使用Windows API实现两个进程间（含窗体）的通信在Windows下的两个进程之间通信通常有多种实现方式，在.NET中，有如命名管道、消息队列、共享内存等实现方式，这篇文章要讲的是使用Windows的API来实现简单的进程间通信，这两个进程既可以都是基于C#开发，也可以都是基于C++开发，也可以是一个C#开发而另一个为C++开发，在C++开发方面，不需要额外调用Windows的API，而是可以直接使用相关方法即可。所以，这里重点要讲的就是在C#中是如何做的，而至于在C++中是如何做的将给出例子，并不做详述。

对于接收消息，只需要重写DefWndProc函数即可，对于发送消息，笔者编写了一个类MsgHandler来实现。要顺利实现消息的接收与发送，使用了Windows的API：FindWindow、SendMessage等。在C#环境中，通过DllImport来引入相应的API，代码示例如下：

// FindWindow method, using Windows API [DllImport("User32.dll", EntryPoint = "FindWindow")] private static extern int FindWindow(string lpClassName, string lpWindowName); // IsWindow method, using Windows API [DllImport("User32.dll", EntryPoint = "IsWindow")] private static extern bool IsWindow(int hWnd); // SendMessage method, using Windows API [DllImport("User32.dll", EntryPoint = "SendMessage")] private static extern int SendMessage( int hWnd, // handle to destination window int Msg, // message int wParam, // first message parameter ref COPYDATASTRUCT lParam // second message parameter ); // SendMessage method, using Windows API [DllImport("User32.dll", EntryPoint = "SendMessage")] private static extern int SendMessage( int hWnd, // handle to destination window int Msg, // message int wParam, // first message parameter string lParam // second message parameter );

笔者查阅了相关网络资源，发现很少有提及使用自定义消息来发送和接收消息的，几乎都是使用了系统消息WM_COPYDATA来实现。在本例中，笔者除了使用系统消息WM_COPYDATA来收发消息外，还将使用自定义消息来实现收发消息。不过，值得注意的是，笔者在测试过程中发现，使用自定义的消息来收发结构体时发生了一些异常，该异常提示说内存不能读，对于该问题，还有待进一步解决，当然，若是哪位前辈或朋友有遇到过该问题并已顺利解决的话，不妨告知，笔者将洗耳恭听。

消息发送类MsgHandler的代码示例如下：

using System; using System.Text; using System.Windows.Forms; using System.Diagnostics; using System.Runtime.InteropServices; namespace CommunicationTest { /// <summary> /// Send message handler class. /// Call the method "SendMessageToTargetWindow" to send /// the message what we want. /// </summary> class MsgHandler { /// <summary> /// System defined message /// </summary> private const int WM_COPYDATA = 0x004A; /// <summary> /// User defined message /// </summary> private const int WM_DATA_TRANSFER = 0x0437; // FindWindow method, using Windows API [DllImport("User32.dll", EntryPoint = "FindWindow")] private static extern int FindWindow(string lpClassName, string lpWindowName); // IsWindow method, using Windows API [DllImport("User32.dll", EntryPoint = "IsWindow")] private static extern bool IsWindow(int hWnd); // SendMessage method, using Windows API [DllImport("User32.dll", EntryPoint = "SendMessage")] private static extern int SendMessage( int hWnd, // handle to destination window int Msg, // message int wParam, // first message parameter ref COPYDATASTRUCT lParam // second message parameter ); // SendMessage method, using Windows API [DllImport("User32.dll", EntryPoint = "SendMessage")] private static extern int SendMessage( int hWnd, // handle to destination window int Msg, // message int wParam, // first message parameter string lParam // second message parameter ); /// <summary> /// CopyDataStruct /// </summary> private struct COPYDATASTRUCT { public IntPtr dwData; public int cbData; public IntPtr lpData; } /// <summary> /// Send message to target window /// </summary> /// <param name="wndName">The window name which we want to found</param> /// <param name="msg">The message to be sent, string</param> /// <returns>success or not</returns> public static bool SendMessageToTargetWindow(string wndName, string msg) { Debug.WriteLine(string.Format("SendMessageToTargetWindow: Send message to target window {0}: {1}", wndName, msg)); int iHWnd = FindWindow(null, wndName); if (iHWnd == 0) { string strError = string.Format("SendMessageToTargetWindow: The target window [{0}] was not found!", wndName); MessageBox.Show(strError, "Error", MessageBoxButtons.OK, MessageBoxIcon.Error); Debug.WriteLine(strError); return false; } else { byte[] bytData = null; bytData = Encoding.Default.GetBytes(msg); COPYDATASTRUCT cdsBuffer; cdsBuffer.dwData = (IntPtr)100; cdsBuffer.cbData = bytData.Length; cdsBuffer.lpData = Marshal.AllocHGlobal(bytData.Length); Marshal.Copy(bytData, 0, cdsBuffer.lpData, bytData.Length); // Use system defined message WM_COPYDATA to send message. int iReturn = SendMessage(iHWnd, WM_COPYDATA, 0, ref cdsBuffer); if (iReturn < 0) { string strError = string.Format("SendMessageToTargetWindow: Send message to the target window [{0}] failed!", wndName); MessageBox.Show(strError, "Error", MessageBoxButtons.OK, MessageBoxIcon.Error); Debug.WriteLine(strError); return false; } return true; } } /// <summary> /// Send message to target window /// </summary> /// <param name="wndName">The window name which we want to found</param> /// <param name="wParam">first parameter, integer</param> /// <param name="lParam">second parameter, string</param> /// <returns>success or not</returns> public static bool SendMessageToTargetWindow(string wndName, int wParam, string lParam) { Debug.WriteLine(string.Format("SendMessageToTargetWindow: Send message to target window {0}: wParam:{1}, lParam:{2}", wndName, wParam, lParam)); int iHWnd = FindWindow(null, wndName); if (iHWnd == 0) { string strError = string.Format("SendMessageToTargetWindow: The target window [{0}] was not found!", wndName); MessageBox.Show(strError, "Error", MessageBoxButtons.OK, MessageBoxIcon.Error); Debug.WriteLine(strError); return false; } else { // Use our defined message WM_DATA_TRANSFER to send message. int iReturn = SendMessage(iHWnd, WM_DATA_TRANSFER, wParam, lParam); if (iReturn < 0) { string strError = string.Format("SendMessageToTargetWindow: Send message to the target window [{0}] failed!", wndName); MessageBox.Show(strError, "Error", MessageBoxButtons.OK, MessageBoxIcon.Error); Debug.WriteLine(strError); return false; } return true; } } } }

