Microsoft KB Archive/178490

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

Article ID: 178490

Article Last Modified on 5/13/2003

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APPLIES TO


 * Microsoft Visual Basic 5.0 Learning Edition
 * Microsoft Visual Basic 6.0 Learning Edition
 * Microsoft Visual Basic 5.0 Professional Edition
 * Microsoft Visual Basic 6.0 Professional Edition
 * Microsoft Visual Basic 5.0 Enterprise Edition
 * Microsoft Visual Basic 6.0 Enterprise Edition

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This article was previously published under Q178490



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
 Start a new Standard EXE project in Visual Basic. Form1 is created by default.  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   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  On the Run menu, click Start, or press the F5 key to start the program. Click on a CommandButton to change the style.</ol>

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