在 C# 應用程式與 DLL 交互中使用訊息 (2)：完整實作範例

範例應用完整流程

1. 定義訊息常數

public class Message
{
    public const int USER = 0x0400;           // 對應 MFC WM_USER
    public const int WM_TEST = USER + 101;    // 應用程式自訂訊息
    public const int WM_MSG = USER + 102;     // DLL 傳遞訊息
}

2. 宣告外部函式

在 MsgForm.cs 中宣告：

// 宣告 SendMessage API
[DllImport("User32.dll", EntryPoint = "SendMessage")]
private static extern int SendMessage(
    IntPtr hWnd, uint Msg, uint wParam, uint lParam);

// 宣告 DLL 中啟動訊息發送函式
[DllImport("MessageDLL.dll", EntryPoint = "StartSendMessage")]
private extern static void StartSendMessage(IntPtr hWnd);

3. 處理系統繪圖訊息 (WM_PAINT)

protected override void OnPaint(PaintEventArgs e)
{
    Font font = new Font("黑體", 28);
    SolidBrush bluepen = new SolidBrush(Color.Blue);
    SolidBrush blackpen = new SolidBrush(Color.FromArgb(0xa0, 0xa0, 0xb0));
    e.Graphics.DrawString("VC知識庫", font, blackpen, 65, 25);
    e.Graphics.DrawString("VC知識庫", font, bluepen, 61, 21); // 偏移 4 像素產生立體效果
}

4. 觸發自訂訊息

// 測試應用程式訊息
private void TestAppbutton_Click(object sender, System.EventArgs e)
{
    SendMessage(this.Handle, Message.WM_TEST, 100, 200);
}

// 測試 DLL 訊息
private void TestDLLbutton_Click(object sender, System.EventArgs e)
{
    StartSendMessage(this.Handle);
}

5. 回應與處理自訂訊息

protected override void DefWndProc(ref System.Windows.Forms.Message m)
{
    string message;
    switch (m.Msg)
    {
        case Message.WM_TEST:
            message = string.Format("收到來自應用程式的訊息! 參數為: {0}, {1}", m.WParam, m.LParam);
            MessageBox.Show(message);
            break;
        case Message.WM_MSG:
            message = string.Format("收到來自 DLL 的訊息! 參數為: {0}, {1}", m.WParam, m.LParam);
            MessageBox.Show(message);
            break;
        default:
            base.DefWndProc(ref m); // 呼叫基類處理非自訂訊息
            break;
    }
}

DLL 端訊息定義請參考「VC.Net 從 DLL 傳遞訊息到 DLL」一文。