the blog => anything goes : C#

Wrong compiler warning? Or Not?

Senthil — Tue, 01 Dec 2009 05:27:57 GMT

This piece of code results in a warning.

   1: class Test

   2: {

   3:     public static void Main()

   4:     {

   5:         object obj = "Senthil";

   6:         string myName = "Senthil";

   7:         Console.WriteLine(obj == myName);

   8:     }

   9: }

The compiler says “warning CS0252: Possible unintended reference comparison; to get a value comparison, cast the left hand side to type 'string'” on line 7. The compiler is right; it has no way of knowing that obj is actually a string. It infers the static type of obj to be System.Object, and if you’ve done any programming in C# at all, you’ll know that the default behavior of the == operator is reference comparison. This code is unintentionally comparing reference values of two local variables, and that’s what the compiler is warning about.

Yet try running the code, and you’ll see that it works correctly; it prints True. Is the compiler wrong?

No. The code appears to work fine because string interning masks the problem. The C# compiler figures that there are two constant strings in the program, and both are equal, so instead of creating two distinct string objects, it creates only one. With strings being immutable, reusing string objects for identical strings is safe. And that’s why this code works, both obj and myName refer to the same string object, which means reference comparison will succeed.

Now that we know why it’s working, it’s easy to prove that the compiler warning is right with a small code change.

   1: class Test

   2: {

   3:     public static void Main()

   4:     {

   5:         object obj = "Senthil";

6:

   7:         string myName1 = "Sen";

   8:         string myName2 = "thil";

   9:         string myName = myName1 + myName2;

  10:         Console.WriteLine(obj == myName);

  11:     }

  12: }

Constructing myName dynamically forces the compiler to create another string object; it cannot reuse the interned one. As you’d expect, this piece of code will print False.

Fixing the warning is a mere matter of casting obj to the correct type (string) and then doing the equality check.

   1: class Test

   2: {

   3:     public static void Main()

   4:     {

   5:         object obj = "Senthil";

6:

   7:         string myName1 = "Sen";

   8:         string myName2 = "thil";

   9:         string myName = myName1 + myName2;

  10:         Console.WriteLine(((string)obj) == myName);

  11:     }

  12: }

Or we could use the Equals method, which is virtual and won’t be fooled by the compile time type.

   1: class Test

   2: {

   3:     public static void Main()

   4:     {

   5:         object obj = "Senthil";

6:

   7:         string myName1 = "Sen";

   8:         string myName2 = "thil";

   9:         string myName = myName1 + myName2;

  10:         Console.WriteLine(obj.Equals(myName));

  11:     }

  12: }

Both of them print True, which is what we want.

Anonymous methods as event handlers – Part 2

Senthil — Wed, 09 Sep 2009 05:02:26 GMT

The previous post discussed having anonymous methods as event handlers and ended with a question – why doesn’t unsubscription work while subscription works out alright?

Vivek got the answer spot on – the way the C# compiler handles and translates anonymous methods is the reason.

Here’s the code involved.

   1: public void Initialize()

   2: {

   3:     control.KeyPressed += IfEnabledThenDo(control_KeyPressed);

   4:     control.MouseMoved += IfEnabledThenDo(control_MouseMoved);

   5: }

6:

   7: public void Destroy()

   8: {

   9:     control.KeyPressed -= IfEnabledThenDo(control_KeyPressed);

  10:     control.MouseMoved -= IfEnabledThenDo(control_MouseMoved);

  11: }

12:

  13: public EventHandler<Control.ControlEventArgs> IfEnabledThenDo(EventHandler<Control.ControlEventArgs> actualAction)

  14: {

  15:     return (sender, args) => { if (args.Control.Enabled) actualAction(sender, args); };

  16: }

The compiler translates IfEnabledThenDo into this

   1: public EventHandler<Control.ControlEventArgs> IfEnabledThenDo(EventHandler<Control.ControlEventArgs> actualAction)

