PRB: Invalid Property Value When Binding Masked Edit Control

- Professional Edition of Microsoft Visual Basic for Windows, version 3.0

SYMPTOMS

Binding a masked edit control (MSMASKED.VBX) to a data control can result in the error "Invalid property value" when the form is loaded. Also, the error "Type mismatch" can occur when you attempt to update a record. For example, this problem can occur when a masked edit control is bound to a date field and has the mask ##/##/##.

CAUSE

Visual Basic uses the Text property of a bound masked edit control to transfer data to and from the data control. This requires strict compatibility, character for character, between the Mask property and the format of the data in the database. For example, dates are stored in a database as a numeric value, not as a string of the format dd/mm/yy.

WORKAROUND

Bind an invisible text box to the data control instead of binding the masked edit control. Then transfer data between the text box and the masked edit control. This allows you precise control over the format of the data for the masked edit control making it appear as if the masked edit control is bound.

This alternative approach involves more lines of code and complexity, but it should prove much more flexible and forgiving. Below is an example showing how to use this technique with a date field.

STATUS

This behavior is by design.

MORE INFORMATION

When a bound masked edit control tries to pull data in from a data control, it behaves differently depending upon the PromptInclude property of the masked edit control.

• If PromptInclude is True (the default), the data coming from the data control must match the mask property exactly or an "Invalid property value" error occurs. If a user tries to change a valid value in a bound masked edit control and doesn't entirely fill up the masked edit control, the error "Type mismatch" occurs upon writing values into a numeric data type field of a database.
• If PromptInclude is False, the masked edit control does not require or provide literal characters from the Mask property. In other words, the Text property operates like ClipText. For example, a Mask of ##/##/## receives the date 1/2/93 into the Text property as 12/93/__.

Step-by-Step Example for the Workaround

This example shows how to use the Masked edit control with an Access database. This particular example demonstrates using the masked edit control on a field of data type Date/Time.

Before testing the example, either load DATAMGR.EXE (located in the \VB directory) or run VISDATA.MAK (located in the \VB\SAMPLES\VISDATA directory.) Open the BIBLIO.MDB sample database by selecting it after choosing Open Database from the File menu and selecting the Access database option.

Next, select the Authors table, and click the Design button. Select the Add button, enter 'Dates' for the field name, and select the Date/Time for the field type. Then choose OK to close the Add Field window, and then click the Open button. Add several dates to the 'Dates' field in an 'mm/dd/yy' format. You may leave some of the 'Dates' fields blank, but you should enter at least five different dates of five different records in the Authors table to test the example.

Now, you are ready to complete the example:

1. Start Visual Basic for Windows, or from the File menu, choose New Project (ALT, F, N) if Visual Basic for Windows is already running. Form1 is created by default.

2. Add the following controls with the associated properties to Form1:

      Control       Name           Property Settings
      -------------------------------------------------------------
      Data          Data1          DatabaseName = "BIBLIO.MDB"
                                   RecordSource = "Authors"
   
      MaskedEdit    MaskedEdit1    Mask = "##/##/##"
                                   PromptInclude = False
   
      TextBox       Text1          Visible = False
                                   DataSource = Data1
                                   DataField = Dates   '** this field was
                                      added to BIBLIO.MDB previously
   
   

3. Add the following lines of code to the (general)(declarations) section of Form1:

      Dim UpdFlag As Integer      'Flag to indicate updating the text box from
                                  ' the data control or from the masked edit
                                  ' control.
   
      Const MAXMASKLEN = 6        ' This constant is the maximum number of
                                  ' characters the user can enter in this
                                  ' particular MaskedEdit.
   
      Function IsValidDate% (MyMask As MaskEdBox, MaskFullLen As Integer)
      ' This function checks the validity of a date in a Masked edit control.
      ' It returns a zero if the FormattedText is not a valid date, a one
      ' if the field is empty, and a two if the FormattedText is valid.
      '
      ' Parameters:
      '    MyMask       - the Masked edit control being checked
      '    MaskFullLen  - max. number of chars the Masked edit control can
   hold
   
         If MyMask.Text = "" Then
            IsValidDate% = 1
         ElseIf Len(MyMask.Text) = MaskFullLen Then
            If IsDate(MyMask.FormattedText) Then
               IsValidDate% = 2
            End If
         Else
           IsValidDate% = 0
         End If
      End Function
   
   

4. Add the following code to the Load event of Form1:

      Sub Form_Load ()
   
         UpdFlag = False
   
      End Sub
   
   

5. Add the following code to the Validate Event of the Data1 Control:

      Sub Data1_Validate (Action As Integer, Save As Integer)
   
         Const  DATA_ACTIONCANCEL   = 0
   
         If IsValidDate%(MaskedEdit1, MAXMASKLEN) = False Then
            MsgBox "Not a valid date!"
            Action = DATA_ACTIONCANCEL         'don't allow changes
            MaskedEdit1.SetFocus
            Exit Sub
         End If
   
      End Sub
   
   

6. Add the following code to the KeyPress event of MaskedEdit1:

      Sub MaskedEdit1_KeyPress (KeyAscii As Integer)
   
         UpdFlag = True
   
      End Sub
   
   

7. Add the following code to the Change event to MaskedEdit1:

      Sub MaskedEdit1_Change ()
   
         If UpdFlag = True Then
            Select Case IsValidDate%(MaskedEdit1, MAXMASKLEN)
               Case 1
                  Text1.Text = ""
               Case 2
                  Text1.Text = CVDate(MaskedEdit1.FormattedText)
            End Select
         End If
   
      End Sub
   
   

8. Add the following code to the Change event of Text1:

      Sub Text1_Change ()
         Const DATEFMT = "mmddyy"
   
         ' The invisible text box can get changed two ways: from the
         ' database because it is bound or from the MaskedEdit when pushing
         ' values back into the data control. This condition handles the
         ' situation when the data is coming from the database and the
         ' MaskedEdit needs to be updated.
   
         If Not UpdFlag Then
            If Text1.Text = "" Then            ' If NULL condition  then
               MaskedEdit1.Text = ""           ' Set the MaskedEdit to ""
            Else
               MaskedEdit1.Text = Format$(Text1.Text, DATEFMT) 'Format output.
            End If
         End If
         UpdFlag = False
   
      End Sub
   
   

9. Press the F5 key to run the program. The masked edit control should behave as if it was bound to the data control.

   NOTE: This example verifies that the dates are valid in the Validate event before actually placing the dates in the database.