Unknown Return Type

Unknown Return Type: The return types for the following stored procedures could not be detected.  Set the return type for each stored procedure in the properties window.

The error was displayed when trying to drag a stored procedure to the LINQ designer file. (DBML).

The cause of the error was the use of a temporary table inside the stored procedure (#tmpTable)

Changing the temporary table to table variable solved the problem

A more detailed explanation can be found here
Ritesh Kesharwani: Error: Unknown Return Type, The return types for the following stored procedures could not be detected….(LINQ).