Chaining commands by returning this pointer/reference in C# without assignment

Question

This is mostly a syntax question. Here's a related thread showing different alternatives to achieve the same result: Method-Chaining in C#

Now, in C++ it is possible to chain commands on an object by making methods return the pointer to the object of which the method is a member.

The syntax I'm looking for is:

foo.Transform(bar).TransformDifferently(yay);

In C#, if I return this, the value is copied (edit: half incorrect, read answers). I don't think I can return a reference or a pointer, can I? Is there any other way to achieve the same syntax?

For now my solution is to just assign the result to my object, like so:

foo = foo.Transform(bar).TransformDifferently(yay);

It's however not the optimal solution, because it's both slower (doesn't matter in my case) and more verbose. I'd be very happy for any pointers in the right direction!

p.s.w.g · Accepted Answer · 2013-07-16T11:25:00.157

In C#, if I return this, the value is copied

This is not true, at least for classes. Classes in C# are reference types, that means every variable of that type is actually a reference, so returning this just returns a reference to the current instance.

So to do this, it's essentially the same in C#:

public class Foo
{
    public Foo Transform(Bar bar)
    {
        // transform
        return this;
    }

    public Foo TransformDifferently(Yay yay)
    {
        // transform
        return this;
    }
}

For value types, it should return a new instance of the type. Value types are supposed to be immutable, so if you've designed your struct correctly, there should be no method which modifies the original. Transform should modify a new version of the struct instead.

Perfect, thanks a lot! My mistake - I thought the only difference between structs and classes were the standard visibility modifiers, (once again, as in C++, hehe). — Martin Grönlund, Jul 16 '13 at 15:07

score 0 · Answer 2 · answered Jul 16 '13 at 11:19

0

In C#, if I return this, the value is copied

Incorrect. The this pointer refers to the 'current' object, and it does not create a deep or shallow copy of the object.

So this:

foo.Transform(bar).TransformDifferently(yay);

works fine.

answered Jul 16 '13 at 11:19

Maarten

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Thanks, +1 for mentioning deep and shallow copy! – Martin Grönlund Jul 16 '13 at 15:17

score 0 · Answer 3 · answered Jul 16 '13 at 11:52

0

What type is your foo variable? Is it a class or a struct instance?

If it is a class instance, you can achieve the syntax you're looking for by returning 'this'. The object will not be copied. 'this' is a reference, exactly as you expect.

If foo is a struct instance, then what you're trying to achieve is a bad idea. It is recommanded to make structs immutable so we can reason about them like primitives (integers, booleans, etc.). You'd better return a copy in this case.

answered Jul 16 '13 at 11:52

Reda

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It was a struct, but I've changed it to be a class now. Thanks, and +1 for the tip on immutability for primitives! – Martin Grönlund Jul 16 '13 at 15:13
Never mind what I said, I changed it back to be a struct - just because of what you said about primitives. – Martin Grönlund Jul 16 '13 at 15:26
Are you returning `this` or a new copy of you struct? – Reda Jul 16 '13 at 15:42
`this` - but that seems to become a new copy when I return it. – Martin Grönlund Jul 16 '13 at 17:13

Chaining commands by returning this pointer/reference in C# without assignment

3 Answers3