PRB: "Can't Assign To Read-Only Property" Error Message

• Microsoft Visual Basic Learning, Professional, and Enterprise Editions for Windows, version 5.0

SYMPTOMS

You receive the following error when you attempt to programmatically set a control property:

  Compile Error:
  Can't assign to read-only property

CAUSE

You are attempting to programmatically change a read-only property of a control at run-time.

RESOLUTION

If you need to change a read-only property at run-time, create another copy of the control and set the read-only property at design-time. Then, display the copy of the control THAT is appropriate to your task.

STATUS

This behavior is by design.

MORE INFORMATION

Controls can have properties that can only be set during design-time. For example, the Style property of a Combo box control can only be changed at design-time. Properties are usually read-only at run-time because the style is inherent to the window created by the control. Control developers call this a "pre-hWnd" property.

The following section demonstrates how you can simulate changing a read- only property of a control at run-time. In this example, a control array of three combo boxes is used. Each member of the combo box array has a different style property. When you click a command button, one member of the combo box array is displayed and the other two members are hidden.

Step-by-Step Example

1. Start a new Standard EXE project in Visual Basic. Form1 is created by default.

2. Add a CommandButton, a label control, and an array of three combo boxes to Form1. Set the following properties for the combo box controls:

      Control           Default Name     Property        Setting
      --------------------------------------------------------------------
      ComboBox          Combo1(0)        Style           0-DropDown Combo
      ComboBox          Combo2(1)        Style           1-Simple Combo
      ComboBox          Combo3(2)        Style           2-Dropdown List
   
   

3. Copy the following code to the Code window of Form1 form:

         Option Explicit
   

         Dim bytCount As Byte
         Dim captions(0 To 2) As String
   

         Private Sub Form_Load()
            Dim i As Integer
            bytCount = 0
            Command1.Caption = "Change Style"
            captions(0) = "Dropdown Combo"
            captions(1) = "Simple Combo"
            captions(2) = "Dropdown List"
            Label1.AutoSize = True
            For i = 0 To 2
               With Combo1(i)
                     .Left = 2400
                     .Top = 360
                     .Visible = False
               End With
            Next i
            Label1.Caption = captions(bytCount)
            Combo1(bytCount).Visible = True
            Combo1(1).Height = 2000
            For i = 1 To 10
               Combo1(0).AddItem i
               Combo1(1).AddItem i
               Combo1(2).AddItem i
            Next i
         End Sub
   
         Private Sub Command1_Click()
            Combo1(bytCount).Visible = False
            bytCount = (bytCount + 1) Mod 3
            Combo1(bytCount).Visible = True
            Label1.Caption = captions(bytCount)
         End Sub
   
   

4. On the Run menu, click Start, or press the F5 key to start the program. Click on a CommandButton to change the style.

REFERENCES

"Can't make an assignment to a read-only property" topic in the Visual Basic Online Reference