TProperty>> propertyLambda)
{
  var body = propertyLambda.Body;
  if (!(body is MemberExpression member)
    && !(body is UnaryExpression unary
      && (member = unary.Operand as MemberExpression) != null))
    throw new ArgumentException($"Expression '{propertyLambda}' " +
      "does not refer to a property.");

  if (!(member.Member is PropertyInfo propInfo))
    throw new ArgumentException($"Expression '{propertyLambda}' " +
      "refers to a field, not a property.");

  var type = typeof(TSource);
  if (!propInfo.DeclaringType.GetTypeInfo().IsAssignableFrom(type.GetTypeInfo()))
    throw new ArgumentException($"Expresion '{propertyLambda}' " + 
      "refers to a property that is not from type '{type}'.");

  return propInfo;
}

我正在玩同樣的東西,並努力工作。 它沒有完全測試,但似乎處理值類型的問題(你遇到的一元表達問題)

public static string GetName(Expression<Func<object>> exp)
{
    MemberExpression body = exp.Body as MemberExpression;

    if (body == null) {
       UnaryExpression ubody = (UnaryExpression)exp.Body;
       body = ubody.Operand as MemberExpression;
    }

    return body.Member.Name;
}

我發現一些深入到MemberExpression / UnaryExpression的建議答案不會捕獲嵌套/子屬性。

ex) o => o.Thing1.Thing2返回Thing1而不是Thing1.Thing2

如果您嘗試使用EntityFramework DbSet.Include(...)則此區別非常重要。

我發現只是解析Expression.ToString()似乎工作正常,並且相對較快。 我將它與UnaryExpression版本進行了比較,甚至將ToStringMember/UnaryExpression ,看看它是否更快,但差異可以忽略不計。 如果這是一個可怕的想法,請糾正我。

擴展方法

/// <summary>
/// Given an expression, extract the listed property name; similar to reflection but with familiar LINQ+lambdas.  Technique @via https://.com/a/16647343/1037948
/// </summary>
/// <remarks>Cheats and uses the tostring output -- Should consult performance differences</remarks>
/// <typeparam name="TModel">the model type to extract property names</typeparam>
/// <typeparam name="TValue">the value type of the expected property</typeparam>
/// <param name="propertySelector">expression that just selects a model property to be turned into a string</param>
/// <param name="delimiter">Expression toString delimiter to split from lambda param</param>
/// <param name="endTrim">Sometimes the Expression toString contains a method call, something like "Convert(x)", so we need to strip the closing part from the end</param>
/// <returns>indicated property name</returns>
public static string GetPropertyName<TModel, TValue>(this Expression<Func<TModel, TValue>> propertySelector, char delimiter = '.', char endTrim = ')') {

    var asString = propertySelector.ToString(); // gives you: "o => o.Whatever"
    var firstDelim = asString.IndexOf(delimiter); // make sure there is a beginning property indicator; the "." in "o.Whatever" -- this may not be necessary?

    return firstDelim < 0
        ? asString
        : asString.Substring(firstDelim+1).TrimEnd(endTrim);
}//--   fn  GetPropertyNameExtended

(檢查分隔符甚至可能是矯枉過正)

演示(LinqPad)

演示+比較代碼 - https://gist.github.com/zaus/6992590



這是一個通用實現,用於獲取struct / class / interface / delegate / array的字段/屬性/索引器/方法/擴展方法/委託的字符串名稱。 我已經測試了靜態/實例和非泛型/通用變體的組合。

//involves recursion
public static string GetMemberName(this LambdaExpression memberSelector)
{
    Func<Expression, string> nameSelector = null;  //recursive func
    nameSelector = e => //or move the entire thing to a separate recursive method
    {
        switch (e.NodeType)
        {
            case ExpressionType.Parameter:
                return ((ParameterExpression)e).Name;
            case ExpressionType.MemberAccess:
                return ((MemberExpression)e).Member.Name;
            case ExpressionType.Call:
                return ((MethodCallExpression)e).Method.Name;
            case ExpressionType.Convert:
            case ExpressionType.ConvertChecked:
                return nameSelector(((UnaryExpression)e).Operand);
            case ExpressionType.Invoke:
                return nameSelector(((InvocationExpression)e).Expression);
            case ExpressionType.ArrayLength:
                return "Length";
            default:
                throw new Exception("not a proper member selector");
        }
    };

    return nameSelector(memberSelector.Body);
}

這件事可以寫在一個簡單的while循環中:

