Static Code Analysis

Filed under Development

These days I've been updating the CA settings on some old projects, I wanted to modify the configuration of certain rules that were removed with the shipping of Visual Studio 2008. You can obtain detailed information about what rules are shipped with each version of CA in this post of the FxCop blog.

As you can see in the post, some of the rules were removed because they do not apply anymore or because they were too noisy compared with the benefit introduced. One of the rules that make feel more upset when I knew it was removed was "CA1818 - Do not concatenate strings inside loops". This rule threw an error (I will continue in my imaginary world where everybody sets warnings as errors in production code) with code like:

string s = string.Empty;
for (int i = 0; i < 5; i++)
{
   s += i.ToString();
}

One of the first things you learn in .NET is about the immutability of the strings, you can find lot of literature talking about how to handle properly strings. Even Improving .NET Application Performance and Scalability, one of the best papers I've seen about .NET performance, makes an explicit reference to do not concatenate strings when the number of concatenations is unknown.

So, today that I've been working again with the rule, I had the curiosity (hope) to verify the rule was not removed because it was too noisy but because the CLR was improved to avoid the issue. I know this is again living in my imaginary world, but I'm a bit stubborn...what I've done is to compile a console project with the code above with 2.0 (VS 2005) and 3.5 (VS 2008), first one fires the error, second one doesn't. First step has been to look for differences in the IL generated, both cases have:

   1:  .entrypoint
   2:  // Code size       39 (0x27)
   3:  .maxstack  2
   4:  .locals init ([0] string s,
   5:         [1] int32 i)
   6:  IL_0000:  ldsfld   string [mscorlib]System.String::Empty
   7:  IL_0005:  stloc.0
   8:  IL_0006:  ldc.i4.0
   9:  IL_0007:  stloc.1
  10:  IL_0008:  br.s     IL_001c
  11:  IL_000a:  ldloc.0
  12:  IL_000b:  ldloca.s i
  13:  IL_000d:  call     instance string 
  14:                     [mscorlib]System.Int32::ToString()
  15:  IL_0012:  call     string [mscorlib]System.String::Concat(string,
  16:                                                            string)
  17:  IL_0017:  stloc.0
  18:  IL_0018:  ldloc.1
  19:  IL_0019:  ldc.i4.1
  20:  IL_001a:  add
  21:  IL_001b:  stloc.1
  22:  IL_001c:  ldloc.1
  23:  IL_001d:  ldc.i4.5
  24:  IL_001e:  blt.s    IL_000a
  25:  IL_0020:  call     valuetype [mscorlib]System.ConsoleKeyInfo 
  26:                     [mscorlib]System.Console::ReadKey()
  27:  IL_0025:  pop
  28:  IL_0026:  ret

No improvements on the compiler side. Next step has been to verify the native code generated, for that I used the command !u at WinDbg (in the post Inline Methods you can see how to obtain the native code after the method is jitted). The code for both projects looks like (note that some memory addresses will be different):

   1:  CA1818.Program.Main(System.String[])
   2:  Begin 009d0070, size 55
   3:  009d0070 55           push ebp
   4:  009d0071 8bec         mov  ebp,esp
   5:  009d0073 57           push edi
   6:  009d0074 56           push esi
   7:  009d0075 83ec10       sub  esp,10h
   8:  009d0078 33c0         xor  eax,eax
   9:  009d007a 8945f4       mov  dword ptr [ebp-0Ch],eax
  10:  009d007d 8b3d2c10d602 mov  edi,dword ptr ds:[2D6102Ch] ("")
  11:  009d0083 33d2         xor  edx,edx
  12:  009d0085 8955f4       mov  dword ptr [ebp-0Ch],edx
  13:  009d0088 837df405     cmp  dword ptr [ebp-0Ch],5
  14:  009d008c 7d26         jge  009d00b4
  15:  009d008e 8b75f4       mov  esi,dword ptr [ebp-0Ch]
  16:  009d0091 e80a0cca6f   call mscorlib_ni+0x220ca0 (70670ca0) 
  17:  009d0096 50           push eax
  18:  009d0097 8bce         mov  ecx,esi
  19:  009d0099 33d2         xor  edx,edx
  20:  009d009b e8bad00971   call 
  21:                        mscorwks!LogHelp_TerminateOnAssert+0xb82 
  22:                        (71a6d15a) 
  23:  009d00a0 8bd0         mov  edx,eax
  24:  009d00a2 8bcf         mov  ecx,edi
  25:  009d00a4 e8a7ebc36f   call mscorlib_ni+0x1bec50 (7060ec50)
  26:  009d00a9 8bf8         mov  edi,eax
  27:  009d00ab ff45f4       inc  dword ptr [ebp-0Ch]
  28:  009d00ae 837df405     cmp  dword ptr [ebp-0Ch],5
  29:  009d00b2 7cda         jl   009d008e
  30:  009d00b4 8d4de8       lea  ecx,[ebp-18h]
  31:  009d00b7 33d2         xor  edx,edx
  32:  009d00b9 e8f6381570   call mscorlib_ni+0x6d39b4 (70b239b4)
  33:  009d00be 8d65f8       lea  esp,[ebp-8]
  34:  009d00c1 5e           pop  esi
  35:  009d00c2 5f           pop  edi
  36:  009d00c3 5d           pop  ebp
  37:  009d00c4 c3           ret

