Generic method with specified types

Question

Can I do something like this:

public void Foo<T>(int param) where T: MYCLASS1, MYCLASS2

To specify that T will only be MYCLASS1 or MYCLASS2 instance?

Thank you..

Answer 1

No, when you specify generic type constraints, the generic type argument must satisfy all the constraints, not just one of them. The code you wrote means that T must inherit both MYCLASS1 and MYCLASS2 , which is not possible since C# doesn't support multiple inheritance. The generic type constraints can be a combination of:

• a base class (only one allowed)
• one or several interfaces
• the new() constraint (ie the type must have a parameterless constructor)
• either struct or class (but not both, since a type can't be a value type and a reference type)

Answer 2

You cannot do that.

While adding constraints on a generic type you can list only one class and others have to be interfaces.

This is a valid constraint -

public void Foo<T>(int param) where T: MyClass1, IInterface1, IInterface2

But not this

public void Foo<T>(int param) where T: MyClass1, MyClass2

This is logical, because when you declare a variable of type Foo such as Foo<MyType> , your MyType can derive from MyClass1 , IInterface1 and MyInterface2 but it cannot derive from both MyClass1 and MyClass2 .

Answer 3

No, generic constraints are always ANDed together. You will have to do a runtime check:

public void Foo<T>(int param) {
    if (typeof(T) != typeof(MyClass1) && typeof(T) != typeof(MyClass2))
        throw new ArgumentException("T must be MyClass1 or MyClass2");
    // ...
}

Answer 4

As Thomas points out, you cannot do this. What you can do however is this:

public void Foo<T>(int param) where T: IMyInterface

As long as you know that both MYCLASS1 and MYCLASS2 implement IMyInterface