assign string value to a bit variable

 hi,
i have to assign value to bit varibale.
so i declared bool variable . i am assigning string value to a bool variable . so i converted to boolean & assigned.
Its not converting to bool..
How to assign string value to bit variable.?
any help.. i am getting error.
Object reference not set to an instance of an object 
brevised = Convert.ToBoolean( rbtnRdataY.SelectedItem.Value);
 
 
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