What is the equivalent of a template class in C# that inherits from the specified type?

Question

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Why cannot C# generics derive from one of the generic type parameters like they can in C++ templates?
Generic class that inherits its generic type

I would like to be able to do something like this, only in C#:

template<class MyClass>
class MockComposite : MyClass
{

};

The class above inherits from the specified type. MyClass is a base type, and this is the reason I want this behaviour. I want to be able to instantiate this object with classes derived from MyClass.

I understand you cannot use templates in C#, but is there any way to get the behaviour I want without having to write a new class for each different type of object?

EDIT - I want to be able to instantiate an object that will test the behaviour of any class, as long as that class is derived from a specified type - 'MyClass'.

@SirYakalot: Please describe what you want to achieve with this construct — Daniel Hilgarth, Aug 28 '12 at 14:57
This reminds me of the [CRTP](http://en.wikipedia.org/wiki/Curiously_recurring_template_pattern), but why would you want to do it in C#? — Sergey Kalinichenko, Aug 28 '12 at 15:00

Daniel Hilgarth · Answer 1 · 2012-08-28T15:11:27.237

In C# you have the concept of generics. The following code is equivalent to your C++ code:

class MockComposite<T> : T
    where T : MyClass
{
}

Unfortunatelly, it is invalid. You will get the following compile error:

Cannot derive from 'T' because it is a type parameter

In C# you would usually solve this problem with composition instead of inheritance.

Composition in this case means one of two things:

Your class still has the generic argument but doesn't try to derive from it. Furthermore, it has the new constraint, meaning that the generic argument needs to have a default constructor:
```
class MockComposite<T>
    where T : MyClass, new()
{
    T _toTest;

    public MockComposite()
    {
        _toTest = new T();
    }
}
```
This means you can instantiate the mock class like this: var mock = new MockComposite<ClassA>(); without the need for creating an instance of ClassA outside of the mock class.
```
class MockComposite
{
    MyClass _toTest;

    public MockComposite(MyClass toTest)
    {
        _toTest = toTest;
    }
}
```
This means you have to instantiate the mock class like this: var mock = new MockComposite(new ClassA());

@exacerbatedexpert: Maybe you want to read the whole answer. The C++ code translated to C# generic syntax is the code I posted. BTW: 2 downvotes? Do you have multiple accounts? — Daniel Hilgarth, Aug 28 '12 at 15:15

tenfour · Answer 2 · 2012-08-28T15:07:31.137

This is simply not possible in C#. But this is a case of trying to use one language as another, different, language. In C++ this pattern has its uses. You can accomplish the same things in C#, but using different techniques.

Maybe this will help you:

class Myclass
{
}

class ChildClass : MyClass
{
}

class MockComposite<T> where T : MyClass
{
    public MockComposite(T objectToTest) { ... }
}

Here, you can pass a any class that derives from MyClass into CompositeTester<>. The biggest difference between this and your code is that here you need to pass in real existing non-generic classes that inherit MyClass. You are trying to inherit generically which is not possible.

In C++ this is basically like doing:

class Myclass
{
};

class ChildClass : MyClass
{
};

template<typename T>
class MockComposite
{
public:
    MockComposite(const T& objectToTest) { ... }
};

C# generics might look a bit like C++ templates but it's an entirely different idea. While C++ templates let you do be very flexible almost to the point of being like macros, generics remain rather tight, type-wise. You cannot do anything with a generic type that you have not said it can do explicitly, hence generic constraints like where T : MyClass. By specifying that, MockComposite.MockComposite can use T as a MyType.

I want to be able to write an object with generic testing behaviour that I can use to test any object that is derived from one specific base-class object. — Dollarslice, Aug 28 '12 at 14:58
@SirYakalot: And why does this testing class need to derive from the class that needs to be tested? — Daniel Hilgarth, Aug 28 '12 at 15:01
I suppose it could be written in an entirely different way. I'm converting some code and was wondering if there was an equivalent. — Dollarslice, Aug 28 '12 at 15:02
@SirYakalot: See [my answer](http://stackoverflow.com/a/12162023/572644). Most likely, you will want to use composition. — Daniel Hilgarth, Aug 28 '12 at 15:04

What is the equivalent of a template class in C# that inherits from the specified type?

2 Answers2