如何限制泛型函数只接受某种类型的类

Question

I'm trying to do the following:

public class A
{
}

public class B
{
}

Somewhere along the project I want to have this:

public class C
{
    public T func<T>(T obj) [where T can be either of class A or class B]
    {
        obj.x = 100;

        return obj;
    }
}

I've been trying:

public T func<T>(T obj) where T: A, B

but this gives me:

The type class constraint 'B' must come before any other constraint.

Can someone explain me how to make func accept only class A or class B?

Answer 1

Exactly as it's described in the question, this job is better handled by overload resolution :

public class C
{
    public A func(A obj)
    {
        obj.x = 100;   
        return obj;
    }

    public B func(B obj)
    {
        obj.x = 100;   
        return obj;
    }

}

But I understand that A and B may be placeholders for any number of types, and it could get tedious to account for them all. In that case, you'll need a common interface that's supported by each of your classes:

interface IBase
{
    int x;
}

public class C
{
    public IBase func(IBase obj)
    {
        obj.x = 100;   
        return obj;
    }
}

Note that at this point we still have no need of generics. Additionally, you may need to support a number of types that won't all fit together under a common interface. In this case, still build the interface and put as many types with that interface as possible. If needed, build another interface for a few more types ... and so on... and then between interfaces and specific types you can handle things with overload resolution.

Answer 2

You need some kind of common base for both the classes, either have them implement the same interface as the below code or have them inherit from same class. You can not have a generic constrained to 2 types.

public interface IFooBar
{
    void DoThis();
}

public class Foo : IFooBar
{
    public void DoThis()
    {
        //Do something
    }
}

public class Bar : IFooBar
{
    public void DoThis()
    {
        //Do something
    }
}

public class C
{
    public T func<T>(T obj) where T : IFooBar
    {
        obj.DoThis();
        return obj;
    }
}

Answer 3

The generics classes are just like any other class, you can't (and shouldn't) have multiple inheritance of classes, you can inherit one class and multiple interfaces.

in your case you should apply an interface on both classes and restrict the generics on that interface.

you can see some documentation in: Constraints on Type Parameters (C# Programming Guide)

Answer 4

interface IMarkerInterface{} // there is a such pattern called marker  
                             // interface . No need to have methods if there 
                            // is no need for, A and B can just implement it 

public class A: IMarkerInterface
{
}

public class B: IMarkerInterface
{
}

public class C
{
    public T func<T>(T obj).Where T:IMarkerInterface
    {
        obj.x = 100;
        return obj;
    }
}

Answer 5

public T func<T>(T obj) where T: A, B this means T should extend both A and B , but multiple inheritance is not valid in C# ,so it won't work.

You could do one of the following though :

1. you could make A and B have a common parent via an interface or an abstract class , but that would be code modification.

2. since both A and B have a default no-arg constructor you could use where T: new() .

Also, you can not do obj.x = 100; as there is no way to guarantee that T will have a instance variable x .