We see there are no improvements either on the jitter side. The next to verify is if the strings are being discarded on each iteration creating a new one or not. To do that I used some good profilers that are on the market but in the end I decided to show how I did it with WinDbg because anybody can download it for free.

With !DumpHeap -type System.String we can see all the string instances of our application. This returns a list with the memory address of the string instances, to view the contents of the object we just need to do a !do (dump object) of the address we want to check. So, just taking some samples from the instances with higher memory addresses we can already see the next:

   1:  0:003> !do 01c83ae0 
   2:  Name: System.String
   3:  MethodTable: 706c0a00
   4:  EEClass: 7047d64c
   5:  Size: 24(0x18) bytes
   6:  String: 012
   7:   
   8:  0:003> !do 01c83af8 
   9:  Name: System.String
  10:  MethodTable: 706c0a00
  11:  EEClass: 7047d64c
  12:  Size: 20(0x14) bytes
  13:  String: 3
  14:   
  15:  0:003> !do 01c83b0c
  16:  Name: System.String
  17:  MethodTable: 706c0a00
  18:  EEClass: 7047d64c
  19:  Size: 26(0x1a) bytes
  20:  String: 0123
  21:   
  22:  0:003> !do 01c83b28
  23:  Name: System.String
  24:  MethodTable: 706c0a00
  25:  EEClass: 7047d64c
  26:  Size: 20(0x14) bytes
  27:  String: 4
  28:   
  29:  0:003> !do 01c83b3c 
  30:  Name: System.String
  31:  MethodTable: 706c0a00
  32:  EEClass: 7047d64c
  33:  Size: 28(0x1c) bytes
  34:  String: 01234

From the results above we can see how on each iteration we have two strings, the resulting of i.ToString() and the resulting of concatenation.

Conclusion, we still creating new strings and discarding the previous version for GC on each manipulation of the string. Therefore, I suppose the rule was removed just because it was too noisy.

Once I stopped playing with WinDbg I come back to the earth to say what I wanted to say from the beginning. It's pity to see that a performance issue that has been repeated so many times, now is ignored just because the rule is too noisy. I know lot of people could argue that using the StringBuilder we also discard old versions of strings when the size of the string becomes bigger than the buffer, but still better than discarding all modifications.

I don't understand why a good string handling was that important before and now is just ignored by the main CA tool used by .NET developers.

Singleton Pattern

Filed under Development

Yesterday I was viewing a video (Spanish) that explains about how to implement the Singleton pattern in .NET. After some implementations the video ends up showing the best way to have a thread safe implementation of the singleton pattern.

The problem is the video ends saying that the next two implementations are exactly the same on a functional level, which is not completely true.

Implementation 1

   1:  public sealed class Singleton1
   2:  {
   3:      private DateTime creationTime;
   4:      private static Singleton1 instance = null;
   5:      private static object synchro = new object();
   6:   
   7:      private Singleton1()
   8:      {
   9:          creationTime = System.DateTime.Now;
  10:      }
  11:   
  12:      public static Singleton1 Instance
  13:      {
  14:          get
  15:          {
  16:              if (instance == null)
  17:              {
  18:                  lock (synchro)
  19:                  {
  20:                      if (instance == null)
  21:                      {
  22:                          instance = new Singleton1();
  23:                      }
  24:                  }
  25:              }
  26:   
  27:              return instance;
  28:          }
  29:      }
  30:   
  31:      public string CreationTime
  32:      {
  33:          get
  34:          {
  35:              return creationTime.ToString("hh:mm:ss.ffffff");
  36:          }
  37:      }
  38:  }

