OpenFileDlg 範例
更新：2007 年 11 月
這個範例示範如何呼叫 Unmanaged 函式，其預期含有字串之結構的指標。此外，它也示範如何使用 Managed 類別來表示 Unmanaged 結構、如何套用 InAttribute 和 OutAttribute 屬性來封送處理類別回呼叫端，以及如何宣告和初始化類別的不同欄位來產生正確的 Unmanaged 表示。
OpenFileDlg 範例使用下列 Unmanaged 函式，顯示其原始函式宣告：
從 Comdlg32.dll 匯出 GetOpenFileName。
BOOL GetOpenFileName(LPOPENFILENAME lpofn);
傳遞至前一個函式之 Win32 API 的 LPOPENFILENAME 結構包含下列項目：
typedef struct tagOFN {
DWORD lStructSize;
//…
LPCTSTR lpstrFilter;
//…
LPTSTR lpstrFile;
DWORD nMaxFile;
LPTSTR lpstrFileTitle;
DWORD nMaxFileTitle;
LPCTSTR lpstrInitialDir;
LPCTSTR lpstrTitle;
//…
LPCTSTR lpstrDefExt;
//…
} OPENFILENAME, *LPOPENFILENAME;
在這個範例中，OpenFileName 類別包含原始結構的元素做為類別成員。Unmanaged 結構是宣告為類別而非 Managed 結構，來顯示當 Unmanaged 函式預期結構的指標時，類別如何使用類別。因為 Managed 類別是參考型別 (Reference Type)，當它是以傳值方式傳遞時，類別的指標會被傳遞至 Unmanaged 程式碼。這正是 Unmanaged 函式所預期的。
套用 StructLayoutAttribute 屬性以確保成員是按照出現的順序，循序配置於記憶體中。設定 CharSet 欄位，以便平台叫用可以根據目標平台在 Run Time 於 ANSI 和 Unicode 格式間選擇。
LibWrap 類別包含 GetOpenFileName 方法的 Managed 原型，會將 OpenFileName 類別當成 In/Out 參數傳遞。在這個範例中，透過明確套用 InAttribute 和 OutAttribute，可以確保將 OpenFileName 封送處理為 In/Out 參數，而呼叫端可以查看封送處理回的變更 (由於效能因素，類別的預設方向屬性是 In，避免呼叫端查看封送處理回的變更)。App 類別會建立 OpenFileName 類別的新執行個體，並使用 Marshal.SizeOf 方法決定 Unmanaged 結構的大小 (以位元組為單位)。
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
' Declare a class member for each structure element.
< StructLayout( LayoutKind.Sequential, CharSet:=CharSet.Auto )> _
Public Class OpenFileName
Public structSize As Integer = 0
'…
Public filter As String = Nothing
'…
Public file As String = Nothing
Public maxFile As Integer = 0
Public fileTitle As String = Nothing
Public maxFileTitle As Integer = 0
Public initialDir As String = Nothing
Public title As String = Nothing
'…
Public defExt As String = Nothing
'…
End Class 'OpenFileName
Public Class LibWrap
' Declare managed prototype for the unmanaged function.
Declare Auto Function GetOpenFileName Lib "Comdlg32.dll" ( _
<[In], Out> ByVal ofn As OpenFileName ) As Boolean
End Class 'LibWrap
// Declare a class member for each structure element.
[ StructLayout( LayoutKind.Sequential, CharSet=CharSet.Auto )]
public class OpenFileName
{
public int structSize = 0;
//…
public String filter = null;
//…
public String file = null;
public int maxFile = 0;
public String fileTitle = null;
public int maxFileTitle = 0;
public String initialDir = null;
public String title = null;
//…
public String defExt = null;
//…
}
public class LibWrap
{
// Declare a managed prototype for the unmanaged function.
[ DllImport( "Comdlg32.dll", CharSet=CharSet.Auto )]
public static extern bool GetOpenFileName([ In, Out ]
OpenFileName ofn );
}
呼叫函式
Public Class App
Public Shared Sub Main()
Dim ofn As New OpenFileName()
ofn.structSize = Marshal.SizeOf( ofn )
ofn.filter = "Log files" & ChrW(0) & "*.log" & ChrW(0) & _
"Batch files" & ChrW(0) & "*.bat" & ChrW(0)
ofn.file = New String( New Char( 256 ) {})
ofn.maxFile = ofn.file.Length
ofn.fileTitle = New String( New Char( 64 ) {})
ofn.maxFileTitle = ofn.fileTitle.Length
ofn.initialDir = "C:\"
ofn.title = "Open file called using platform invoke..."
ofn.defExt = "txt"
If LibWrap.GetOpenFileName( ofn ) Then
Console.WriteLine( "Selected file with full path: {0}", _
ofn.file )
End If
End Sub 'Main
End Class 'OpenFileDlgSample
public class App
{
public static void Main()
{
OpenFileName ofn = new OpenFileName();
ofn.structSize = Marshal.SizeOf( ofn );
ofn.filter = "Log files\0*.log\0Batch files\0*.bat\0";
ofn.file = new String( new char[ 256 ]);
ofn.maxFile = ofn.file.Length;
ofn.fileTitle = new String( new char[ 64 ]);
ofn.maxFileTitle = ofn.fileTitle.Length;
ofn.initialDir = "C:\\";
ofn.title = "Open file called using platform invoke...";
ofn.defExt = "txt";
if( LibWrap.GetOpenFileName( ofn ))
{
Console.WriteLine( "Selected file with full path: {0}",
ofn.file );
}
}
}