What steps will reproduce the problem?

Assuming "num_col" is an appropriate numeric column in the database, the following throws a FormatException:

Convert.ToInt32(datareader["num_col"])

What is the expected output? What do you see instead?

With the above example, the expected behavior is for Convert.ToInt32 to return an integer. Instead it throws a FormatException. Generally speaking, the indexer property should behave the same as GetValue(int), which converts boxed java types to CLR types.