Implementation 2

   1:  public sealed class Singleton2
   2:  {
   3:      private DateTime creationTime;
   4:      private static Singleton2 instance = new Singleton2();
   5:      
   6:   
   7:      private Singleton2()
   8:      {
   9:          creationTime = System.DateTime.Now;
  10:      }
  11:   
  12:      public static Singleton2 Instance
  13:      {
  14:          get
  15:          {
  16:              return instance;
  17:          }
  18:      }
  19:   
  20:      public string CreationTime
  21:      {
  22:          get
  23:          {
  24:              return creationTime.ToString("hh:mm:ss.ffffff");
  25:          }
  26:      }
  27:  }

The second implementation is correct, since it shows how you can avoid the lock while creating the object instance of the Singleton class. The reason why this is thread safe is because .NET guarantees that the Type Constructor (or Type Initializer on the ECMA standard) is executed only once per AppDomain and is thread-safe.

Regardless both of them are thread-safe, the difference is that the first one is lazy loaded while the second one is eager loaded. In order to implement the singleton pattern using a lazy loaded version and still take profit of the Type Constructors, you need to add a container for the object instance of Singleton2. Doing this the Type Constructor will only be initialized when you access the property (or any other member making reference to it). Below you can see the correct implementation for it.

   1:  public sealed class LazyLoading
   2:  {
   3:      private DateTime creationTime;
   4:   
   5:      private LazyLoading()
   6:      {
   7:          creationTime = DateTime.Now;
   8:      }
   9:   
  10:      public static LazyLoading Instance
  11:      {
  12:          get
  13:          {
  14:              return SingletonContainer.instance;
  15:          }
  16:      }
  17:   
  18:      public string CreationTime
  19:      {
  20:          get
  21:          {
  22:              return creationTime.ToString("hh:mm:ss.ffffff");
  23:          }
  24:      }
  25:   
  26:      class SingletonContainer
  27:      {
  28:          internal static readonly LazyLoading instance = 
  29:              new LazyLoading();
  30:      }
  31:  }

Docking as Windows 7 in Vista and XP using Windows Hooks

Filed under Development

I was one of those lucky guys who got the Windows 7 virtual machine at the PDC for the first time and I already liked. When the new beta was released I decided to install it on my old laptop and I love it, I really mean it. I think it's great, there are a lot of small features that you immediately like with almost not noticing them. One of them is the cool docking feature that allows you drag a window over the limits of the screen to change it's size and position.

When I saw I immediately wondered how this could be done using C#. The idea would be to detect when a window is being dragged to the limits of our screen, when it reaches them display a frame and finally change the size and position of the window being dragged. 

At a first view, using some Windows Hook and capturing a couple of messages can do the trick. The problem is that we can only install global hooks for WH_KEYBOARD_LL and WH_MOUSE_LL, the other global hooks require a native DLL export to inject itself in another process. This is already a problem because discards the possibility to monitor messages before and after they are processed by a window,  in addition we cannot use either the MouseProc that gives us information about the mouse and more important the value of the HitTest, which is necessary to know what part of the window the user is clicking. What we will do is to try to use the WH_MOUSE_LL with some work around. I already wrote about hooks more than one year ago, you can read it here if you are not familiarized with them.

With that hook we will obtain information about mouse events, we will focus on the messages WM_LBUTTONDOWN, WM_MOUSEMOVE and WM_LBUTTONUP. In the attached code I created a hook helper that captures the hook callbacks and generates .NET events to facilitate its use. You can see below the event handler: 

   1:  void hookHelper_HookMouseEvent(object sender,

   2:      HookMouseEventArgs e)

   3:  {

   4:      switch(e.Message)

   5:      {

   6:          case NativeConstants.WM_LBUTTONDOWN:

   7:              CheckWindow(e.Point);

   8:              break;

   9:    
  10:          case NativeConstants.WM_MOUSEMOVE:

  11:              CheckPosition(e.Point);

  12:              break;

  13:    
  14:          case NativeConstants.WM_LBUTTONUP:

  15:              ResizeWindow(e.Point);

  16:              break;

  17:      }

  18:  }

In the method CheckWindow we decide if we Window where the user has clicked is a window we are interested to monitor, to do it we retrieve the window handle using the API WindowFromPoint and doing a Hit test over the window. The Hit test can be done by sending the message WM_NCHITTEST specifying the handle of the window and the coordinates of the point, this will return us a value indicating what part of the window has been clicked, in our case we are interested only if the user clicks the Title of the window, which value is HTCAPTION. 

