How do I use Rundll32 to swapmousebutton?

Thanks so much for the C# solution. It worked like a charm.

I made a slight alteration so that I could simply click on a desktop short-cut to toggle the primary mouse button, without passing in arguments. In case my approach helps someone else, here is that version:

using System.Runtime.InteropServices;
using System;

class SwapMouse
{
    [DllImport("user32.dll")]
    public static extern Int32 SwapMouseButton(Int32 bSwap);

    static void Main(string[] args)
    {
        int rightButtonIsAlreadyPrimary = SwapMouseButton(1);
        if (rightButtonIsAlreadyPrimary != 0)
        {
            SwapMouseButton(0);  // Make the left mousebutton primary
        }
    }
}

You do not use rundll32 for that.

Q164787: INFO: Windows Rundll and Rundll32 Interface

[...] Rundll and Rundll32 programs do not allow you to call any exported function from any DLL. For example, you can not use these utility programs to call the Win32 API (Application Programming Interface) calls exported from the system DLLs. The programs only allow you to call functions from a DLL that are explicitly written to be called by them.

---

If you have the .NET Framework Runtime installed, it comes with compilers for several languages (for example, %SystemRoot%\Microsoft.NET\Framework64\v3.5\csc.exe for the v3.5 C# compiler on 64-bit systems). You can write a program in C#:

using System.Runtime.InteropServices;
using System;

class SwapMouse {
    [DllImport("user32.dll")]
    public static extern Int32 SwapMouseButton(Int32 bSwap);
    static void Main(string[] args) {
        if (args.Length > 0 && String.Compare(args[0], "/u", true) == 0)
            SwapMouseButton(0);
        else
            SwapMouseButton(1);
    }
}

Compile with:

"%SystemRoot%\Microsoft.NET\Framework64\v3.5\csc" swap.cs

Swap/unswap buttons:

swap
swap /u

There is hack, which can be used to configure the layout of the mouse for a left-handed user. Just run the following command:

rundll32.exe user32.dll,SwapMouseButton

Run this command to reconfigure the layout of the mouse for a left-handed user.

I am going to give you an explanation for this kind of behaviour:

Functions, which are called by rundll32.exe, have the following function prototype:

void CALLBACK  EntryPoint(HWND hwnd, HINSTANCE hinst, LPSTR lpszCmdLine, int nCmdShow);

SwapMouseButton has the following function prototype:

BOOL WINAPI SwapMouseButton(
  _In_ BOOL fSwap
)

SwapMouseButton uses the same calling convention (__stdcall) as every function called by rundll32.exe does.

If you call SwapMouseButton using rundll32.exe with an additional command-line, this command-line will be passed to this function as lpszCmdLine and will be ignored.

If you call that functions using rundll32, rundll32 automatically passes a valid window handle (HWND) as the first argument of the called function.

hwnd - window handle that should be used as the owner window for any windows your DLL creates

The function SwapMouseButton function called by rundll32 requires TRUE as the first argument in order to configure the layout of the mouse for a left-handed user. The valid window handle passed by rundll32.exe to SwapMouseButton within user32.dll is unequal to 0 and is defined as TRUE, when using a BOOL value.

You can find details on rundll32.exe and the prototype used by functions called by this executable here: INFO: Windows Rundll and Rundll32 Interface

You can find details about the function SwapMouseButton here: SwapMouseButton function (Windows)