I have three forms: A, B and C

There is a button on A to load B, and a button on B to load C, which has a datagrid with checkbox column.

I use showDialog() to load the forms.

When I click the checkbox on C's datagrid, i get error on A's button click event "An unhandled exception of type 'System.InvalidCastException' occurred in system.windows.forms.dll

Additional information: Specified cast is not valid."

Any idea? Thanks.