   2: {

   3:     <>c__DisplayClass1 CS$<>8__locals2 = new <>c__DisplayClass1();

   4:     CS$<>8__locals2.actualAction = actualAction;

   5:     return new EventHandler<Control.ControlEventArgs>(CS$<>8__locals2.<IfEnabledThenDo>b__0);

   6: }

Now the problem should be fairly obvious – every time the function is called, a new object gets created, and the event handler returned actually refers to a method (<IfEnabledThenDo>b__0) on the new instance. And that’s what breaks unsubscription. –= will not remove a delegate of a different instance of the same class from the invocation list – if it did, the consequences would not be pleasant if multiple instances of the same class subscribe to an event.

But why does the compiler translate our lambda expression this way? Raymond Chen has a great blog post explaining why, but the short answer is that it is needed to “hold” actualAction (the method parameter to IfEnabledThenDo) so that it is available when the event handler actually executes.

Now that we know why, the way to get around this issue is to cache the delegate instance returned by IfEnabledThenDo and use the same instance for subscription and unsubscription.

   1: EventHandler<Control.ControlEventArgs> keyPressed;

   2: EventHandler<Control.ControlEventArgs> mouseMoved;

3:

   4: public void Initialize()

   5: {

   6:    keyPressed = IfEnabledThenDo(control_KeyPressed);

   7:    mouseMoved = IfEnabledThenDo(control_MouseMoved);

8:

   9:    control.KeyPressed += keyPressed;

  10:    control.MouseMoved += mouseMoved;

  11: }

12:

  13: public void Destroy()

  14: {

  15:    control.KeyPressed -= keyPressed;

  16:    control.MouseMoved -= mouseMoved;

  17: }

Knowing how things work under the hood has its advantages, I guess :)

PS : A very small syntactic change to the original example would have made the code work right away. If you’ve followed along this far, you should be able to figure out why.

   1: public void Initialize()

   2: {

   3:    actualAction = control_KeyPressed;

   4:    control.KeyPressed += IfEnabledThenDo();

   5: }

6:

   7: public void Destroy()

   8: {

   9:     control.KeyPressed -= IfEnabledThenDo();

  10: }

11:

  12: EventHandler<Control.ControlEventArgs> actualAction;

  13: public EventHandler<Control.ControlEventArgs> IfEnabledThenDo()

  14: {

  15:     return (sender, args) => { if (args.Control.Enabled) actualAction(sender, args); };

  16: }

Anonymous methods as event handlers – Part 1

Senthil — Tue, 01 Sep 2009 20:22:40 GMT

The syntactic sugar offered by anonymous methods makes them great candidates for writing event handlers; together with smart type inference, they reduce the amount of code written by an order of magnitude.

And that’s without considering the power offered by closures. With event handlers, closures allow you to kind of “stuff” extra parameters into the handler, without changing the actual number of formal parameters. This blog post shows a situation where an anonymous method acting as an event handler makes code simpler, and then goes on to show a gotcha with un-subscription and anonymous methods.

Here’s a simple Control class that fires a bunch of events.

   1: class Control

   2: {

   3:     public class ControlEventArgs : EventArgs

   4:     {

   5:         public Control Control {get;set;}

   6:     }

7:

   8:     public bool Enabled { get; set; }

9:

  10:     public event EventHandler<ControlEventArgs> KeyPressed;

  11:     public event EventHandler<ControlEventArgs> LeftButtonClicked;

  12:     public event EventHandler<ControlEventArgs> RightButtonClicked;

  13:     public event EventHandler<ControlEventArgs> MouseMoved;

  14: }

Let’s say you’re developing a GUI application with this class, and you want to handle events only if the originating control is visually enabled i.e., Enabled set to true. Pretty reasonable constraint, but tedious to implement, if you go the standard way of adding the check to the start of each of your event handlers.