//iteration based
public static string GetMemberName(this LambdaExpression memberSelector)
{
    var currentExpression = memberSelector.Body;

    while (true)
    {
        switch (currentExpression.NodeType)
        {
            case ExpressionType.Parameter:
                return ((ParameterExpression)currentExpression).Name;
            case ExpressionType.MemberAccess:
                return ((MemberExpression)currentExpression).Member.Name;
            case ExpressionType.Call:
                return ((MethodCallExpression)currentExpression).Method.Name;
            case ExpressionType.Convert:
            case ExpressionType.ConvertChecked:
                currentExpression = ((UnaryExpression)currentExpression).Operand;
                break;
            case ExpressionType.Invoke:
                currentExpression = ((InvocationExpression)currentExpression).Expression;
                break;
            case ExpressionType.ArrayLength:
                return "Length";
            default:
                throw new Exception("not a proper member selector");
        }
    }
}

我喜歡遞歸方法,儘管第二個方法可能更易於閱讀。 人們可以這樣稱呼它:

someExpr = x => x.Property.ExtensionMethod()[0]; //or
someExpr = x => Static.Method().Field; //or
someExpr = x => VoidMethod(); //or
someExpr = () => localVariable; //or
someExpr = x => x; //or
someExpr = x => (Type)x; //or
someExpr = () => Array[0].Delegate(null); //etc

string name = someExpr.GetMemberName();

打印最後一個成員。

注意:

  1. 如果是ABC等鍊式表達式,則返回“C”。

  2. 這不適用於const ,數組索引器或enum (不可能涵蓋所有情況)。


這是另一種基於此答案獲取PropertyInfo的方法。 它消除了對象實例的需要。

/// <summary>
/// Get metadata of property referenced by expression. Type constrained.
/// </summary>
public static PropertyInfo GetPropertyInfo<TSource, TProperty>(Expression<Func<TSource, TProperty>> propertyLambda)
{
    return GetPropertyInfo((LambdaExpression) propertyLambda);
}

/// <summary>
/// Get metadata of property referenced by expression.
/// </summary>
public static PropertyInfo GetPropertyInfo(LambdaExpression propertyLambda)
{
    // https://.com/questions/671968/retrieving-property-name-from-lambda-expression
    MemberExpression member = propertyLambda.Body as MemberExpression;
    if (member == null)
        throw new ArgumentException(string.Format(
            "Expression '{0}' refers to a method, not a property.",
            propertyLambda.ToString()));

    PropertyInfo propInfo = member.Member as PropertyInfo;
    if (propInfo == null)
        throw new ArgumentException(string.Format(
            "Expression '{0}' refers to a field, not a property.",
            propertyLambda.ToString()));

    if(propertyLambda.Parameters.Count() == 0)
        throw new ArgumentException(String.Format(
            "Expression '{0}' does not have any parameters. A property expression needs to have at least 1 parameter.",
            propertyLambda.ToString()));

    var type = propertyLambda.Parameters[0].Type;
    if (type != propInfo.ReflectedType &&
        !type.IsSubclassOf(propInfo.ReflectedType))
        throw new ArgumentException(String.Format(
            "Expression '{0}' refers to a property that is not from type {1}.",
            propertyLambda.ToString(),
            type));
    return propInfo;
}

它可以這樣調用:

var propertyInfo = GetPropertyInfo((User u) => u.UserID);

那麼,沒有必要調用.Name.ToString() ,但大致就是這樣,是的。 你可能需要考慮的唯一問題是x.Foo.Bar應該返回“Foo”,“Bar”還是一個異常 - 也就是說你是否需要迭代。

(重新評論)了解更多有關靈活分類的信息,請參閱here





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