You could use the inheritance tree to shorten the code:
private string GetControlValue(string controlId)
{
var control = FindControl(controlId);
if(control is ITextControl)
{
return ((ITextControl) control).Text; // works also for the RadComboBox since it returns the currently selected item's text
}
else if(control is ICheckBoxControl)
{
return ((ICheckBoxControl)control).Checked.ToString();
}
else
{
return null;
}
}