Can you explain in more details your scenario and what you are trying to achieve?
As far as we can see you are trying to cast .Value
and you are getting the following error: "Unable to cast object of type..."
Since you have implemented IConvertible, is it true that you have an implementation for FilterElement to String conversion? If you have a custom class, create a constructor which accepts a string and use the code as follows:
FilterElement condition = new FilterElement (Value);
As to your second question about the string: when the filterExpression is build, it is a string in order to be sent to the SQL server as a string.
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