Category: Compiler

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Construct something else! (C#)

Please read my follow-up post after reading this one.

Quoth rjw on stackoverflow…

Given the following client code:

    var obj = new Class1();

Is there any way to modify the constructor of Class1 so that it will actually return a subclass (or some other alternate implementation) instead?

C# compiler guru, Eric Lippert commented…

We are considering adding a feature “extension new” which would essentially allow you to make a static factory method that would be called when the “new” operator is used, much as extension methods are called when the “.” operator is used. It would be a nice syntactic sugar for the factory. If you have a really awesome scenario where this sort of pattern would be useful, I’d love to see an example.

I have one!

Version one of our DLL had a class that wrapped a connection to a remote server.

    using (var connect = new ExampleConnection("service.example.com"))
    {
        connect.DoStuff(42);
    }

It worked great. Our customers were very happy with it and developed lots of code to use our little DLL. Life was good.

Time passes and our customers ask us to add support for a different type of server that does a similar job but with a very different protocol. No problem, we develop a new class called DifferentConnection and just to be helpful, both ExampleConnection and DifferentConnection implement a common interface object.

We’re about to release version two to our customers, but a common response comes back;

“This is good, but we were hoping your library would automatically detect which variety of server it’s talking to. Also, we really don’t want to change our code. We want to just drop your updated DLL into the install folder, but we’ll recompile our EXE if we really have to.”

With these new requirements, ExampleConnection had to become a class that supported both varieties of remote server. The constructor has to perform the auto-detect, and all of the public functions now all begin with an if statement, selecting for which variety of remote server is in use.

If we had a bit more foresight, we should have supplied a static Connect function that wrapped a private constructor. That way, version two of this function could have returned a subclass object instead. But we didn’t. There are costs to writing code that way, so you wouldn’t do it unless there was a clear point to it. If a normal constructor could return a subclass instead, there would be no problem.

Mr Lippert, I hope this provides the justification you need to add this to .NET 5, but I’d much rather have destructors on structs instead. I also want a pony.

Picture credit: ‘LEGO Mini Construction Site’ by flickr user ‘bucklava’.
(I don’t really want a pony.)

UPDATE: Someone submitted this to reddit. Lots of discussion there.
UPDATE(2): Follow-up post.

Posted on

Wishing for a destructor (C#)

 

I like the C# programming language. It feels like C++ done right, divesting itself of much of the C legacy that complicates matters so much. When I do programming, I prefer to use this language. Having to go back and deal with C++ just doesn’t give me that warm feeling like it used to.

But, I have a pet peeve that I miss from C++.

Choose… Choose the form of the Destructor!

Quick recap. Here’s a brief C++ function…

 void MyFunction()
 {
    string x;
    MyOtherFunction(x);    /* Pass by value. */
 }

Doesn’t look like much is going on, but there’s five function calls in there.

  1. A constructor function is called to build x.
  2. A copy constructor function is called to copy x for the function call.
  3. ‘MyOtherFunction’ is called.
  4. A destructor function is called to tidy up the copy of x.
  5. The same destructor function is called to tidy up x itself.

The clever bit is that the compiler has worked out when objects go “out of scope” and inserted calls to that’s objects destructor function in exactly the right place. Even “anonymous” objects are tidied up. Say a function is called that returns an object, but the caller just ignores the return value. The compiler spots it and inserts the destructor call in just the right place.

C# doesn’t do this. Instead, from the very beginning of the language, unused objects are “garbage collected”. Every so often, some code will run that goes over everything built by the program and sees if its being used anywhere. Anything that can’t be traced to running code is removed. Doing it this way allows the programmer to share objects between two different areas of code, without having to worry about which one has responsibility for tidying up.

I imagine that when the very clever people at Microsoft designed the C# language, they had already decided to use garbage collection, and so concluded that this messing about with destructors was no longer needed. No need to insert a function call into code, just leave the object lying around and the garbage collector will deal with it.

This would all great if memory was the only resource we have to keep track of. Open file handlers, database connections, etc. All must closed in a deterministic manner, instead of at some unknown time in the future when memory is about to run out.

