generics - Passing dynamic object to C# method changes return type

Question

I created a class that inherits DynamicObject and want to create a static method that can create new instances with pre-determined properties (stored in the Dictionary).

public class CustomDynamic : DynamicObject
{
    protected Dictionary<string, object> InnerDictionary;

    public static T Create<T>(Dictionary<string, object> dictionary) where T : CustomDynamic , new()
    {
        return new T
        {
            InnerDictionary = dictionary
        };
    }
}

Usage:

dynamic d = new Dictionary<string, object>();

var realPlayer = CustomDynamic.Create<Player>(d as Dictionary<string, object>);
var dynaPlayer = CustomDynamic.Create<Player>(d);

realPlayer // Player type according to VS2013
dynaPlayer // dynamic type according to VS2013

Since there is only one method signature, why does passing in a dynamic return a dynamic object? Or is actually just Visual Studio 2013 getting confused?

Answer 1

This is because almost any operation involving a dynamic value is resolved dynamically at execution time. There are no exceptions made for cases where actually there's only one method present at compile-time; the language is simpler that way. (For certain calls, the compiler does perform enough resolution at compile-time to ensure that there is at least one method with a suitable number of parameters - this is specified in the C# 5 spec in section 7.5.4, but that doesn't affect the effective return type.)

From the C# 5 spec, section 7.6.5:

An invocation-expression is dynamically bound if at least one of the following holds:

• The primary-expression has compile-time type dynamic.
• At least one argument of the optional argument-list has compile-time type dynamic and the primary-expression does not have a delegate type.

In this case the compiler classifies the invocation-expression as a value of type dynamic. [...]

There are a few operations involving dynamic values which still have a non-dynamic overall type. For example:

• d is Foo is always bool
• d as Foo is always Foo
• new Foo(d) is always Foo even though the exact constructor to use is determined at execution time

But any method call is treated as having a return type of dynamic.