   1:  private void CheckWindow(Point point)

   2:  {

   3:      Win32.NativeStructs.Point p =

   4:          new Win32.NativeStructs.Point();

   5:      p.x = point.X;

   6:      p.y = point.Y;

   7:    
   8:      windowHandle = NativeMethods.WindowFromPoint(p);

   9:      if (windowHandle != IntPtr.Zero)

  10:      {

  11:          IntPtr lParam = new IntPtr(65536 * p.y + p.x);

  12:          IntPtr hitTestPtr = NativeMethods.SendMessage(windowHandle,

  13:              (uint)NativeConstants.WM_NCHITTEST, (uint)0, lParam);

  14:          if ((int)hitTestPtr == NativeConstants.HTCAPTION)

  15:          {

  16:              dragging = true;

  17:          }

  18:      }

  19:  }

The messages, return values and functions are declared in the header Winuser.h, the easier to have all the information about the windows API is to go to MSDN, of course, but also to download the Windows SDK.

Next step is to give to the user the visual clue that something will happen with the window, to do it Windows 7 shows an animation below the pointer and a frame simulating where the window will be placed. To do it we will not code any fancy thing, we can simulate it by just opening a standard .NET form setting its properties ControlBox, ShowInTaskBar and TransparencyKey to our desired values. To know the size we need to give to our frame we just need to check the property WorkingArea from the .NET class System.Windows.Forms.SystemInformation. The only trick we will do is to display the form with the function SetWindowPos, because it allow us controlling the Z-Order of the window and to open the form without activate it, this is handy to avoid our fake frame is placed on top of the window we are actually moving. To do it we use the function SetWindowPos including the handle of the window to insert after and the parameter SWP_NOACTIVATE. 

   1:  NativeMethods.SetWindowPos(frame.Handle,

   2:      windowHandle,

   3:      0, 0,

   4:      SystemInformation.WorkingArea.Width / 2,

   5:      SystemInformation.WorkingArea.Height,

   6:      NativeConstants.SWP_SHOWWINDOW |

   7:      NativeConstants.SWP_NOACTIVATE);

We are near to the end, we just need to complete the last step, change the size and position of the window to the desired the values using again the function SetWindowPos for the left and right placement. To maximize we just use the function ShowWindow passing as parameter SW_SHOWMAXIMIZED.

Windows 7 also restores the original size of the window after you drag away from the docked position, this could be done by capturing first the size and position of the window using the functions GetWindowPlacement and SetWindowPlacement, if you use them do not forget to set the length of your WindowPlacement structure using Marshal.SizeOf().

If you are not familiarized with P/Invoke in .NET you can make use of tools like PInvoke Interop Assistant from Microsoft CLR Interop Team or the P/Invoke wiki.

Now that all seems to be under control, if you download the code you will see that we have a big problem. When we release the button the size doesn't seem to change, but it actually changes to the size and position we have given.  The problem is that the window is in the moving state and when we release the button after set our size, it comes back to the last size the window had before releasing the mouse button. If we make use of Microsoft Spy++ we can see that after set the position the window still receives the messages like WM_WINDOWPOSCHANGED or WM_EXITSIZEMOVE.

I've tried with some of the messages captured previously using the function SendMessage to see if I could exit the moving/sizing state but without success. So, at the end I couldn't resolve it using C#, if you have ideas on how this can be solved they will be welcome. As I was joking with a friend a couple of days ago I feel I've lost my "mojo".

It is curious to see how our mind works, when I completely stopped thinking about what message send in order to make it work, I thought I could try the last trick by setting the size after the resize was completed. The way to do that is pretty simply, we just need to enable a timer that starts after we capture the WM_LBUTTONUP.

MadAboutNet.WindowsDocker.zip (404.63 kb)

Post Event: Descubriendo el CLR

Filed under Development

On Tuesday I did an online webcast about the CLR. I hope the people who joined us enjoyed the event and found the contents interesting.

The event was recorded and you can download it from the Microsoft Events website (Spanish).

