implicit/explicit conversion operator not called by attribute (System.ComponentModel.DataAnnotation.dll)

Question

We have a custom LocalizedString type used in our domain model. We want to decorate the properties with validation attributes like `MaxLength`. For this, we added implicit operators to enable casts required by this attribute. Strangely, the operator seems to never get called and an InvalidCastException get's thrown in the attributes `IsValid` method. Performing this cast in our own project works. Is there a special cast behavior compiler magix going on in this system clr ngen'ed attribute or something? ``` // 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:…

Accepted Answer

Operators of any kind only apply if the type of object is known at compile time. They are not applied "on the fly" to `object`.

You could try using `dynamic` which does do that.

Examples:

```
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!)
    }
}
```

implicit/explicit conversion operator not called by attribute (System.ComponentModel.DataAnnotation.dll)

Solution

Problem