how to use SendInput in .net?

Hi @mc , Welcome to Microsoft Q&A,

You need to set the appropriate dwFlags in the MOUSEINPUT structure for mouse down and mouse up events. Additionally, ensure the dx and dy fields are set to 0 if you don't want to move the mouse cursor.

using System;
using System.Runtime.InteropServices;
using System.Threading.Tasks;

public class Program
{
    [DllImport("user32.dll")]
    public extern static uint SendInput(uint nInputs, INPUT[] pInputs, int cbSize);

    [StructLayout(LayoutKind.Sequential)]
    public struct INPUT
    {
        public int type;
        public InputUnion U;
    }

    [StructLayout(LayoutKind.Explicit)]
    public struct InputUnion
    {
        [FieldOffset(0)]
        public MOUSEINPUT mi;
        [FieldOffset(0)]
        public KEYBDINPUT ki;
        [FieldOffset(0)]
        public HARDWAREINPUT hi;
    }

    [StructLayout(LayoutKind.Sequential)]
    public struct MOUSEINPUT
    {
        public int dx;
        public int dy;
        public int mouseData;
        public uint dwFlags;
        public uint time;
        public IntPtr dwExtraInfo;
    }

    [StructLayout(LayoutKind.Sequential)]
    public struct KEYBDINPUT
    {
        public ushort wVk;
        public ushort wScan;
        public uint dwFlags;
        public uint time;
        public IntPtr dwExtraInfo;
    }

    [StructLayout(LayoutKind.Sequential)]
    public struct HARDWAREINPUT
    {
        public uint uMsg;
        public ushort wParamL;
        public ushort wParamH;
    }

    private const int INPUT_MOUSE = 0;
    private const uint MOUSEEVENTF_LEFTDOWN = 0x0002;
    private const uint MOUSEEVENTF_LEFTUP = 0x0004;

    public static async Task Main(string[] args)
    {
        // Simulate a left mouse button click
        var input = new INPUT[2];

        // Mouse down
        input[0] = new INPUT
        {
            type = INPUT_MOUSE,
            U = new InputUnion
            {
                mi = new MOUSEINPUT
                {
                    dwFlags = MOUSEEVENTF_LEFTDOWN,
                }
            }
        };

        // Mouse up
        input[1] = new INPUT
        {
            type = INPUT_MOUSE,
            U = new InputUnion
            {
                mi = new MOUSEINPUT
                {
                    dwFlags = MOUSEEVENTF_LEFTUP,
                }
            }
        };

        // Send the input
        SendInput((uint)input.Length, input, Marshal.SizeOf(typeof(INPUT)));

        // Optional delay
        await Task.Delay(300);
    }
}

Best Regards,

Jiale

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