Adam Nathan's Blog : Quizzes

Quiz: Searching in a managed array

Adam Nathan — Fri, 22 Aug 2003 21:21:00 GMT

Sorry for my recent lack of blogging. There's been a lot going on these days!

As the banner on my blog now indicates, you'll be able to catch me at two upcoming conferences. As with TechEd in June, Sonja Keserovic and I will be giving an Interop presentation at this year's PDC in October. This presentation will be brand new, though. Feel free to make suggestions if you're dying to hear about certain topics! One month before that, I'll be giving two presentations at WinSummit in Switzerland - one on PInvoke, and one on COM Interoperability. Hopefully I'll see you there!

Yesterday I learned about something that surprised me. Maybe it won't surprise you, but I'm going to share it anyway. If it prevents just one person from introducing a subtle bug in their code, then it was worth it!

If you're writing managed code and you need to efficiently find elements in an array, you'll be happy to know that you don't have to write your own binary search algorithm. System.Array has a static BinarySearch method! (System.Collections.ArrayList has an instance BinarySearch method, too.)

So what's wrong with the following C# code? Why does it print "Not found!"?

    // WARNING: This code has a bug!
    int [] numbers = {10, 9, 8, 7, 6, 5, 4, 3, 2, 1};
    int index = Array.BinarySearch(numbers, 3);

    if (index < 0)
      Console.WriteLine("Not found!");
    else
      Console.WriteLine("Found at index " + index);

Quiz: Gotcha with Exceptions and HRESULTs

Adam Nathan — Sat, 14 Jun 2003 04:01:00 GMT

The C# code below, when executed, prints the following:

0x80004002

Who can figure out why the second line printed isn't 0x80004003?

using System;

using System.Runtime.InteropServices;

public class Quiz

{

const int E_NOINTERFACE = unchecked((int)0x80004002);

const int E_POINTER = unchecked((int)0x80004003);

public static void Main ()

{

try

{

Marshal.ThrowExceptionForHR(E_NOINTERFACE);

}

catch (Exception ex)

{

Console.WriteLine("0x{0:x}", Marshal.GetHRForException(ex));

}

try

{

Marshal.ThrowExceptionForHR(E_POINTER);

}

catch (Exception ex)

{

Console.WriteLine("0x{0:x}", Marshal.GetHRForException(ex));

}

CLR SPY and Customer Debug Probes: The Object Not Kept Alive and Buffer Overrun Probes (A Quiz)

Adam Nathan — Fri, 06 Jun 2003 01:11:00 GMT

The Object Not Kept Alive and Buffer Overrun probes are unlike any other CDPs, because they do not output any messages to report bugs in your code. Instead, they change general CLR behavior with the goal of forcing non-deterministic bugs that can be almost impossible to reproduce into bugs that will happen every time you run your application. As a developer or tester, that's obviously a good thing. The lack of a specific probe message might make such bugs somewhat harder to diagnose, but once you understand the probes' behavior, you can learn to be successful with them.

The Object Not Kept Alive probe forces a garbage collection (then waits at most a second for pending finalizers) immediately before every transition from managed to unmanaged code. This is done to surface problems that could occur if a managed object happens to be collected during the small time window when transitioning to unmanaged code. I find this to be one of the most useful of all the CDPs.

The Buffer Overrun probe forces a garbage collection (then waits at most a second for pending finalizers) immediately after every transition from managed to unmanaged code. The intent of this is to catch buffer overruns, which unmanaged code has the power to cause. (Managed code does too -- even verifiable managed code if it uses methods of the Marshal class, which require unmanaged code permission.) If unmanaged code writes into memory that it's not supposed to, it could potentially corrupt the GC heap. The next time the garbage collector compresses the heap (as part of a collection), it's likely to run into an access violation. So by forcing a collection after every call into unmanaged code, the corruption can hopefully be noticed at the source of the problem.

Due to the large number of collections forced by these probes, they will significantly slow down most applications.

Now consider the following harmless-looking C# code that uses the Microsoft.Win32.RegistryKey class:

// This code has a bug!

public static void Main ()

{

// Open the key for read-only access

RegistryKey key = Registry.LocalMachine.OpenSubKey(

"Software\\Microsoft\\.NETFramework");

Console.WriteLine("Install root: " +

key.GetValue("InstallRoot"));

}

Most likely, this code will run fine if you don't enable any probes. If you enable the Object Not Kept Alive debug probe and compile this code with debugging information ("csc /debug ObjectNotKeptAlive.cs" from a command prompt, or the Debug solution configuration in Visual Studio .NET), you'll still get the output that you'd expect when running the program. For example:

Install root: c:\windows\microsoft.net\framework

But if you compile it without debugging information ("csc ObjectNotKeptAlive.cs" from a command prompt, or the Release solution configuration in Visual Studio .NET) with the Object Not Kept Alive probe still enabled, you get surprising output:

Install root:

Weird, huh?

Do you see any bugs in the code? Who can tell me why this happens with the probe enabled? Why does it only happen in the debug configuration? My last quiz was answered pretty quickly, so please contribute any of your thoughts!

Quiz: What's wrong with the following code?

Adam Nathan — Wed, 07 May 2003 00:31:00 GMT

The following C# code has the goal of enabling managed code to call CreateDC, but it's incorrect. Calling all Interop aficionados... Can you see what's wrong?

using System;

using System.Runtime.InteropServices;

internal class DeviceContext

{

[DllImport("gdi32.dll", CharSet=CharSet.Auto)]

internal static extern IntPtr CreateDC(

string lpszDriver, string lpszDevice,

string lpszOutput, ref DEVMODE lpInitData);

}

[StructLayout(LayoutKind.Sequential)]

internal class DEVMODE

{

[MarshalAs(UnmanagedType.ByValArray, SizeConst=32)]

public char [] dmDeviceName;

public short dmSpecVersion;

public short dmDriverVersion;

public short dmSize;

public short dmDriverExtra;

public int dmFields;

public DEVMODE_UNION u;

public short dmColor;

public short dmDuplex;

public short dmYResolution;

public short dmTTOption;

public short dmCollate;

[MarshalAs(UnmanagedType.ByValArray, SizeConst=32)]

public byte [] dmFormName;

public short dmLogPixels;

public int dmBitsPerPel;

public int dmPelsWidth;

public int dmPelsHeight;

public int dmDisplayFlagsOrdmNup;

public int dmDisplayFrequency;

public int dmICMMethod;

public int dmICMIntent;

public int dmMediaType;

public int dmDitherType;

public int dmReserved1;

public int dmReserved2;

public int dmPanningWidth;

public int dmPanningHeight;

}

[StructLayout(LayoutKind.Explicit)]

internal struct DEVMODE_UNION

{

[FieldOffset(0)]

public short dmOrientation;

[FieldOffset(2)]

public short dmPaperSize;

[FieldOffset(4)]

public short dmPaperLength;

[FieldOffset(6)]

public short dmPaperWidth;

[FieldOffset(8)]

public short dmScale;

[FieldOffset(10)]

public short dmCopies;

[FieldOffset(12)]

public short dmDefaultSource;

[FieldOffset(14)]

public short dmPrintQuality;

[FieldOffset(0)]

public int dmPosition_x;

[FieldOffset(4)]

public int dmPosition_y;

[FieldOffset(8)]

public int dmDisplayOrientation;

[FieldOffset(12)]

public int dmDisplayFixedOutput;

}