   1: class GUIApp

   2: {

   3:     public void Initialize()

   4:     {

   5:         Control control = new Control();

   6:         control.KeyPressed += new EventHandler<Control.ControlEventArgs>(control_KeyPressed);

   7:         control.MouseMoved += new EventHandler<Control.ControlEventArgs>(control_MouseMoved);

   8:     }

9:

  10:     void control_MouseMoved(object sender, Control.ControlEventArgs e)

  11:     {

  12:         if (e.Control.Enabled)

  13:         {

  14:             ///

  15:         }

  16:     }

17:

  18:     void control_KeyPressed(object sender, Control.ControlEventArgsEventArgs e)

  19:     {

  20:         if (e.Control.Enabled)

  21:         {

  22:             ///

  23:         }

  24:     }

  25: }

With an anonymous method, you could write a far more terse and easy to maintain version

   1: class GUIApp

   2: {

   3:     public void Initialize()

   4:     {

   5:         Control control = new Control();

   6:         control.KeyPressed += IfEnabledThenDo(control_KeyPressed);

   7:         control.MouseMoved += IfEnabledThenDo(control_MouseMoved);

   8:     }

9:

  10:     public EventHandler<Control.ControlEventArgs> IfEnabledThenDo(EventHandler<Control.ControlEventArgs> actualAction)

  11:     {

  12:         return (sender, args) => { if (args.Control.Enabled) { actualAction(sender, args); } };

  13:     }

14:

  15:     void control_MouseMoved(object sender, Control.ControlEventArgs e)

  16:     {

  17:         ///

  18:     }

19:

  20:     void control_KeyPressed(object sender, Control.ControlEventArgs e)

  21:     {

  22:         ///

  23:     }

  24: }

IfEnabledThenDo returns an anonymous function that first checks whether the control is enabled before calling the actual function. The code is much shorter, and the condition is checked only in one place, which makes it easy to modify or add additional logic without having to remember to change every single event handler. Plus, the reads like an English statement – subscribe to the event and if enabled, then do whatever else is necessary.

Great, but unless you are a masochist who revels in littering the code base with hard to reproduce bugs that bomb your app only when demoing to your most important customer, you must, of course, write code to unsubscribe. But there’s no method name to refer to, so you do it the same way as you did when subscribing.

   1: public void Initialize()

   2: {

   3:     control.KeyPressed += IfEnabledThenDo(control_KeyPressed);

   4:     control.MouseMoved += IfEnabledThenDo(control_MouseMoved);

   5: }

6:

   7: public void Destroy()

   8: {

   9:     control.KeyPressed -= IfEnabledThenDo(control_KeyPressed);

  10:     control.MouseMoved -= IfEnabledThenDo(control_MouseMoved);

  11: }

This, unfortunately, won’t work – the application will still remain subscribed to those events. Can you figure out why?

Answer and more in the next blog post.

Remoting IEnumerables

Senthil — Sat, 29 Aug 2009 14:49:45 GMT

You're writing this really cool and innovative class to calculate the first hundred thousand natural numbers. You think about the API, and you realize that returning an array of the numbers a) might take a long time to complete, and b) is going to cause memory usage to spike up like mad.

So you decide to stream them instead, returning an IEnumerable<T> instance instead of an array. Being a crack C# developer, you use the incredibly powerful yield keyword, rather than rolling your own implementation of the IEnumerable<T> interface.

        1: class MyCoolMathEngine

       2: {

       3:     public IEnumerable<int> GetFirstHundredThousandNaturalNumbers()

       4:     {

       5:         for (int i = 0; i < 100000; ++i)

       6:             yield return i;

       7:     }

       8: }

People start downloading your class and it becomes so popular that they want you to host it in an external service, as a remote component.

Fine, you say, and you pick .NET remoting to provide remote access. You pat yourself on the back for thinking ahead and using an enumerator model – it scales rather nicely. You write the plumbing code to host the class, and with a victorious smile on your lips, you write a test client that instantiates and accesses the method.

