Here's a fairly nasty error message with a (sometimes) pretty subtle solution. Let's say you want to insert a null value into a nullable database column with ADO.NET using parameterized SQL. The insert statement could look something like this:
INSERT INTO TableName (ColumnName) VALUES (@NullableValue)
Let's say I setup my SqlParameter array as follows:
SqlParameter[] sqlParameterArray = new SqlParameter[1];
SqlParameter oneSqlParameter = new SqlParameter("@NullableValue", SqlDbType.Int);
oneSqlParameter.Value = null; // I want to insert the value null
If I ran the insert using the SqlCommand.ExecuteNonQuery method, I would get this exception:
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System.Data.SqlClient.SqlException
Prepared statement '(@NullableValue int)INSERT INTO TableName (ColumnName) VALUES (@' expects parameter @NullableValue, which was not supplied.
--
What's wrong? Apparently, "null" is not a valid value for the Value of a SqlParameter. You need to do this instead:
oneSqlParameter.Value = DBNull.Value;
Is that error message helpful? I don't think so.
There are plenty of other reasons why that exception could be thrown, but I've been using ADO.NET for a while and I still stumbled upon this particular under documented problem recently. Very, very subtle. Crystal clear error messages from software are so important, yet so uncommon in the real world.