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Textbox Line Character Limit

, 31 Oct 2005
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Limit the number of characters in a multi line textbox.


Recently, while I was working on a project, there was a need to limit the amount of text in each line of a multi line textbox to 72 characters. The challenge was to limit the text while the user is typing and to maintain the required formatting. To get this to work, I had to create a class that inherited from TextBox. In the constructor, I placed a global keyboard trap on the TextBox. When the text reaches the designated length the program will stop all keyboard characters from displaying on the screen forcing the user to hit the enter key and start a new line.

The code

In the constructor, the program calls the function HookKeyboard. In this function, the program will trap all keyboard events for the TextBox:

Public Sub New()
    'This call is required by the Windows Form Designer.
    'Hook all Keyboard Events
End Sub

The function isHooked in the Keyboard module, where the actual fun begins:

'This function is where you capture the key pressed. 
Public Function IsHooked(ByRef Hookstruct As KBDLLHOOKSTRUCT) As Boolean
    Dim zKey As Keys
    Dim control As Control
    Dim hInstance As Integer
        Dim ActiveWin As Integer
        'GetActiveWindow determines if the application has focus
        ActiveWin = GetActiveWindow
        If ActiveWin <> 0 Then
            If UBound(frmM.Lines) >= 0 Then
                If frmM.Lines(GetTextBoxCurrentLine(frmM) - 1).Length > _
                                                          MaxLengthLine Then
                'Accept Enter Button
                    If (Hookstruct.vkCode = zKey.Enter) Then
                        Return True
                    End If
                    'Accept BackSpace
                    If (Hookstruct.vkCode = zKey.Back) Then
                        Return True
                    End If
                    'Accept Up Arrow
                    If (Hookstruct.vkCode = zKey.Up) Then
                        Return True
                    End If
                    'Accept Down Arrow
                    If (Hookstruct.vkCode = zKey.Down) Then
                        Return True
                    End If
                    'Accept Left Arrow
                    If (Hookstruct.vkCode = zKey.Left) Then
                        Return True
                    End If
                    'Accept Right Arrow
                    If (Hookstruct.vkCode = zKey.Right) Then
                        Return True
                    End If
                    Return True
                End If
            End If
        End If
        Return False
    Catch ex As Exception
        MsgBox(ex.Message + vbCrLf + ex.StackTrace)
        Return False
    End Try
End Function


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About the Author

Ryan Rader
Web Developer
United States United States
Specialize in interfacing SAP and .net applications.

Comments and Discussions

QuestionInsert in data gride Pinmemberjayesht31-Mar-07 16:24 
Questioncontrol doesnt work when using two on the same form Pinmemberzastremi29-Aug-06 11:54 
AnswerRe: control doesnt work when using two on the same form Pinmemberrader1919-Sep-06 14:16 
GeneralRe: control doesnt work when using two on the same form [modified] Pinmembermanfred5120-Nov-06 0:06 
GeneralTake care of with the width of the textbox PinmemberJorgeRuiz11-Aug-06 12:02 
QuestionCan I use this with my browser Pinmembernachtegaal999925-Apr-06 10:37 
GeneralPainting a Warning Line PinmemberIan MacLean6-Dec-05 15:35 
GeneralRe: Painting a Warning Line Pinmemberrader197-Dec-05 3:06 

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