Question

Possible Duplicate:
Impossible recursive generic class definition?

I just discovered that

public class Foo<T> where T : Foo<T>
{

}

is legal. What exactly does it mean? It seems
recursive and is it possible to instantiate something
like this?

Answer 1

I wouldn’t say that this is useless. Let’s observe the below example how to support fluent syntax. In cases, that you are creating some base implementation in a Parent and would like to provide fluent declarations… you can use this constraint this way

public class Parent<TChild>
    where TChild : Parent<TChild>
{
    public string Code { get; protected set; }

    public TChild SetCode(string code)
    {
        Code = code;
        return this as TChild; // here we go, we profit from a constraint
    }
}

public class Child : Parent<Child>
{
    public string Name { get; protected set; }

    public Child SetName(string name)
    {
        Name = name;
        return this // is Child;
    }
}

[TestClass]
public class TestFluent
{
    [TestMethod]
    public void SetProperties()
    {
        var child = new Child();
        child
            .SetCode("myCode") // now still Child is returned
            .SetName("myName");

        Assert.IsTrue(child.Code.Equals("myCode"));
        Assert.IsTrue(child.Name.Equals("myName"));
    }
}

Please, take it just an example, of how this constraint could be used