Generics and returning class objects

I am trying to return an object of a class using the generics.

This is the generic class

public class ClientBase <S>
{
    protected S CreateObject()
    {
        return default(S)  ;
    }
}

This is how I am trying to use it...

public class ClientUser : ClientBase <SomeClass>
{

    public void call()
    {
        var client = this.CreateObject();
        client.SomeClassMethod();
     }
}

While I get the SomeClassMethod() in the client object, when running the code it gives an error at the line:

client.SomeClassMethod();

Error is 'Object reference not set to an instance of an object'. I know there is something missing in the generic class ClientBase's CreateObject() method; just cant figure that bit out. Could someone help me here please?

Thanks for your t开发者_高级运维ime...

default(S) where S is a reference type is null. In your case, default(SomeClass) returns null. When you try to invoke a method on a null reference, that's when you get your exception.

Are you trying to return a default instance of SomeClass? You may want to use a new() constraint and return new S() in your generic class instead, like so:

public class ClientBase<S> where S : new()
{
    protected S CreateObject()
    {
        return new S();
    }
}

If S needs to be a reference type you can also constrain it to class:

public class ClientBase<S> where S : class, new()
{
    protected S CreateObject()
    {
        return new S();
    }
}

See what default(T) does: http://msdn.microsoft.com/en-us/library/xwth0h0d.aspx

In your case, default(S) is going to return null (because it's a class) - this is not an instance of the class.

You either need to call new S() or some other S constructor or override CreateObject in your derived class.