隐式/显式转换运算符未被属性调用(System.ComponentModel.DataAnnotation.dll)

Question

我们在域模型中使用了自定义LocalizedString类型.我们想用验证属性来装饰属性MaxLength.为此,我们添加了隐式运算符以启用此属性所需的强制转换.

奇怪的是,运算符似乎永远不会被调用,并且在属性IsValid方法中抛出了InvalidCastException .在我们自己的项目中执行此演员表.

在这个系统clr ngen'ed属性中有没有特殊的强制转换行为编译器magix？

// Custom type
public class LocalizedString
{
    public string Value
    {
        get { return string.Empty; }
    }

    public static implicit operator Array(LocalizedString localizedString)
    {
        if (localizedString.Value == null)
        {
            return new char[0];
        }

        return localizedString.Value.ToCharArray();
    }
}

// Type: System.ComponentModel.DataAnnotations.MaxLengthAttribute
// Assembly: System.ComponentModel.DataAnnotations, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35
// Assembly location: C:\Windows\Microsoft.NET\Framework\v4.0.30319\System.ComponentModel.DataAnnotations.dll
public override bool IsValid(object value)
{
  this.EnsureLegalLengths();
  if (value == null)
  {
    return true;
  }
  else
  {
    string str = value as string;
    int num = str == null ? ((Array) value).Length : str.Length;
    if (-1 != this.Length)
      return num <= this.Length;
    else
      return true;
  }
}


[TestMethod]
public void CanCallIsValidWithLocalizedString()
{
    // Arrange
    var attribute = new MaxLengthAttribute(20);
    var localized = new LocalizedString { Value = "123456789012345678901" };

    // Act
    var valid = attribute.IsValid(localized);

    // Assert
    Assert.IsFalse(valid);
}

谢谢你的帮助.

编辑

Das Objekt des Typs "Nexplore.ReSearch.Base.Core.Domain.Model.LocalizedString" kann nicht in Typ "System.Array" umgewandelt werden.
bei System.ComponentModel.DataAnnotations.MaxLengthAttribute.IsValid(Object value)
bei Nexplore.ReSearch.Base.Tests.Unit.Infrastructure.CodeFirst.MaxLengthLocalizedAttributeTests.CanCallIsValidWithLocalizedString() in d:\Nexplore\SourceForge\Nexplore.ReSearch.Base\Source\Nexplore.ReSearch.Base.Tests.Unit\Infrastructure.CodeFirst\MaxLengthLocalizedAttributeTests.cs:Zeile 40.

Answer 1

任何类型的运算符仅在编译时已知对象类型时才适用.它们不是"动态"应用于object.

您可以尝试使用dynamic哪个可以做到这一点.

例子:

using System;

class Foo
{
    public static implicit operator Array(Foo foo)
    {
        return new int[0]; // doesn't matter
    }
    static void Main()
    {
        Foo foo = new Foo();
        Array x = (Array)foo; // implicit operator called via compiler
        dynamic dyn = foo;
        Array y = (Array)dyn; // implicit operator called via dynmic
        object obj = foo;
        Array z = (Array)obj; // implicit operator NOT called
                              // - this is a type-check (BOOM!)
    }
}