Only to find that it crashes with a System.Runtime.Serialization.SerializationException that says a type that you didn’t even write is not marked serializable.

That’s when it hits you, or at least that’s when it hit me, when I was modifying FinalizeTypeFinder to get it load on a different AppDomain.

Because yield return was used, what actually gets back to the caller is an instance of a compiler generated class that implements IEnumerable<int>, and that autogenerated class is not marked serializable. Nor does it derive from MarshalByRefObject, so there’s no way it can be remoted.

The only real solution I can think of is to write a custom implementation of IEnumerable<T>, like we had to do back in the C# 1.1 days. Yet another instance where compiler magic doesn’t quite work out in a particular scenario, I suppose.

Overload resolution and null

Senthil — Wed, 08 Jul 2009 06:28:15 GMT

My colleague Soundar discovered this rather interesting behavior.

  1: class Test
  2: {
  3:     public static void Main()
  4:     {
  5:         Test test = null;
  6: 
  7:         Console.WriteLine("{0}", test);
  8:         Console.WriteLine("{0}", null);
  9:     }
 10: }

If you run this code, you’ll find that while line 7 prints an empty line, line 8 causes an ArgumentNullException. Note that the test reference is also null, so it should certainly surprise you that the two lines result in different behavior at runtime.

It certainly surprised me enough to make me dig deeper into the reason for the difference. I reasoned that given that the parameter values are identical at runtime, the discrepancy must happen because of a compiler operation – probably method overloading. And sure enough, the two calls resolve to different overloads.

Line 7 resolves to

public static void WriteLine(string format, object arg0);

whereas Line 8 resolves to

public static void WriteLine(string format, params object[] arg);

A peek at the source code using Reflector showed that the second overload throws if arg is null, whereas the first one packs arg0 into an object array and calls the second overload.

Ok, but why did the compiler pick different overloads?

Intuitively, for a method call with a single parameter, you’d expect the overload resolution algorithm to choose a single parameter method over a method with variable number of arguments. And that’s what the compiler did on line 7.

On line 8, the situation is different – null is directly assignable to arg0 and to arg. The overload resolution algorithm had to choose the best function, and it chose the one with the object array.

That appears counter intuitive, until you have code like

  1: class Test
  2: {
  3:     public static void Main()
  4:     {
  5:         SubTest subTest = null;
  6:         M(subTest);
  7:     }
  8: 
  9:     static void M(Test t) { Console.WriteLine("Test"); }
 10:     static void M(SubTest s) { Console.WriteLine("SubTest"); }
 11: }
 12: 
 13: class SubTest : Test { }

You wouldn’t be surprised if the call at line 6 resolved to M(SubTest), would you?

The C# spec’s rules for determining the best match say that

“ Given an implicit conversion C1 that converts from a type S to a type T1, and an implicit conversion C2 that converts from a type S to a type T2, the better conversion of the two conversions is determined as follows:

If T1 and T2 are the same type, neither conversion is better.
If S is T1, C1 is the better conversion.
If S is T2, C2 is the better conversion.
If an implicit conversion from T1 to T2 exists, and no implicit conversion from T2 to T1 exists, C1 is the better conversion.
If an implicit conversion from T2 to T1 exists, and no implicit conversion from T1 to T2 exists, C2 is the better conversion.
… “

In this case, SubTest (T1) is implicitly convertible to Test (T2), and therefore the compiler picks M(SubTest).

Now in our case, the compiler was trying to pick the best conversion between null to object and null to object[]. Applying the same rule as above, object[] is implicitly convertible to object, and therefore the overload resolution algorithm chose WriteLine(string format, params object[] arg). The params modifier didn’t play a part in overload resolution in this (null) case.

Interesting, ain’t it?

