Invalid cast exception when reading result from SQLDataReader

My guess is that the value is being returned as a boxed double instead of float. When you unbox the type has to be exactly right. So assuming I'm right and it's not decimal or something like that, you could use:

float monday = (float) (double) reader["Monday"];

and it would work. That's pretty ugly though. If you use SqlDataReader.GetFloat it should get it right if it's genuinely a single-precision value, and it's clearer (IMO) what's going on.

On the other hand, your data could actually be coming back from the database as a double, in which case you should (IMO) use:

float monday = (float) reader.GetDouble(column);

As an aside, are you sure that float is actually the most appropriate type here in the first place? Often decimal is more appropriate...

A sql float is a .NET Double, see on the msdn. Try casting to a double.