During the presentation I did a demo in which I was showing how we can see with WinDBG when a method has been jitted or not. We saw it for the constructor but I stopped before show it for other methods to avoid wasting too much time on the same thing.

One of the attendees has asked me today why the method was shown as not jitted even after it was clearly executed. The problem was simply that I attached WinDGB to the release version of my demo application and the JIT optimized the method replacing it by an inlined version. If I had attached the debug version this would not happen and the method would be shown as jitted. If you want to have more information about it I widely explained in a previous post about Inline methods where I used the same demo application as in the webcast.

Static Fields

Filed under Development

"Types may declare locations that are associated with the type rather than any particular value of the type. Such locations are static fields of the type. As such, static fields declare a location that is shared by all values of the type. Just like non-static (instance) fields, a static field is typed and that type never changes. Static fields are always restricted to a single application domain basis, but they may also be allocated on a per-thread basis."

The paragraph above is the definition of static field extracted from the CLI specification. The text means that when we create a static field "f" inside a class "C", the value will not belong to any of the instances we create of C, instead its value is shared across all the instances and therefore "belongs" to the type C itself.

I've created a very simple class with one static and two instance fields, which you can see below.

   1: public class BusinessLogic
   2: {
   3:     public int instanceField1;
   4:     public int instanceField2;
   5:  
   6:     public static int staticField3 = 3;
   7:  
   8:     public BusinessLogic()
   9:     {
  10:         instanceField1 = 1;
  11:         instanceField2 = 2;
  12:     }
  13: }

I've run the code and I've created 5 instances of the class above, which we will check with WinDBG. To do it we can execute the next command:

!DumpHeap -type BusinessLogic

That will show something similar to the next:

Address               MT     Size
01dfa98c      006668c4      16    
01dfe898     006668c4       16    
01e027a4     006668c4       16    
01e0668c     006668c4       16    
01e13104     006668c4       16    
total 5 objects
Statistics:
      MT    Count    TotalSize   Class Name
006668c4        5           80        BusinessLogic
Total 5 objects

Once we have the address of the 5 instances we created we can dump the objects one by one by using the command !DumpObj [Address]. If we take the first one it will display something similar to:

Name: BusinessLogic
MethodTable: 001c68bc
EEClass: 00281abc
Size: 16(0x10) bytes
Fields:
          MT      Field   Offset   Type                VT   Attr         Value    Name
6f642b38  4000001        4      System.Int32     1    instance  1           instanceField1
6f642b38  4000002        8      System.Int32     1    instance  2           instanceField2
6f642b38  4000003       1c     System.Int32     1    static       3           staticField3

What we have done with this command is to examine the fields of one of the instances in memory of BusinessLogic. We can see that the object has the fields instanceField1, instanceField2 and staticField3 and its values are 1,2 and 3 respectively. So, all is as expected.

Lets focus now on the cool thing of static fields. We can see how "staticField3" is shared across al the instances we create of BusinessLogic by examing the EEClass of the objects. The 5 instances we created before have the same EEClass: "00281abc". If we examine it with the command !DumpClass 00281abc we will see that "staticField3" is present at EEClass level and its value is already initialized.

Class Name: BusinessLogic
mdToken: 02000002
Parent Class: 6f3d3ef0
Module: 001c64f0
Method Table: 001c68bc
Vtable Slots: 4
Total Method Slots: 6
Class Attributes: 100001 
NumInstanceFields: 2
NumStaticFields: 1
      MT          Field  Offset   Type                 VT    Attr         Value   Name
6f642b38    4000001        4      System.Int32   1     instance              instanceField1
6f642b38    4000002        8      System.Int32   1     instance              instanceField2
6f642b38  4000003      1c    System.Int32   1     static           3     staticField3

At this point we have demonstrated how the static fields and their values are shared across all the instances we create of a class.

In this sample we have used value types for the static field, but this also applies for reference types. This is very cool, because it allow us creating class designs where objects with a heavy load construction can be instantiated just once  i.e.

static object staticField;
....
if (staticField == null)
       staticField = new object();

This simple code will allow sharing the same instance of staticField across the entire application domain helping with the performance if staticField is hard to construct.

This does not mean that from now you must create all your "hard to construct" fields as static, because like always the gold hammer does not exist and static fields have a downside.

As we have seen the static field values are related to the EEClass, which are allocated on the loader heaps that are AppDomain specific, so this means they will be in memory until the AppDomain is unloaded.