How to get Guid from SqlDataReader uniqueidentifier

There is something wrong either with your data or your SQL. The first and third approaches should work - personally I'd use the first form, as it's the clearest in my view.

But look at the stack trace:

... at System.Data.SqlClient.SqlDataReader.ReadInternal(Boolean setTimeout) at System.Data.SqlClient.SqlDataReader.Read() at Simego.DataSync.DynamicColumns.DataSourceRowOverride.get_EpochSchemeHistoryID()

Note that it's the Read() call which is failing, not GetGuid or the indexer.

My guess is that your property is being fetched several times, and sometimes it works - which is why you were getting a cast exception in your second approach - but for some rows, it fails due to some problem with the data or the SQL. As we've no idea where your data is coming from, we can't help you beyond that diagnosis, but that's where you should look next.

That error is being generated from SQL. In other words there something wrong with you SQL command text. Not your c# code.