Volatile and local

Senthil — Fri, 03 Jul 2009 05:16:54 GMT

If you’ve done any multithreading programming at all, you must be aware of the volatile modifier. When a field is marked volatile, it tells

1. the JIT compiler that it can’t hoist the field because it may be modified by multiple threads

2. the CLR that the field must be read to and written from with acquire and release semantics.

Given what you’ve read above, the post’s title doesn’t make sense. A local variable, by definition, cannot be accessed from multiple threads. An object referred to by a local variable can be shared among threads, but never the variable itself.

Well, that was true as long as local variables remained just that – local variables. The 2.0 release of C# brought closures to the language, and C# implements capturing of local variables by making them members of a generated class. Now do you see the problem?

  1: public static void Main()
  2: {
  3:     bool stopRunning = false;
  4: 
  5:     Thread t = new Thread(() =>
  6:         {
  7:             while (!stopRunning)
  8:                 Console.WriteLine("Hello");
  9:         });
 10:     t.Start();
 11:     DoSomethingElse();
 12:     stopRunning = true;
 13: }

Nothing out of the ordinary here – I’m creating a thread, passing a lambda to the Thread constructor, and capturing stopRunning inside the lambda.

This code isn’t correct though – for the reasons mentioned in the initial paragraph of this post, stopRunning needs to be declared with the volatile modifier. Unfortunately, you can’t make stopRunning volatile – the compiler complains that local variables cannot be marked volatile.

Oops.

Making stopRunning a member of the class will solve the immediate problem – you can then mark the field volatile, and all is good. However, left at that, it now makes the class non-threadsafe – two threads could call Main, and stopRunning will be shared between them.

I guess this is the price to pay for compiler magic – magic that enables seamless access to local variables from anonymous methods.

Mutating value types – Part 1

Senthil — Thu, 21 May 2009 05:13:48 GMT

Take a look at the following short snippet of code.

  1: using System;
  2: 
  3: struct S
  4: {
  5:     public int X;
  6: }
  7: 
  8: class C
  9: {
 10:     /* More code here */
 11: }
 12: 
 13: class Test
 14: {
 15:     public static void Main()
 16:     {
 17:         C c = new C();
 18:         c.S.X = 1;
 19:     }
 20: }

Without knowing the type definition of C, can you tell whether the code will compile, much less work?

Turns out that you can’t. It depends on whether the S in c.S is defined as a field or a property.

class C
{
    public S S;
}

class C
{
    public S S { get; private set; }
}

The first type definition will compile, the second won’t (error CS1612: Cannot modify the return value of 'C.S' because it is not a variable)

Can you guess why?

Let’s assume the compiler does allow the second type definition to compile. Would you expect the value of X in the instance of S inside C to change? That is, what would be the output of

public static void Main()
{
    C c = new C();
    c.S.X = 1;
    Console.WriteLine(c.S.X);
}

If you’re expecting it to be 1, then you have just broken the value semantics of a struct, S might as well have been defined as a class. The above code is logically identical to

public static void Main()
{
    C c = new C();
    S temp = c.S;
    temp.X = 1;
    Console.WriteLine(c.S.X);
}

Because S is a struct, the property getter always returns a copy (temp), and changing the copy will have no effect on the original instance. With c.S.X = 1, you can’t access the copy either, so the only effect of executing that statement will be to make the poor developer’s eyes go wide as he steps through the code in the debugger, wondering why a simple field assignment refuses to work.

So, the C# compiler is being helpful here by not allowing this kind of code to compile.

Right, so why does it compile if S is defined as a field rather than a property? We’ll see why in the next post. Meanwhile, feel free to post why you think it works.

Confusion++

Senthil — Thu, 26 Feb 2009 13:09:44 GMT

I'll bet a hundred bucks that any entry level C++ interview or exam will somehow drift into questions about the pre and post increment operators. It's almost become a canonical, rite of passage sort of thing.