消息接收重写了DefWndProc方法，其代码示例如下：

/// <summary> /// Override the DefWndProc function, in order to receive the message through it. /// </summary> /// <param name="m">message</param> protected override void DefWndProc(ref System.Windows.Forms.Message m) { switch (m.Msg) { // Here, we use WM_COPYDATA message to receive the COPYDATASTRUCT case WM_COPYDATA: COPYDATASTRUCT cds = new COPYDATASTRUCT(); cds = (COPYDATASTRUCT)m.GetLParam(typeof(COPYDATASTRUCT)); byte[] bytData = new byte[cds.cbData]; Marshal.Copy(cds.lpData, bytData, 0, bytData.Length); this.ProcessIncomingData(bytData); break; // Here, we use our defined message WM_DATA_TRANSFER to receive the // normal data, such as integer, string. // We had try to use our defined message to receive the COPYDATASTRUCT, // but it didn't work!! It told us that we try to access the protected // memory, it usually means that other memory has been broken. case WM_DATA_TRANSFER: int iWParam = (int)m.WParam; string sLParam = m.LParam.ToString(); this.ProcessIncomingData(iWParam, sLParam); break; default: base.DefWndProc(ref m); break; } }

消息的接收与发送最终通过一个WinForm展现出来，其代码实现如下：

using System; using System.Text; using System.Windows.Forms; using System.Runtime.InteropServices; namespace CommunicationTest { public partial class FrmTest : Form { /// <summary> /// System defined message /// </summary> private const int WM_COPYDATA = 0x004A; /// <summary> /// User defined message /// </summary> private const int WM_DATA_TRANSFER = 0x0437; /// <summary> /// CopyDataStruct /// </summary> public struct COPYDATASTRUCT { public IntPtr dwData; public int cbData; public IntPtr lpData; } public FrmTest() { InitializeComponent(); } /// <summary> /// Override the DefWndProc function, in order to receive the message through it. /// </summary> /// <param name="m">message</param> protected override void DefWndProc(ref System.Windows.Forms.Message m) { switch (m.Msg) { // Here, we use WM_COPYDATA message to receive the COPYDATASTRUCT case WM_COPYDATA: COPYDATASTRUCT cds = new COPYDATASTRUCT(); cds = (COPYDATASTRUCT)m.GetLParam(typeof(COPYDATASTRUCT)); byte[] bytData = new byte[cds.cbData]; Marshal.Copy(cds.lpData, bytData, 0, bytData.Length); this.ProcessIncomingData(bytData); break; // Here, we use our defined message WM_DATA_TRANSFER to receive the // normal data, such as integer, string. // We had try to use our defined message to receive the COPYDATASTRUCT, // but it didn't work!! It told us that we try to access the protected // memory, it usually means that other memory has been broken. case WM_DATA_TRANSFER: int iWParam = (int)m.WParam; string sLParam = m.LParam.ToString(); this.ProcessIncomingData(iWParam, sLParam); break; default: base.DefWndProc(ref m); break; } } /// <summary> /// Process the incoming data /// </summary> /// <param name="data">incoming data</param> private void ProcessIncomingData(byte[] bytesData) { string strRevMsg = "Receive message: " + Encoding.Default.GetString(bytesData); lstReceivedMsg.Items.Add(strRevMsg); } /// <summary> /// Process the incoming data /// </summary> /// <param name="iWParam">a integer parameter</param> /// <param name="sLParam">a string parameter</param> private void ProcessIncomingData(int iWParam, string sLParam) { StringBuilder strBuilder = new StringBuilder(); strBuilder.Append("wParam: "); strBuilder.Append(iWParam.ToString()); strBuilder.Append(", lParam: "); strBuilder.Append(sLParam); lstReceivedMsg.Items.Add(strBuilder.ToString()); } private void FrmTest_Load(object