C# 无法将“System.Linq.Expressions.UnaryExpression”类型的对象转换为“System.Linq.Expressions.MemberExpression”

Unable to cast object of type 'System.Linq.Expressions.UnaryExpression' to type 'System.Linq.Expressions.MemberExpression'

提问by Nikhil Agrawal

I created a method in C#to get methodname

我在 C# 中创建了一个方法来获取方法名

public string GetCorrectPropertyName<T>(Expression<Func<T, string>> expression)
{
   return ((MemberExpression)expression.Body).Member.Name; // Failure Point
}

and calling it as

并将其称为

string lcl_name = false;
public string Name
{
get { return lcl_name ; }
set 
    {
        lcl_name = value;
        OnPropertyChanged(GetCorrectPropertyName<ThisClassName>(x => x.Name));
}
}

This works fine if property is string and for all other types gives this exception:

如果属性是字符串并且对于所有其他类型给出此异常，则这可以正常工作：

Unable to cast object of type 'System.Linq.Expressions.UnaryExpression' to type 'System.Linq.Expressions.MemberExpression'.

无法将“System.Linq.Expressions.UnaryExpression”类型的对象转换为“System.Linq.Expressions.MemberExpression”类型。

1. I changed string to object in method signature, but then it fails again.
2. I changed calling from x => x.PropertyNameto x => Convert.ToString(x.PropertyName)and it still fails

1. 我在方法签名中将字符串更改为对象，但随后又失败了。
2. 我将呼叫从 更改x => x.PropertyName为x => Convert.ToString(x.PropertyName)，但仍然失败

Where am I wrong?

我哪里错了？

采纳答案by Jon Egerton

You need a separate line to extract the Member where the input expression is a Unary Expression.

您需要一个单独的行来提取成员，其中输入表达式是一元表达式。

Just converted this from VB.Net, so might be slightly off - let me know if I need to make any minor tweaks:

刚刚从 VB.Net 转换了这个，所以可能会稍微偏离 - 如果我需要做任何小的调整，请告诉我：

public string GetCorrectPropertyName<T>(Expression<Func<T, Object>> expression)
{
    if (expression.Body is MemberExpression) {
        return ((MemberExpression)expression.Body).Member.Name;
    }
    else {
        var op = ((UnaryExpression)expression.Body).Operand;
        return ((MemberExpression)op).Member.Name;
    }                
}

The VB version is:

VB版本是：

Public Shared Function GetCorrectPropertyName(Of T) _
             (ByVal expression As Expression(Of Func(Of T, Object))) As String
    If TypeOf expression.Body Is MemberExpression Then
        Return DirectCast(expression.Body, MemberExpression).Member.Name
    Else
        Dim op = (CType(expression.Body, UnaryExpression).Operand)
        Return DirectCast(op, MemberExpression).Member.Name
    End If
End Function

Note that the input expression does not return string necessarily - that constrains you to only reading properties that return strings.

请注意，输入表达式不一定返回字符串 - 这限制了您只能读取返回字符串的属性。

回答by Nikhil Agrawal

After asking this question(yes I am OP) i received comments on question from Jon

问这个问题后（是的，我是 OP），我收到了Jon对问题的评论

and I came up with this

我想出了这个

public string ResolvePropertyName<TEntity>(Expression<Func<TEntity>> expression)
{
try {
    if (expression == null) {
        Throw New ArgumentNullException("propertyExpression")
    }

    object memberExpression = expression.Body as MemberExpression;
    if (memberExpression == null) {
        Throw New ArgumentException("The expression is not a member access expression.", "propertyExpression")
    }

    object property = memberExpression.Member as PropertyInfo;
    if (property == null) {
        Throw New ArgumentException("The member access expression does not access a property.", "propertyExpression")
    }

    object getMethod = property.GetGetMethod(true);
    if (getMethod.IsStatic) {
        Throw New ArgumentException("The referenced property is a static property.", "propertyExpression")
    }
    return memberExpression.Member.Name;
} catch (Exception ex) {
    return string.Empty;
}
}

回答by Scott Munro

This is apparentlyrelated to boxing/unboxing. Lambda expressions returning value types that require boxing will be represented as UnaryExpressions whereas those that return reference types will be represented as MemberExpressions.

这显然与装箱/拆箱有关。返回需要装箱的值类型的 Lambda 表达式将表示为 UnaryExpressions，而返回引用类型的表达式将表示为 MemberExpressions。