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Lesson 3: Using Unmanaged Code
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private const String KernelReference = "kernel32.dll"; private const String UserReference = "user32.dll"; private const Int32 MessageSize = 255; [DllImport(KernelReference)] private static extern Int32 FormatMessage(Int32 dwFlags, Int32 lpSource, Int32 intdwMessageId, Int32 dwLanguageId, ref String lpBuffer, Int32 nSize, Int32 Arguments); [DllImport(UserReference, SetLastError=true)] private static extern Int32 MessageBox(IntPtr hWnd, String pText, String pCaption, Int32 uType); public static void ThrowMessageBoxException() { IntPtr ProblemCauser = (IntPtr)(-100); MessageBox(ProblemCauser, "This won't work", "Caption - This won't work", 0); Int32 ErrorCode = Marshal.GetLastWin32Error(); Console.WriteLine("Error Code: " + ErrorCode.ToString()); Console.WriteLine("Real Error Code: " + GetLastErrorMessage(ErrorCode)); } public static String GetLastErrorMessage(Int32 errorValue) { // This order doesn't matter but should be kept // for logical consistency Int32 FORMAT_MESSAGE_ALLOCATE_BUFFER = 0x00000100; Int32 FORMAT_MESSAGE_IGNORE_INSERTS = 0x00000200; Int32 FORMAT_MESSAGE_FROM_SYSTEM = 0x00001000; String lpMsgBuf = String.Empty; Int32 dwFlags = FORMAT_MESSAGE_ALLOCATE_BUFFER | FORMAT_MESSAGE_FROM_SYSTEM | FORMAT_MESSAGE_IGNORE_INSERTS; Int32 ReturnValue = FormatMessage(dwFlags, 0, errorValue, 0, ref lpMsgBuf, MessageSize, 0); if (ReturnValue == 0) { return null; } else{return lpMsgBuf;} } } }
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If this code is run correctly, you ll get a return value indicating that there was a bad pointer instead of just a numeric value.
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Interoperation
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Limitations of Unmanaged Code
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Since the advent of .NET, there have been some shortcomings with using unmanaged code. They are largely related to inherent differences between .NET and previous development methodologies. Following is a list of those shortcomings:
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Performance I mention this with hesitation because a common misconception
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is that the performance Interop code is much less than its .NET equivalent. This might be the case, but performance varies from one instance to another. Code that isn t managed by a runtime will typically have the ability to perform faster than equivalent code that is managed. However, this benefit might not necessarily be realized. This is because of the overhead associated with marshaling information between the unmanaged code and the .NET 2.0 runtime. It s important to remember that unmanaged code can easily introduce issues such as memory leaks.
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Type safety Unmanaged code is sometimes not type safe. This deficiency can have multiple implications, including decreased readability and security issues. One of the most widely touted benefits of Visual Basic 2005 over previous versions is enhanced type strength, so this point should not be trivialized. Moreover, there s no guarantee that type library definitions are accurate, so depending on the metadata, there could be some definitions that are inaccurate or missing. Code security The .NET Framework security model didn t exist previously.
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There s no way that code written prior to this model can take advantage of it. Features such as declarative security are not available in unmanaged code, which can mean your new .NET code will be forced to accommodate this inconsistency.
Versioning As is the case with security, versioning (which incidentally was a
huge issue in prior development environments) didn t exist in the form it does now. Therefore, side-by-side execution might not be available when using unmanaged code.
Lab: Call Windows DLL Functions
In this lab, you create a simple method to call a Windows DLL. If you encounter a problem completing an exercise, the completed projects are available on the companion CD in the Code folder.
Exercise 1: Calling a Windows API
In this exercise, you ll call a MessageBox Windows API.
Lesson 3: Using Unmanaged Code
1. Open Visual Studio 2005, and create a new C# or Visual Basic 2005 console application. 2. Open Program.cs or Module1.vb, depending on your language of preference. 3. Add the following code definition or definitions for whichever class or classes you decided to write:
VB Public Const UserReference As String = "user32.dll" <DllImport(UserReference)> _ Private Function MessageBox(ByVal hWnd As Int32, _ ByVal pText As String, ByVal pCaption As String, _ ByVal uType As Int32) As Int32 End Function C# public const String UserReference = "user32.dll"; [DllImport(UserReference, SetLastError=true)] private static extern Int32 MessageBox(IntPtr hWnd, String pText, String pCaption, Int32 uType);
4. Build the project, and resolve any errors.
Lesson Summary
The .NET Framework provides a mechanism to call Windows API calls and unmanaged code through Platform Invoke. To use P/Invoke, you use the DllImport attribute and the name of the DLL you are referencing. You must use the private and static/shared attributes for P/Invoke calls. To allow default positioning in a structure that s to be marshaled, you can use the Layout.Sequential attribute. To specify a value for the positioning in a structure, you can use the Layout .Explicit attribute. Error messages from unmanaged code behave differently from managed exceptions. To trap them correctly, the Windows API can be used.
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