Detailed explanation of parameter passing of SendMessage and PostMessage in C#

In C#, you can use SendMessage and PostMessage provided by Window API to pass parameters. The difference between the two is briefly introduced: the difference in return value, let's first take a look at the statement in MSDN:

LRESULT SendMessage(

HWND hWnd,

UINT Msg,


WPARAM wParam,

LPARAM lParam

);


BOOL PostMessage(

HWND hWnd,

UINT Msg,


WPARAM wParam,

LPARAM lParam

);

The meaning of the four parameters is the same, but the return value type is different ( In fact, from the data point of view, they are the same 32-bit number, but the meaning is different), LRESULT represents the return value after the message is processed, and BOOL represents whether the message is Posted successfully.


2. PostMessage is asynchronous and SendMessage is synchronous.
PostMessage only puts the message into the queue and returns regardless of whether the message is processed. The message may not be processed; and SendMessage waits for the message to be processed before returning. If the message is not processed, The thread sending the message will always be blocked.


3. If SendMessage sends a message in the same thread, the USER32.DLL
module calls the message handler of the target window and returns the result. . SendMessage sends a message in the same thread and does not put it into the thread message queue. PostMessage
When sending a message, the message must first be put into the thread's message queue, and then dispatched to the target window (DispatchMessage) through the message loop.


If in different threads, SendMessage sends the message to the message queue of the thread to which the target window belongs, and then the thread sending the message monitors and waits for message processing in the USER32.DLL
module until the target window Return after processing. SendMessage also does a lot of work before returning, such as responding to other threads sending Message to it. When Posting to other threads, it is best to use PostThreadMessage instead of
PostMessage. The hWnd parameter of PostMessage can be
NULL, which is equivalent to PostThreadMessage +Get
CurrentThreadId. When Post WM_QUIT, PostQuitMessage should be used instead.
4. The system only marshals system messages (messages between 0 and WM_USER). When sending user messages (WM_USER and above) to other processes, you need to marshal them yourself.

When using PostMessage, SendNot
ifyMessage, SendMessageCallback and other asynchronous functions to send system messages, pointers cannot be used in the parameters because The sender does not wait for the message to be processed before returning, and the pointer is released before the receiver processes it. 5. In Windows 2000/XP, each message queue can only store up to 10,000 Post messages. The excess messages that have not been processed will not be processed and will be discarded directly. This value can be changed to a larger value: [HKEY_LOCAL_MACHINE/SOFTWARE/ Microsoft/Windows NT/CurrentVersion/Windows] USERPostMessageLimit, the minimum can be 4000. PostMessage is only responsible for placing messages in the message queue. It is not sure when and whether to process SendMessage. It must wait until it receives the return code of message processing (DWord type) before continuing. PostMessage will return immediately after execution. SendMessage must wait for the message. It will be returned after being processed. The following is a small example to illustrate the differences between these two methods for parameter passing: //Win32 API class

using System;
using System.Runtime.InteropServices;


namespace TestHwnd
{
    public class Win32API
    {
        
        [DllImport("User32.dll", EntryPoint = "FindWindow")]
        public static extern IntPtr FindWindow(string lpClassName, string lpWindowName);


        [DllImport("User32.dll", EntryPoint = "FindWindowEx")]
        public static extern IntPtr FindWindowEx(IntPtr hwndParent, IntPtr hwndChildAfter, string lpClassName, string lpWindowName);


        /// <summary>
        /// 自定义的结构
        /// </summary>
        public struct My_lParam
        {
            public int i;
            public string s;
        }
        /// <summary>
        /// 使用COPYDATASTRUCT来传递字符串
        /// </summary>
        [StructLayout(LayoutKind.Sequential)]
        public struct COPYDATASTRUCT
        {
            public IntPtr dwData;
            public int cbData;
            [MarshalAs(UnmanagedType.LPStr)]
            public string lpData;
        }
        //消息发送API
        [DllImport("User32.dll", EntryPoint = "SendMessage")]
        public static extern int SendMessage(
            IntPtr hWnd,        // 信息发往的窗口的句柄
            int Msg,            // 消息ID
            int wParam,         // 参数1
            int lParam          //参数2
        );


        //消息发送API
        [DllImport("User32.dll", EntryPoint = "SendMessage")]
        public static extern int SendMessage(
            IntPtr hWnd,        // 信息发往的窗口的句柄
            int Msg,            // 消息ID
            int wParam,         // 参数1
            ref My_lParam lParam //参数2
        );
        
        //消息发送API
        [DllImport("User32.dll", EntryPoint = "SendMessage")]
        public static extern int SendMessage(
            IntPtr hWnd,        // 信息发往的窗口的句柄
            int Msg,            // 消息ID
            int wParam,         // 参数1
            ref  COPYDATASTRUCT lParam  //参数2
        );


        //消息发送API
        [DllImport("User32.dll", EntryPoint = "PostMessage")]
        public static extern int PostMessage(
            IntPtr hWnd,        // 信息发往的窗口的句柄
            int Msg,            // 消息ID
            int wParam,         // 参数1
            int lParam            // 参数2
        );
        
        
        
        //消息发送API
        [DllImport("User32.dll", EntryPoint = "PostMessage")]
        public static extern int PostMessage(
            IntPtr hWnd,        // 信息发往的窗口的句柄
            int Msg,            // 消息ID
            int wParam,         // 参数1
            ref My_lParam lParam //参数2
        );
        
