Question

I have a C# application, using ADO.Net to connect to MSSQL

I need to create the table (with a dynamic number of columns), then insert many records, then do a select back out of the table.

Each step must be a separate C# call, although I can keep a connection/transaction open for the duration.

Answer 1

There are two types of temp tables in SQL Server, local temp tables and global temp tables. From the BOL:

Prefix local temporary table names with single number sign (#tablename), and prefix global temporary table names with a double number sign (##tablename).

Local temp tables will live for just your current connection. Globals will be available for all connections. Thus, if you re-use (and you did say you could) the same connection across your related calls, you can just use a local temp table without worries of simultaneous processes interfering with each others’ temp tables.

You can get more info on this from the BOL article, specifically under the "Temporary Tables" section about halfway down.