CommonDialog.HookProc(IntPtr, Int32, IntPtr, IntPtr) Method

Reference

Definition

Namespace:: System.Windows.Forms

Assembly:: System.Windows.Forms.dll

Important

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Defines the common dialog box hook procedure that is overridden to add specific functionality to a common dialog box.

protected:
 virtual IntPtr HookProc(IntPtr hWnd, int msg, IntPtr wparam, IntPtr lparam);

protected virtual IntPtr HookProc (IntPtr hWnd, int msg, IntPtr wparam, IntPtr lparam);

abstract member HookProc : nativeint * int * nativeint * nativeint -> nativeint
override this.HookProc : nativeint * int * nativeint * nativeint -> nativeint

Protected Overridable Function HookProc (hWnd As IntPtr, msg As Integer, wparam As IntPtr, lparam As IntPtr) As IntPtr

Parameters

hWnd: IntPtr
nativeint

The handle to the dialog box window.

msg: Int32

The message being received.

wparam: IntPtr
nativeint

Additional information about the message.

lparam: IntPtr
nativeint

Additional information about the message.

Returns

IntPtr

nativeint

A zero value if the default dialog box procedure processes the message; a nonzero value if the default dialog box procedure ignores the message.

Examples

The following code example demonstrates how to override the HookProc method. The example consists of a class that inherits the CommonDialog class. In the class's HookProc override, the example evaluates the method's msg parameter against constant values for particular Windows messages. If the msg parameter equals the specified constant, the example writes trace output identifying the Windows message that was passed to the HookProc method. This example assumes that the class in which the HookProc method is declared inherits the CommonDialog class.

private:
   // Defines the constants for Windows messages.
   literal int WM_SETFOCUS = 0x0007;
   literal int WM_INITDIALOG = 0x0110;
   literal int WM_LBUTTONDOWN = 0x0201;
   literal int WM_RBUTTONDOWN = 0x0204;
   literal int WM_MOVE = 0x0003;

protected:
   // Overrides the base class hook procedure...
   [SecurityPermission(SecurityAction::Demand, Flags=SecurityPermissionFlag::UnmanagedCode)] 
   virtual IntPtr HookProc( IntPtr hWnd, int msg, IntPtr wParam, IntPtr lParam ) override
   {
      // Evaluates the message parameter to determine the user action.
      #if defined(TRACE)
      switch ( msg )
      {
         case WM_INITDIALOG:
            System::Diagnostics::Trace::Write( "The WM_INITDIALOG message was received." );
            break;
         case WM_SETFOCUS:
            System::Diagnostics::Trace::Write( "The WM_SETFOCUS message was received." );
            break;
         case WM_LBUTTONDOWN:
            System::Diagnostics::Trace::Write( "The WM_LBUTTONDOWN message was received." );
            break;
         case WM_RBUTTONDOWN:
            System::Diagnostics::Trace::Write( "The WM_RBUTTONDOWN message was received." );
            break;
         case WM_MOVE:
            System::Diagnostics::Trace::Write( "The WM_MOVE message was received." );
            break;
      }
      #endif
     
      // Always call the base class hook procedure.
      return FontDialog::HookProc( hWnd, msg, wParam, lParam );
   }


// Defines the constants for Windows messages.

const int WM_SETFOCUS = 0x0007;
const int WM_INITDIALOG = 0x0110;
const int WM_LBUTTONDOWN = 0x0201;
const int WM_RBUTTONDOWN = 0x0204;
const int WM_MOVE = 0x0003;

// Overrides the base class hook procedure...
protected override IntPtr HookProc(IntPtr hWnd, int msg, IntPtr wParam, IntPtr lParam)
{
    // Evaluates the message parameter to determine the user action.

    switch(msg)
    {

        case WM_INITDIALOG:
            System.Diagnostics.Trace.Write("The WM_INITDIALOG message was received.");
            break;
        case WM_SETFOCUS:
            System.Diagnostics.Trace.Write("The WM_SETFOCUS message was received.");
            break;
        case WM_LBUTTONDOWN:
            System.Diagnostics.Trace.Write("The WM_LBUTTONDOWN message was received.");
            break;
        case WM_RBUTTONDOWN:
            System.Diagnostics.Trace.Write("The WM_RBUTTONDOWN message was received.");
            break;
        case WM_MOVE:
            System.Diagnostics.Trace.Write("The WM_MOVE message was received.");
            break;
    }

    // Always call the base class hook procedure.

    return base.HookProc(hWnd, msg, wParam, lParam);
}

    ' Defines the constants for Windows messages.

    Const WM_SETFOCUS = &H7
    Const WM_INITDIALOG = &H110
    Const WM_LBUTTONDOWN = &H201
    Const WM_RBUTTONDOWN = &H204
    Const WM_MOVE = &H3
<System.Security.Permissions.PermissionSetAttribute(System.Security.Permissions.SecurityAction.Demand, Name:="FullTrust")> _
    Protected Overrides Function HookProc(ByVal hWnd As IntPtr, ByVal msg As Integer, ByVal wParam As IntPtr, ByVal lParam As IntPtr) As IntPtr


        ' Evaluates the message parameter to determine the user action.

        Select Case msg

            Case WM_INITDIALOG
                System.Diagnostics.Trace.Write("The WM_INITDIALOG message was received.")
            Case WM_SETFOCUS
                System.Diagnostics.Trace.Write("The WM_SETFOCUS message was received.")
            Case WM_LBUTTONDOWN
                System.Diagnostics.Trace.Write("The WM_LBUTTONDOWN message was received.")
            Case WM_RBUTTONDOWN
                System.Diagnostics.Trace.Write("The WM_RBUTTONDOWN message was received.")
            Case WM_MOVE
                System.Diagnostics.Trace.Write("The WM_MOVE message was received.")

        End Select

        ' Always call the base class hook procedure.

        Return MyBase.HookProc(hWnd, msg, wParam, lParam)

    End Function

Remarks

A hook procedure is a mechanism by which a function can intercept events before they reach an application. When you override the HookProc method for a CommonDialog class, the operating system invokes your override of the function to post operating system messages to the window.

By default, the hook procedure centers the dialog box on the screen in response to a WM_INITDIALOG message.

Notes to Inheritors

Inheriting classes can override this method to add specific functionality to a common dialog box. When overriding HookProc(IntPtr, Int32, IntPtr, IntPtr) in a derived class, be sure to call the base class's HookProc(IntPtr, Int32, IntPtr, IntPtr) method.

Applies to

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