ADO.NET 101

I'll file this under the "stupid mistakes" category, but I'll admit to it here just in case someone else didn't realize this until it was too late like I did.

When you have a SqlDataReader open, you can't reuse the underlying SqlConnection. In my case, I just changed to using a DataSet instead of a reader (I don't remember why I used a reader in the first place... the query always returned 1 row). Another solution would be to open a second connection.

This support document puts it quite succinctly:

While the SqlDataReader object is in use, the associated SqlConnection object serves the SqlDataReader, and you cannot perform any other operations on the SqlConnection object other than to close it. This is true until you call the Close method of the SqlDataReader object.