        //异步消息发送API
        [DllImport("User32.dll", EntryPoint = "PostMessage")]
        public static extern int PostMessage(
            IntPtr hWnd,        // 信息发往的窗口的句柄
            int Msg,            // 消息ID
            int wParam,         // 参数1
            ref  COPYDATASTRUCT lParam  // 参数2
        );


    }
}

//Main form, send Message

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Runtime.InteropServices;


namespace TestHwnd
{
    public partial class Main : Form
    {

        public IntPtr hwndTest;
        public int IwndTest;
        public IntPtr hwndfrmTest;
        
        public Main()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            Test test = new Test();
            test.Show(this);
        }


        private void timer1_Tick(object sender, EventArgs e)
        {
            string strTest = "25425";
            Win32API.COPYDATASTRUCT cds;
            cds.dwData = (IntPtr)100;
            cds.lpData = strTest;
            byte[] sarr = System.Text.Encoding.UTF8.GetBytes(strTest);
            int len = sarr.Length;
            cds.cbData = len + 1;
            
            Win32API.My_lParam lp=new Win32API.My_lParam();
            lp.i=3;
            lp.s="test";
            
            if(hwndTest!=(IntPtr)0)
            {
                if (DateTime.Now.Second % 2 == 0)
                {
                    Win32API.SendMessage(hwndTest, 0x60, 1, 3);//传递2个整型参数成功
                }
                if(DateTime.Now.Second % 3 == 0)
                {
                    Win32API.SendMessage(hwndTest, 0x61, 5, ref lp);//传递整型参数和结构类型成功，这个方法加以改变后可以传递对象
                }
                if(DateTime.Now.Second % 5 == 0)
                {
                    Win32API.SendMessage(hwndTest, 0x62, 5, ref cds);//传递整型参数和不定长的字符串成功
                }
                if(DateTime.Now.Second % 7 == 0)
                {
                    Win32API.PostMessage(hwndTest, 0x63, 5, 6);//传递2个整型参数成功
                }
                if(DateTime.Now.Second % 9 == 0)
                {
                    Win32API.PostMessage(hwndTest, 0x64, 3, ref lp);//传递整型参数成功，但是传递参数lp失败，3可以传递成功。
                }
                if(DateTime.Now.Second % 11 == 0)
                {
                    Win32API.PostMessage(hwndTest, 0x65, 3, ref cds);//传递整型参数成功，传递参数cds失败，3可以传递成功。
                }
            }
            
            
        }
    }
}

//Subform
Receives message and parameters

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Runtime.InteropServices;


namespace TestHwnd
{
    public partial class Test : Form
    {
        Main main;
        public Test()
        {
            InitializeComponent();
        }


        private void Test_Load(object sender, EventArgs e)
        {
            main = this.Owner as Main;
            main.hwndTest = this.Handle;
            
        }

        ///重写窗体的消息处理函数DefWndProc，从中加入自己定义消息的检测的处理入口
        protected override void DefWndProc(ref Message m)
        {
            switch (m.Msg)
            {
                    //接收自定义消息MYMESSAGE，并显示其参数
                case 0x60:
                    {
                        label1.Text = DateTime.Now.ToString() + "-" + m.WParam.ToInt32().ToString() + "-" + m.LParam.ToInt32().ToString();
                    }
                    break;
                case 0x61:
                    {
                        Win32API.My_lParam ml = new Win32API.My_lParam();
                        Type t = ml.GetType();
                        ml = (Win32API.My_lParam)m.GetLParam(t);
                        label2.Text = DateTime.Now.ToString() + "-" + m.WParam.ToInt32().ToString() + "-" + ml.i.ToString()+":"+ml.s;
                    }
                    break;
                    case 0x62:
                    {
                        Win32API.COPYDATASTRUCT mystr = new Win32API.COPYDATASTRUCT();
                        Type mytype = mystr.GetType();
                        mystr = (Win32API.COPYDATASTRUCT)m.GetLParam(mytype);
                        string str2 = mystr.lpData;
                        label3.Text = DateTime.Now.ToString() + "-" + m.WParam.ToInt32().ToString() + "-" + str2;
                    }
                    break;
                    case 0x63:
                    {
                        label4.Text = DateTime.Now.ToString() + "-" + m.WParam.ToInt32().ToString() + "-" + m.LParam.ToInt32().ToString();
                    }
                    break;
                    case 0x64:
                    {
                        Win32API.My_lParam ml = new Win32API.My_lParam();
                        Type t = ml.GetType();
                        ml = (Win32API.My_lParam)m.GetLParam(t);
                        label5.Text = DateTime.Now.ToString() + "-" + m.WParam.ToInt32().ToString() + "-" + ml.i.ToString()+":"+ml.s;
                    }
                    break;
                    case 0x65:
                    {
                        Win32API.COPYDATASTRUCT mystr = new Win32API.COPYDATASTRUCT();
                        Type mytype = mystr.GetType();
                        mystr = (Win32API.COPYDATASTRUCT)m.GetLParam(mytype);
                        string str2 = mystr.lpData;
                        label6.Text = DateTime.Now.ToString() + "-" + m.WParam.ToInt32().ToString() + "-" + str2;
                    }
                    break;
                default:
                    base.DefWndProc(ref m);
                    break;
            }
        }

        private void button1_Click(object sender, EventArgs e)
        {
            main.hwndTest = (IntPtr) (0);
            this.Close();
        }

    }

}

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