Microsoft didn’t leave us completely out on the branch, classes that need to be tidied up can be written to implement the IDisposable interface. This allows the using block to work.

 using (SqlConnection con = new SqlConnection(db))
 {
    /* Use con. */
 } /* Dispose con. */

With the using block, just like with the C++ destructors shown above, the compiler inserts a call to the tidy-up function at the end of the block. Even if there’s a return or throw statement in the middle, it’ll make sure everything is tidied up when the code leaves the using block.

But why have the using block at all? If you forget to include the using block, the tidy-up code won’t be called (unless you invoke it manually) and you won’t even get a compiler warning. (You don’t get the warning for very good reasons which I won’t go into right now.)

Even when you use using correctly, adding a using block to a function means introducing an additional block, with all the block-visibility issues and additional indenting that implies.

Structs to the rescue

Fortunately, C# and .NET come with a type of object called structs. These are similar to classes except they are solid value types rather than references to data floating in the ether. The practical difference is that when a struct value is copied (such as when passed into a function as a parameter) and you change the value of the one of the copies, the other copy stays the same.

In contrast, when you copy a class value, you’re instead just making a copy of the reference, so both point to same data. Change the contents of one, and the other changes value too, because there is no “other”.

So what if, when a struct appears in code, it came with an automatic using block attached? That way, we could open files or database connections just by introducing one in code and it would be tidied up in a deterministic way.

To complete the job, we would need mechanisms to support copy constructors and assignment as well as the final destructor call, just like the C++ people are used to.

I’ve been nursing this peeve and whining about it for so long that I’m even boring myself. I plan this to be my last word on the topic and in future I’ll just post links to this article. Enjoy.

Picture credits
“staypuft_3feb2009_0621” by patrick h. lauke on flickr
“choose determinism” by alyceobvious on flickr
“John E. Cox Memorial Bridge” by Elizabeth Thomsen on flickr

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is, then as (C#)

The C# language has a pair of related operators, is and as. Say you have an object, x, and you want to know if its really type Form underneath.

(x is Form) will be true if it is, or false if it isn’t. The as operator will actually perform the conversion, returning null if it can’t be converted to that type.

One common piece of advice to C# programmers is to avoid using the is operator. Here’s how both of these operators are typically used together.

    if (x is Form)
    {
        Form frm = x as Form;
        /* Use frm. */
    }

While the code works, all the is operator is doing is performing an as operation, and checking if the result is null or not. In other words, (x is Form) is equivalent to (x as Form != null). This means the code above is really saying…

    if (x as Form != null)
    {
        Form frm = x as Form;
        /* Use frm. */
    }

See that. The same as operation was done twice. What a waste! So, the standard advice is to re-write the code to use just one as operation.

    Form frm = x as Form;
    if (frm != null)
    {     
        /* Use frm. */
    }

Simple. So what’s the point of this article? Why use is at all?

The problem for me is that is just looks so darn nice. Beginners very quickly understand what’s going on when they see it. The re-written code that checks for null just doesn’t look as pretty and isn’t as obvious what’s going on.

You may be wondering what the cost of this additional operation is. I knocked together a quick test which looped is-then-as and then looped the optimized version the same number of times and reported the difference in time. When I increased the count to 100,000,000, I saw approx one second difference build up. That’s a lot of looping.

But hey, that cost is non-zero, and I’m wondering why. Compilers are pretty good these days. I recall advice from long ago that its better to use an XOR operation to check if two values are equal, or to use shift operations instead of multiplying. These days, best practice to write what you mean and leave micro-optimizations to the compiler. So why doesn’t Microsoft’s C# compiler optimize is-then-as into a single operation?

I don’t know. It could be that its not worth the development team’s time, or maybe there’s a subtle reason that escapes me right now. I wish I knew. But if we could eliminate the repeated operation and use is freely, code would be lot nicer to look at.

Picture credit: Fruit balance by Pickersgill Reef.
Many thanks to Eric Lippert and the commentators to his article that inspired this piece.
Update 18/Jan/2010: Introduction modified to be better understood by non C# programmers. (Thanks Andrew.)