Question

Given this class with an implicit cast operator:

public class MyDateTime
{
    public static implicit operator MyDateTime(System.Int64 encoded)
    {
        return new MyDateTime(encoded);
    }

    public MyDateTime(System.Int64 encoded)
    {
        _encoded = encoded;
    }
    System.Int64 _encoded;
}

I can now do the following:

long a = 5;
MyDateTime b = a;

But NOT the following:

long f = 5;
object g = f;
MyDateTime h = g;

This gives a compile time:

Cannot implicitly convert type ‘object’ to ‘MyDateTime’.

Makes sense to me.

Now I modify the previous example as follows:

long f = 5;
object g = f;
MyDateTime h = (MyDateTime)g;

This compiles fine. Now I get a runtime InvalidCastException:

Unable to cast object of type ‘System.Int64′ to type MyDateTime’.

This tells me that C# implicit cast operators are applied at compile time only, and are not applied when the .NET runtime is attempting to dynamically cast an object to another type.

My questions:

1. Am I correct?
2. Is there some other way to do this?

By the way, the full application is that I’m using Delegate.DynamicInvoke() to call a function that takes a MyDateTime parameter, and the type of the argument I’m passing to DynamicInvoke is a long.

Answer 1

Am I correct?

Yes, yes you are. To be nit-picky, you should be saying “user-defined implicit conversion” rather than “implicit cast” — a cast is (almost) always explicit. But your deduction that overload resolution chooses which user-defined conversion to call at compile time and not at run time is correct.

Is there some other way to do this?

Yes. In C# 4 if you type your “object” as “dynamic” then we start up the compiler again at runtime and re-perform all the analysis on the operands as though their compile-time types were the current run-time types. As you might imagine, this is not cheap, though we are very smart about caching and re-using the results should you do this in a tight loop.