TriStateBound
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Provide three-state (grayable) values in boolean columns.

By default, the grid will show DBNulls as grayed checkboxes, but
once you change them it will toggle between true and false (that's what
a boolean is!). That's the MS Access behavior.

If you want to allow null values in the table, you can handle the 
ValidateEdit event and assign the underlying value directly, allowing
for DBNulls. That's the MS DataGrid behavior.
