How do I add Generic constraints?

Question

I have the following situation in C#:

class MyGenericClass<T>
{
    public void do()
    {
    }
}

class SpecificFooImpl : MyGenericClass<Foo>
{
     public void otherStuff()
     {
     }
}

Now I want to write a generic method which can return only MyGenericClass<T> or specific implementations. I would write something like:

var v1 = GetMyClass<MyGenericClass<Foo>>();
var v2 = GetMyClass<MyGenericClass<Bar>>();
var v3 = GetMyClass<SpecificFooImpl>();

I could use the following signature but it have no constraints on the type:

public T GetMyClass<T>();
//I don't want to write
//var v4 = GetMyClass<AnyOtherTypesWhichNotExtendMyGenericClass>();

Is there any elegant mode to solve the problem?

so add a constraint msdn.microsoft.com/en-us/library/d5x73970.aspx — Ric
– Ric, Commented Nov 18, 2014 at 11:05

dcastro · Accepted Answer · 2014-11-18 11:28:16Z

2

Add a where : clause after the definition, then you can define what should be obeyed.
I've said it must be a class but you can add a base class or interface as a constraint.

class MyGenericClass<T> where T : class, IYourCommonInterface
{
    public void do()
    {
    }
}

References:
See MSDN on constraints: http://msdn.microsoft.com/en-us/library/d5x73970.aspx

edited Nov 18, 2014 at 11:28

dcastro

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answered Nov 18, 2014 at 11:02

Ralph Willgoss

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2 Comments

Fabio Cometti Over a year ago

How can I write the where clause? I cannot find a way to achieve it!

Ralph Willgoss Over a year ago

Copy what I have, then define an interface or another base class as a constraint

Jon Egerton · Accepted Answer · 2014-11-18 11:10:49Z

2

This gets a little tricky, as you can't leave the type constraint open - it has to be concrete.

So what you'd like to do is:

public T GetMyClass<T>() where T: MyGenericClass<>

However the closest you'd get is to include a second generic type that makes MyGenericClass concrete:

public T GetMyClass<T,T2>() where T: MyGenericClass<T2>

However this makes the caller need to know too much about the implementation, especially where you're using SpecificFooImpl.

Instead, consider using an interface to remove your inner generic type:

interface MyInterface
{
    void Stuff();
}

class MyGenericClass<T> : MyInterface
{
    public void Stuff()
    {
    }
}

Then you can have:

public T GetMyClass<T>() where T : MyInterface

answered Nov 18, 2014 at 11:10

Jon Egerton

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1 Comment

Jon Egerton Over a year ago

All of the desired usages will work (eg var v3 = GetMyClass<SpecificFooImpl>();) since they all derive from MyGenericClass<T> which now implements MyInterface.

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