Constraint on class type parameters to say types must not be equal?

I wondered if there's any type constraint in C# .NET that states that one type parameter doesn't equal another type parameter; something like this...

public class FooBar<TA,TB> where TA notequalto TB

The reason I want to do this is so that I can create 2 addition operators as per the following...

public class FooBar<TA,TB> where TA notequalto TB
{
    public static TA operator +(TB b, FooBar<TA,TB> fooBar)
    {
        return fooBar.Foo(b);
    }

    public static TB operator +(TA a, FooBar<TA,TB> fooBar)
    {
        return fooBar.Bar(a);
    }

    public TA Foo(TB b) { ... }

    public TB Bar(TA a) { ... }
}

The problem I face is that if TA = TB, it would be ambiguous in terms of which operator should be called, and thus whether Foo() or Bar() should be called! Although it's not essential, I wish to be able to implement this if possible...