>I have a combobox on a form and I want to set the ControlSource property
>when the form is activated. I have set up a PUBLIC variable which I want
>to set the ControlSource to.

First, why use a public variable? Why not use a form property as the control source. If you are using Codemine (which the category suggests you are), then use a form based on frmModalDialogCustom (or frmModalDataChildCustom). Then, do as follows :-

m.oDialog = createobject('frmMyDialog')
m.oDialog.Show()

*// After clicking OK in the dialog, the dialog closes but is not released.
*// Assign the value of the combo's control source property to a variable.
m.uValueIWant = m.oDialog.cboMyCombo.Value

*// Call the dialog's release method after reading the combo's property value.
m.oDialog.Release()

HTH

-=Gary