MissingMethodException calling a method with generic parameter

I have a library containing a class with two methods (and some others that aren't relevant):

public T Foo<T>()
{
    // Real version does some other things, but this is the gist.
    return (T)this.Foo();
}

public object Foo()
{
    // Do stuff and return something.
}

So far so good. This library compiles.

Yet when calling .Foo<string>() I get a MissingMethodException. What could be causing this? Everything compiles fine.

For reference the Foo without the generic is the legacy method, I am introducing the generic version to help with casting etc.

Solution

I agree that the MissingMethodException is due to old dlls. However, once you fix that, this still won't work. You can't implicit cast an object to a string. At runtime, your call .Foo<string>() will call (String)Foo(), where Foo is of type Object. This will basically never work unless the T you provide is already object.

What is it you are trying to do with this piece of code? If you expect to always be using .Foo<string>() or a subset of a few types then you could just define explicit casts for them instead.