Now using the operators is one thing, overloading them for your own types is another. In C++, you write something like

class X {
    int val;
public:

    X() : val(0){ }

    X operator++() { val++; return *this; }
    X operator++(int) { X pX = *this; val++; return pX; };

    int Value() { return val; }
};

Fairly straightforward stuff - C++ uses the int overload to distinguish between pre and post, and the post increment overload copies itself before incrementing and returns the copy.

Now let's see how to do this in C#.

class X
{
    public int Value { get; set; }

    public static X operator ++(X p)
    {
        int x = p.Value;
        return new X() { Value = x + 1 };
    }
}

No, there's no separate overload for the other one - the same method works for both pre and post increment operations. The compiler does the work of generating code that exhibits correct pre and post increment behavior. For code like

X x = new X();
X y = x++;

it generates

X x = new X();
X y = x;
x = X.op_Increment(x);

and for code like

X x = new X();
X y = ++x;

it generates

X x = new X();
X y = x = X.op_Increment(x);

Nifty, eh?

But wait, did you notice the difference between the C++ and C# overload implementation? The C# overload does not modify the original at all and always returns a copy, whereas the C++ code always modifies the current instance and returns a copy only for the post increment overload. Looking at the generated code, it should be easy to understand why - the compiler can't do its magic if the overload tinkers with the original instance.

Did you notice that X is a reference type?

X x = newX();
Xy = x;

x++;

Given that x and y are referring to the same object after executing the second line, shouldn't the increment be visible from y as well? It doesn't happen though, because the overload returns a new instance that then gets assigned to x, leaving y referring to the "old" unmodified instance.

All this confusion will not arise if X is a value type. The assignment of x to y will create a copy, so there's no possibility of changes in x reflecting in y. And modifying the passed instance from within the operator overload will have no effect on the original instance either.

Moral of the story : watch out when overloading operators on reference types.

Writing your own SkypeBot

Senthil — Thu, 05 Feb 2009 18:32:13 GMT

If you use Skype, do you know that you can program against it? Head over to developer.skype.com if you're interested. There's a COM API, one for Java and even one for Python.

Just to show how easy it is, we'll write a bot in .NET that will simply echo whatever is sent to it.

You first need to download Skype4COM, a COM library provided by Skype developers to control Skype.

Create a WinForms application in your favorite version of Visual Studio (>= 2005), and in the default Form class, paste the following piece of code

public partial class MainForm : Form
{
    Skype skype;
    Dictionary<string, Session> userSessions = new Dictionary<string, Session>();

    public MainForm()
    {
        InitializeComponent();
    }

    private void Form1_Load(object sender, EventArgs e)
    {
        skype = new SkypeClass();
        skype.Attach(5, false);
        skype.MessageStatus += new _ISkypeEvents_MessageStatusEventHandler(skype_MessageStatus);
    }

    void skype_MessageStatus(ChatMessage pMessage, TChatMessageStatus Status)
    {
        if (Status == TChatMessageStatus.cmsReceived)
        {
            string text = pMessage.Body;
            string response = "Echoing " + text;
            skype.SendMessage(pMessage.FromHandle, response);
        }
    }

    private void Form1_FormClosing(object sender, FormClosingEventArgs e)
    {
        skype.MessageStatus -= new _ISkypeEvents_MessageStatusEventHandler(skype_MessageStatus);
    }
}

Add a reference to Skype4COM and build the code. Make sure Skype is running before launching this application. You'll need to accept accessing Skype from this application by hitting the "Accept" button when Skype asks- the Skype developers put it in there to avoid malicious code taking control.

That's all there is to it. If you now send a message to the currently signed in user, you should get back the same message text prefixed by "Echoing".

There are a lot of other things you could do with the library, but the documentation is pretty thin, so you'll have to experiment a bit to get things working.

Still, it's pretty neat and I can see plenty of situations where a bot running on Skype would be useful. Imagine chatting with your bot running on your home PC, asking it the list of running applications. Skype takes care of NAT, firewall and other network related issues for you. How cool would it be to ask your build server bot the version number of the previous week's build?