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Strange behaviour with ProcessKeyPreview and ProcessCmdKey

Posted on 2006-11-16
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Last Modified: 2011-10-03
Using .NET windows Forms, .NET 1.1 version

I have a custom datagrid, inherited from a windows form datagrid. The grid has rows and columns. I prevent the user from using the arrow keys (up, down, left, right) to go to certain rows or columns. I used the ProcessKeyPreview function of the datagrid to control the arrow keys in another project. In this project, I have more or less the same exact custom datagrid and the ProcessKeyPreview doesn't get fired at all. The strange thing is the ProcessCmdKey is what gets fired. Anyone experienced this? Below is the code. The same exact definition is in ProcessKeyPreview and ProcessCmdKey

**************ProcessKeyPreview
 Protected Overrides Function ProcessKeyPreview(ByRef m As System.Windows.Forms.Message) As Boolean

        If Not _dt Is Nothing Then
            GetKeyStatus()

            If (m.Msg = WM_KEYDOWN Or (m.Msg = WM_SYSKEYDOWN)) Then
                'ignore the key if invalid edit entered

                Select Case m.WParam.ToInt32
                    Case Keys.Up
                        If _invalidUpKey = True Then
                            Return True
                        Else
                            Return MyBase.ProcessKeyPreview(m)
                        End If

                    Case Keys.Down
                        If _invalidDownKey = True Then
                            Return True
                        Else
                            Return MyBase.ProcessKeyPreview(m)
                        End If

                    Case Keys.Left
                        If _invalidLeftKey = True Then
                            Return True
                        Else
                            Return MyBase.ProcessKeyPreview(m)
                        End If

                    Case Keys.Right
                        If _invalidRightKey = True Then
                            Return True
                        Else
                            Return MyBase.ProcessKeyPreview(m)
                        End If
                End Select


'****************ProcessCmdKey
Protected Overrides Function ProcessCmdKey(ByRef msg As System.Windows.Forms.Message, ByVal keyData As System.Windows.Forms.Keys) As Boolean
        If Not _dt Is Nothing Then
            GetKeyStatus()

            If (msg.Msg = WM_KEYDOWN Or (msg.Msg = WM_SYSKEYDOWN)) Then
                'ignore the key if invalid edit entered

                Select Case msg.WParam.ToInt32
                    Case Keys.Up
                        If _invalidUpKey = True Then
                            Return True
                        Else
                            'Return MyBase.ProcessKeyPreview(m)
                            Return MyBase.ProcessCmdKey(msg, keyData)
                        End If

                    Case Keys.Down
                        If _invalidDownKey = True Then
                            Return True
                        Else
                            'Return MyBase.ProcessKeyPreview(m)
                            Return MyBase.ProcessCmdKey(msg, keyData)
                        End If

                    Case Keys.Left
                        If _invalidLeftKey = True Then
                            Return True
                        Else
                            'Return MyBase.ProcessKeyPreview(m)
                            Return MyBase.ProcessCmdKey(msg, keyData)
                        End If

                    Case Keys.Right
                        If _invalidRightKey = True Then
                            Return True
                        Else
                            'Return MyBase.ProcessKeyPreview(m)
                            Return MyBase.ProcessCmdKey(msg, keyData)
                        End If
                End Select

            End If
        End If
    End Function

0
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Question by:Sanmarie
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2 Comments
 
LVL 14

Accepted Solution

by:
jake072 earned 500 total points
ID: 17965580
If you read the following articles you will note that ProcessCmdKey PREPROCESSES messages i.e., it processes the messages before they are translated to the form, whereas ProcessKeyPreview handles messages received from the window.  ProcessCmdKey will always hook the event first, and if you handle the event in that method, then ProcessKeyPreview will never fire because the event was consumed in ProcessCmdKey.

Reference:

http://msdn2.microsoft.com/en-us/library/system.windows.forms.control.processkeypreview.aspx
http://msdn2.microsoft.com/en-us/library/system.windows.forms.control.processcmdkey.aspx

Jake
0
 

Author Comment

by:Sanmarie
ID: 17967018

Thanks for your response.

I was not handling the ProcessCmdKey method at all. I didn't even have a definition for it. All I had was the ProcessKeyPreview and it was not being fired. That's when I decided to try adding the ProcessCmdKey to see if that would work and it did.

The strange thing is that I used the ProcessKeyPreview to handle the arrow keys in a previous project and it worked fine. Whereas now, in this project it isn't being fired at all. ProcessCmdKey was not used in either project. So, it couldn't be the case that it was preventing the ProcessKeyPreview from being fired.

Thanks for your input though. I will read up more on both

San



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