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MORE INFO Definition of type safe
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Although the term type safe has different meanings in different environments, in general it refers to verification of a given type so that mismatches cannot present themselves. An in-depth discussion is available at Wikipedia (http://en.wikipedia.org/wiki/Type_safety).
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To use Delegate objects correctly, perform the following steps: 1. Create a Delegate object with the same signature as the callback. 2. Substitute the Delegate for the callback, and make the call. The following listing is an example of using an unmanaged callback:
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' VB Imports System.Text Imports System.Runtime.InteropServices Imports System.Runtime.CompilerServices Public Class UnmanagedCallbackDemoVB Public Delegate Function DemoCallback(ByVal hWnd As IntPtr, _ ByVal lParam As Int32) As Boolean Private Const UserReference As String = "user32.dll" Private Const BufferSize As Int32 = 100 <DllImport(UserReference)> _ Public Shared Function EnumWindows(ByVal callback As DemoCallback, _ ByVal param As Int32) As Int32 End Function <DllImport(UserReference)> _ Public Shared Function GetWindowText(ByRef hWnd As IntPtr, _ ByRef lpString As StringBuilder, ByVal nMaxCount As Int32) As Int32 End Function
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Interoperation
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Public Shared Function DisplayWindowInfo(ByVal hWnd As IntPtr, _ ByVal lParam As Int32) As Boolean Dim DemoBuilder As New StringBuilder(BufferSize) If GetWindowText(hWnd, DemoBuilder, BufferSize) <> 0 Then Console.WriteLine("Demo Output: " + DemoBuilder.ToString()) End If Return True End Function Public Shared Sub RunDemo() EnumWindows(AddressOf DisplayWindowInfo, 0) Console.WriteLine("Beginning process...") Console.ReadLine() End Sub End Class // C# using System; using System.Runtime.CompilerServices; using System.Runtime.InteropServices; namespace NetForComDemoCS { public class UnmanagedCallbackDemo { public delegate Boolean DemoCallback(IntPtr hWnd, Int32 lParam); private const String UserReference = "user32.dll"; private const Int32 BufferSize = 100; [DllImport(UserReference)] public static extern Int32 EnumWindows(DemoCallback callback, Int32 param); [DllImport(UserReference)] public static extern Int32 GetWindowText(IntPtr hWnd, StringBuilder lpString, Int32 nMaxCount); public static Boolean DisplayWindowInfo(IntPtr hWnd, Int32 lParam) { StringBuilder DemoBuilder = new StringBuilder(BufferSize); if (GetWindowText(hWnd, DemoBuilder, BufferSize) != 0) { Console.WriteLine("Demo Output: " + DemoBuilder.ToString()); } return true; } public static void RunDemo() { EnumWindows(DisplayWindowInfo, 0);
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Lesson 3: Using Unmanaged Code
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Console.WriteLine("Beginning process..."); Console.ReadLine(); } } }
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When the application is run, the output should approximate that shown in Figure 13-4.
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Figure 13-4
Output from unmanaged callback
Exception Handling in Managed Code
Exceptions in unmanaged code are markedly different from those thrown in managed code. In the earlier COM days, you could use the GetLastError function to get the last error that was raised. This approach won t work in a managed application because the return value of GetLastError might not be the correct one. Why Because the GetLastError method can be set by either a .NET Framework object or the common language runtime (CLR). Because you can t use the GetLastError method, you need to do something else. After all, the only thing more problematic than no exception handling is bad exception handling. Here is an example of error handling in managed code:
' VB Imports System.Runtime.CompilerServices Imports System.Runtime.InteropServices Public Class UnmanagedErrorDemo Private Const KernelReference As String = "kernel32.dll" Private Const UserReference As String = "user32.dll"
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Private Const MessageSize As Int32 = 255 <DllImport(KernelReference)> _ Private Shared Function FormatMessage(dwFlags as Int32, _ lpSource as Int32, dwMessageId as Int32, dwLanguageId as Int32, _ lpBuffer ByRef, nSize as Int32, Arguments as Int32 ) As Int32 End Function <DllImport(UserReference)> _ Private Shared Function MessageBox(ByVal hWnd As Int32, _ ByVal pText As String, ByVal pCaption As String, _ ByVal uType As Int32) As Int32 End Function Public Shared Sub ThrowMessageBoxException() Dim ProblemCauser As IntPtr = CType(-100, IntPtr) MessageBox(ProblemCauser, "This won't work", _ "Caption - This won't work", 0) Dim ErrorCode As Int32 = Marshal.GetLastWin32Error() Console.WriteLine("Error Code: " & ErrorCode.ToString()) Console.WriteLine("Real Error Code: " & _ GetLastErrorMessage(ErrorCode)) End Sub Public Shared Function GetLastErrorMessage( _ ByVal errorValue As Int32) As String 'This order doesn't matter but should be kept 'for logical consistency Dim FORMAT_MESSAGE_ALLOCATE_BUFFER As Int32 = &H100 '0x00000100 Dim FORMAT_MESSAGE_IGNORE_INSERTS As Int32 = &H200 ' 0x00000200 Dim FORMAT_MESSAGE_FROM_SYSTEM As Int32 = &H1000 ' 0x00001000 Dim lpMsgBuf As String = String.Empty Dim dwFlags As Int32 = FORMAT_MESSAGE_ALLOCATE_BUFFER And _ FORMAT_MESSAGE_FROM_SYSTEM And FORMAT_MESSAGE_IGNORE_INSERTS Dim ReturnValue As Int32 = FormatMessage(dwFlags, 0, errorValue, _ 0, lpMsgBuf, MessageSize, 0) If ReturnValue = 0 Then Return Nothing Else Return lpMsgBuf End If End Function // C# using System.Runtime.CompilerServices; using System.Runtime.InteropServices; namespace NetForComDemoCS { public class UnmanagedErrorDemo {
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