Delphi - ReadOnly Components

In some applications, I use ReadOnly components to display data that the user is not allowed to modify. In other applications, the state of the component depends on various factors.

In general, many components provide a ReadOnly property ... however, in other cases there is no obvious solution.

Components with a "ReadOnly" Property

Most components based on TCustomEdit will have a ReadOnly property ... including

TEdit  TMemo  TRichEdit

This is sample code I use to change the ReadOnly status of these components

procedure TForm_PortConfiguration.Protect_Fields(ReadOnly: boolean);
var
  Color : TColor;
begin
  if ReadOnly then Color := clScrollBar  // grey  indicates readonly
              else Color := clWindow;    // white indicates that the value can be edited


  Edit1.ReadOnly := ReadOnly;
  Memo1.ReadOnly := ReadOnly;

  Edit1.Color := Color;
  Memo1.Color := Color;

end;

TComboBox

TComboBox does NOT have a ReadOnly property ... therefore, you will have to fake it.

These commands call the Windows api directly. They will keep a user from typing a new value, but not from selecting a new value from the list.

  // Set ReadOnly
SendMessage(GetWindow(ComboBox1.Handle,GW_CHILD), EM_SETREADONLY, 1, 0);

  // Remove ReadOnly
SendMessage(GetWindow(ComboBox1.Handle,GW_CHILD), EM_SETREADONLY, 0, 0);

You could set Enabled false, but then

The text color is wrong
You can not highlight the text and copy it to the clipboard (which is allowed in other ReadOnly controls)

This kludge works

procedure TForm_abc.Protect_Fields(ReadOnly: boolean);
begin

  if ReadOnly then begin
    ComboBox1.Tag := Port_Barcode_UIComboBox.Items.IndexOf(ComboBox1.Text);
    SendMessage(GetWindow(ComboBox1.handle, GW_Child), EM_SetReadOnly, 1, 0);
  end else begin
    ComboBox1.Tag := -1;
    SendMessage(GetWindow(ComboBox1.handle, GW_Child), EM_SetReadOnly, 0, 0);
  end;
end;


procedure TForm_abc.ReadOnly_UIComboBoxChange(Sender: TObject);
var
  xx : TComboBox;
begin
  // This helps to make ComboBoxes read only when tag<>-1
  xx := Sender as TComboBox;
  if xx.Tag <> -1 then begin
    xx.ItemIndex := xx.Tag;
  end;
end;

The Tag property is used to indicate which item in the list is selected, as a result, this technique does not work if the text is not located in the list
OnChange keeps the user from selecting another option from the list ... and fixes MOST typing
If SendMessage is left out, there are instances where single characters can be typed
An alternative would be to store the current text in Hint and set ShowHint and ParentShowHint to false
HelpKeyword could also be used (not available in Delphi 5)

Experiments using - Style := csDropDownList - did not work - it hid the list and stopped the user from typing ... but the other list options could still be selected using the arrow keys.

Author: Robert Clemenzi - clemenzi@cpcug.org