MsgBox 範例
這個範例示範如何以傳值方式將字串型別當成 In 參數傳遞，以及何時使用 EntryPoint、CharSet 和 ExactSpelling 欄位。
MsgBox 範例使用下列 Unmanaged 函式，顯示其原始函式宣告：
從 User32.dll 匯出的 MessageBox。
int MessageBox(HWND hWnd, LPCTSTR lpText, LPCTSTR lpCaption, UINT uType);
在這個範例中，LibWrap 類別包含由 MsgBoxSample 類別呼叫的每一個 Unmanaged 函式的 Managed 原型。對於相同的 Unmanaged 函式，Managed 原型方法 MsgBox、MsgBox2 和 MsgBox3 有不同的宣告。
MsgBox2 的宣告會在訊息方塊中產生不正確的輸出，因為以 ANSI 指定的字元型別與進入點 (Entry Point) MessageBoxW (即 Unicode 函式的名稱) 不相符。MsgBox3 的宣告會在 EntryPoint、CharSet 和 ExactSpelling 欄位之間建立一個不相符。當呼叫時，MsgBox3 會擲回例外狀況。如需字串命名和名稱封送處理的詳細資訊，請參閱指定字元集。
下列程式碼範例的原始程式碼是由 .NET Framework 平台叫用技術範例所提供。
宣告原型
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 );
}
呼叫函式
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!" );
}
}
}