Mike, 

Thanks for the reply. 

Now that's interesting. Where is the list repopulated from? As it's definately not being populated with the right values.

 I don't mean while it's already open. I'm seeing the wrong values when I open the drop-down. Interestingly enough if I resize the column and then do the drop-down it updates to reflect the new values. Or if I click on a row in the grid first and then on the drop down it also updates. The grid is read-only and it's when the data it's bound to changes that the drop-down is not updated.

 I'll have a look at how we're doing the binding, perhaps we're doing something slightly odd.

What's the data source? Are you seeing the actual rows in the grid update? It sounds to me like your data source is not notifying the grid that an update has taken place. The dropdown gets it's values from the actual data in the column of the grid. This should be the same data that is in the dat source - unless your data source is an IList and doesn't notify the grid of changes. 

 I am seeing the actual rows in the grid update, so I assume that the data source must be notifying the grid of changes. The data source is a System.Windows.Forms.BindingSource, that uses a    System.ComponentModel.BindingList<TItem> as it's DataSource, the BindingList is initialized from a List<TItem>. (Hope that chain made sense!). Then when one of the TItems is updated we replace the instance in the BindingSource with the new version. I didn't originally create this code... Does it look right to you? is there a 'better' way?

Simplified versions of the 2 relevant methods are below.

public void PopulateGrid(List<TItem> itemList)
{
BindingList<TItem> boundList = new BindingList<TItem>(itemList);
gridBindingSource.DataSource = boundList;//This is of type BindingSource
grid.DataSource = gridBindingSource;
} 

public void AddOrUpdateItem(TItem item)
{
int idx;
if ((idx = gridBindingSource.IndexOf(item)) != -1)
{//the item already exists in the list so replace it
gridBindingSource[idx] = item;
}
else
{
//add the new item
idx = gridBindingSource.Add(item);
}
}