Question

I am having a problem calling DefWindowsProc on Windows 8 from a C# winform. I have this form that I need it to be dragabble from anywhwere inside the form.

Here is my code.

[DllImport("user32.dll")]
static extern IntPtr DefWindowProc(IntPtr hWnd, uint uMsg, UIntPtr wParam, IntPtr lParam);
[DllImport("user32.dll")]
static extern bool ReleaseCapture(IntPtr hwnd);

const uint WM_SYSCOMMAND = 0x112;
const uint MOUSE_MOVE = 0xF012;

public void Drag()
{
    DefWindowProc(this.Handle, WM_SYSCOMMAND, (UIntPtr)MOUSE_MOVE, IntPtr.Zero);
}


private void OnMainPanelMouseDown(object sender, MouseEventArgs e)
{
    Control ctrl = sender as Control;
    ReleaseCapture(ctrl.Handle);

    this.Drag(); // put the form into drag mode.
}

DefWindowProc always return 0 yet I am unable to drag my window. This call works on XP, Vista and 7 but not on 8. I am guessing it has something to do with the decleration of DefWindowProc that is not working well on Windows 8.

Please note that on Windows 8 I am building my application with the .NET 4.0 framework yet on other platforms I am using the 2.0 version to build the software.

Answer 1

I achieved your functionality with a slightly different approach.

I’m running Windows 8 and Visual Studio 2012.

I tested my solution an all versions of the framework, versions 2, 3, 4, 4.5.

Of course your main form will need to trap the mouse_down() method.

  private void InitializeComponent()
    {
        this.SuspendLayout();
        // 
        // Form1
        // 
        this.AutoScaleDimensions = new System.Drawing.SizeF(8F, 16F);
        this.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font;
        this.ClientSize = new System.Drawing.Size(282, 253);
        this.Name = "Form1";
        this.Text = "Form1";
        this.Load += new System.EventHandler(this.Form1_Load);
        this.MouseDown += new System.Windows.Forms.MouseEventHandler(this.mouse_down);
        this.ResumeLayout(false);

    }

First I defined a class called UnsafeNativeMethods:

[SuppressUnmanagedCodeSecurity]
internal static class UnsafeNativeMethods
{
    public const int WM_NCLBUTTONDOWN = 0xA1;
    public const int HT_CAPTION = 0x2;

    [DllImport("User32.dll", EntryPoint = "SendMessage")]
    public static extern int SendMessage(IntPtr hwnd, uint Msg, int wParam, int lParam);

    [DllImport("User32.dll", EntryPoint = "ReleaseCapture")]
    public static extern bool ReleaseCapture();

}

Inside the form you wish to move with mouse from anywhere:

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
    }
    private void mouse_down(object sender, MouseEventArgs e)
    {
        if (e.Button == MouseButtons.Left)
        {
            UnsafeNativeMethods.ReleaseCapture();
            UnsafeNativeMethods.SendMessage(Handle, UnsafeNativeMethods.WM_NCLBUTTONDOWN, UnsafeNativeMethods.HT_CAPTION, 0);
        }
    }


}