Even though you can not return the UDTT from a function, you can return a table variable and receive it in a UDTT as long as the schema match. The following code is tested in SQL Server 2008 R2
— Create the UDTT
CREATE TYPE dbo.MyCustomUDDT AS TABLE
(
FieldOne varchar (512),
FieldTwo varchar(1024)
)
— Declare your variables
DECLARE @uddt MyCustomUDDT;
DECLARE @Modifieduddt MyCustomUDDT;
// Call the function
INSERT INTO @Modifieduddt SELECT * FROM dbo.MyUDF(@uddt);
Function signature
CREATE FUNCTION dbo.MyUDF(@localUDDT MyCustomUDDT)
RETURNS @tableVar TABLE
(
FieldOne varchar (512),
FieldTwo varchar(1024)
)
AS
BEGIN
--Modify your variable here
RETURN
END
Hopefully this will help somebody.