MsgBox 示例

项目
09/04/2008

更新：2007 年 11 月

此示例演示如何通过值将字符串类型作为 In 参数传递以及何时使用 EntryPoint、CharSet 和 ExactSpelling 字段。

MsgBox 示例使用以下非托管函数（这里同时显示其原始函数声明）：

从 User32.dll 导出的 MessageBox。

int MessageBox(HWND hWnd, LPCTSTR lpText, LPCTSTR lpCaption, 
   UINT uType);

在该示例中，LibWrap 类包含 MsgBoxSample 类调用的每一个非托管函数的托管原型。托管原型方法 MsgBox、MsgBox2 和 MsgBox3 对于同一个非托管函数具有不同的声明。

MsgBox2 的声明将在消息框中产生不正确的输出，原因是被指定为 ANSI 的字符类型与按 Unicode 函数命名的入口点 MessageBoxW 不匹配。MsgBox3 的声明将在 EntryPoint、CharSet 和 ExactSpelling 字段之间产生不匹配现象。在调用时，MsgBox3 将引发异常。有关字符串命名和名称封送处理的详细信息，请参见指定字符集。

下面的代码示例的源代码由 .NET Framework 平台调用技术示例提供。

声明原型

Public Class LibWrap
   ' Declares managed prototypes for unmanaged functions.
   Declare Auto Function MsgBox Lib "User32.dll" Alias "MessageBox" ( _
   ByVal hWnd As Integer, ByVal txt As String, ByVal caption As String, _
   yVal typ As Integer ) As Integer
   
   ' Causes incorrect output in the message window.
   Declare Ansi Function MsgBox2 Lib "User32.dll" Alias "MessageBoxW" ( _
   ByVal hWnd As Integer, ByVal txt As String, ByVal caption As String, _
   ByVal type As Integer ) As Integer 
   
   ' Causes an exception to be thrown.
   ' ExactSpelling is True by default in Visual Basic 2005 when 
   ' Ansi or Unicode is used.
   Declare Ansi Function MsgBox3 Lib "User32.dll" Alias "MessageBox" ( _
   ByVal hWnd As Integer, ByVal txt As String, ByVal caption As String, _
   ByVal typ As Integer ) As Integer
End Class

public class LibWrap 
{
   // Declares managed prototypes for unmanaged functions.
   [ DllImport( "User32.dll", EntryPoint="MessageBox", 
      CharSet=CharSet.Auto )]
   public static extern int MsgBox( int hWnd, String text, String caption, 
      uint type );
   // Causes incorrect output in the message window.
   [ DllImport( "User32.dll", EntryPoint="MessageBoxW", 
      CharSet=CharSet.Ansi )]
   public static extern int MsgBox2( int hWnd, String text, 
      String caption, uint type );
   // Causes an exception to be thrown. EntryPoint, CharSet, and 
   // ExactSpelling fields are mismatched.
   [ DllImport( "User32.dll", EntryPoint="MessageBox", 
      CharSet=CharSet.Ansi, ExactSpelling=true )]
   public static extern int MsgBox3( int hWnd, String text, 
      String caption, uint type );
}

调用函数

Public Class MsgBoxSample
   Public Shared Sub Main()
   
      LibWrap.MsgBox( 0, "Correct text", "MsgBox Sample", 0 )
      LibWrap.MsgBox2( 0, "Incorrect text", "MsgBox Sample", 0 )
      
      Try
         LibWrap.MsgBox3( 0, "No such function", "MsgBox Sample", 0 )
      Catch e As EntryPointNotFoundException
         Console.WriteLine( "EntryPointNotFoundException thrown _
           as expected!" )
      End Try
   End Sub 
End Class

public class MsgBoxSample
{
   public static void Main()
   {
      LibWrap.MsgBox( 0, "Correct text", "MsgBox Sample", 0 );
      LibWrap.MsgBox2( 0, "Incorrect text", "MsgBox Sample", 0 );
      try
      {
         LibWrap.MsgBox3( 0, "No such function", "MsgBox Sample", 0 );
      }
      catch( EntryPointNotFoundException )
      {
         Console.WriteLine( "EntryPointNotFoundException thrown as 
           expected!" );
      }
   }
}