sender, EventArgs e) { this.Text = "First Test Form"; this.txtCurrentWndName.Text = "First Test Form"; this.txtTargetWndName.Text = "Second Test Form"; } private void txtCurrentWndName_TextChanged(object sender, EventArgs e) { this.Text = txtCurrentWndName.Text; } /// <summary> /// Send message to the target window with system defined message WM_COPYDATA /// </summary> private void btnSend1_Click(object sender, EventArgs e) { string strWndName = this.txtTargetWndName.Text; // Check the target window name is null/empty or not if (string.IsNullOrEmpty(strWndName)) { MessageBox.Show("The target window name must not be null or empty!", "Information", MessageBoxButtons.OK, MessageBoxIcon.Exclamation); return; } string strMsg = this.txtSendingMsg.Text; // Check the sending message is null/empty or not if (string.IsNullOrEmpty(strMsg)) { MessageBox.Show("The sending message must not be null or empty!", "Information", MessageBoxButtons.OK, MessageBoxIcon.Exclamation); return; } // Send message to the target window bool bReturn = MsgHandler.SendMessageToTargetWindow(strWndName, strMsg); } /// <summary> /// Send message to the target window with user defined message WM_DATA_TRANSFER /// </summary> private void btnSend2_Click(object sender, EventArgs e) { string strWndName = this.txtTargetWndName.Text; // Check the target window name is null/empty or not if (string.IsNullOrEmpty(strWndName)) { MessageBox.Show("The target window name must not be null or empty!", "Information", MessageBoxButtons.OK, MessageBoxIcon.Exclamation); return; } string strWParam = this.txtWParam.Text; string strLParam = this.txtLParam.Text; // Check the sending message is null/empty or not if (string.IsNullOrEmpty(strWParam) || string.IsNullOrEmpty(strLParam)) { MessageBox.Show("The sending message must not be null or empty!", "Information", MessageBoxButtons.OK, MessageBoxIcon.Exclamation); return; } int iWParam = 0; // Convert string to integer try { iWParam = Int32.Parse(strWParam); } catch (Exception ex) { MessageBox.Show("Convert string to integer exception: " + ex.Message, "Error", MessageBoxButtons.OK, MessageBoxIcon.Error); return; } // Send message to the target window bool bReturn = MsgHandler.SendMessageToTargetWindow(strWndName, iWParam, strLParam); } } }

通过上述的C#部分的代码，已经可以实现两个C#窗体间的通信，其界面截图如下图所示：

那么，在C++中又是如何实现的呢？这个其实也是很简单的。要实现消息的收发，同理也要重写WindowProc以便于接收消息，而通过调用方法亦可实现消息的发送。
    对于消息接收，如果是系统消息，可以通过OnCopyData(CWnd* pWnd, COPYDATASTRUCT* pCopyDataStruct)事件完成；如果是自定义消息，可以通过重写WindowProc完成。代码示例如下：

BOOL CTestDlg::OnCopyData(CWnd* pWnd, COPYDATASTRUCT* pCopyDataStruct) { // TODO: 在此添加消息处理程序代码和/或调用默认值 DWORD dwSize = pCopyDataStruct->cbData; CString szData;// = (LPCTSTR)(pCopyDataStruct->lpData); TCHAR* ptchar = new TCHAR[dwSize+1]; memcpy( ptchar, pCopyDataStruct->lpData, dwSize ); ptchar[dwSize] = TEXT('/0'); szData = ptchar; // TODO: Add some code here to handler the message delete[] ptchar; return CDialog::OnCopyData(pWnd, pCopyDataStruct); } LRESULT CTestDlg::WindowProc(UINT message, WPARAM wParam, LPARAM lParam) { // TODO: 在此添加专用代码和/或调用基类 COPYDATASTRUCT szCpyData; switch ( message ) { case WM_DATA_TRANSFER: szCpyData = *((PCOPYDATASTRUCT)lParam); // TODO: Add some code here to handler the message break; default: break; } return CDialog::WindowProc(message, wParam, lParam); }
    对于消息发送，只需要调用形如SendMessage(m_hwndMsg, WM_DATA_TRANSFER, wParam, lParam)方法即可实现，lParam参数可以是PCOPYDATASTRUCT等。

通过上面的介绍，相信已经可以轻松实现两个进程间（含窗体）的通信的，使用这样的方法，既简单又能够满足大部分的应用需求，不失为一种简便的方法。