Error Handling in T-SQL Scalar Function - sql-server-2005

Question

This question could easily take multiple paths, so I will hit the more specific path first. While working with SQL Server 2005, I'm trying to create a scalar function that acts as a 'TryCast' from varchar to int. Where I encounter a problem is when I add a TRY block in the function;
CREATE FUNCTION u_TryCastInt
(
#Value as VARCHAR(MAX)
)
RETURNS Int
AS
BEGIN
DECLARE #Output AS Int
BEGIN TRY
SET #Output = CONVERT(Int, #Value)
END TRY
BEGIN CATCH
SET #Output = 0
END CATCH
RETURN #Output
END
Turns out theres all sorts of things wrong with this statement including "Invalid use of side-effecting or time-dependent operator in 'BEGIN TRY' within a function" and "Invalid use of side-effecting or time-dependent operator in 'END TRY' within a function". I can't seem to find any examples of using try statements within a scalar function, which got me thinking, is error handling in a function is possible?
The goal here is to make a robust version of the Convert or Cast functions to allow a SELECT statement carry through depsite conversion errors. For example, take the following;
CREATE TABLE tblTest
(
f1 VARCHAR(50)
)
GO
INSERT INTO tblTest(f1) VALUES('1')
INSERT INTO tblTest(f1) VALUES('2')
INSERT INTO tblTest(f1) VALUES('3')
INSERT INTO tblTest(f1) VALUES('f')
INSERT INTO tblTest(f1) VALUES('5')
INSERT INTO tblTest(f1) VALUES('1.1')
SELECT CONVERT(int,f1) AS f1_num FROM tblTest
DROP TABLE tblTest
It never reaches point of dropping the table because the execution gets hung on trying to convert 'f' to an integer. I want to be able to do something like this;
SELECT u_TryCastInt(f1) AS f1_num FROM tblTest
fi_num
__________
1
2
3
0
5
0
Any thoughts on this? Is there anything that exists that handles this? Also, I would like to try and expand the conversation to support SQL Server 2000 since Try blocks are not an option in that scenario.

Answer 1

Check if you can convert to int first, check out the IsInteger function here: IsNumeric, IsInt, IsNumber It will work on 2000 and up

Answer 2

And, to answer in general: No, you can't use try-catch logic in a function. I can sort of see why - or at least it's clearly preferable to avoid it, given the huge performance penalty that would come with it.
However, I think it is odd that one also cannot RAISE an error in a function... that's something built-in functions already do. I suppose one has to get by returning NULL.

Answer 3

The TRY…CATCH construct cannot be used in a user-defined function in SQL 2012!
See this:
http://msdn.microsoft.com/en-us/library/ms175976.aspx
When I try to use this script:
CREATE FUNCTION u_TryCastInt
(
#Value as VARCHAR(MAX)
)
RETURNS Int
AS
BEGIN
DECLARE #Output AS Int
BEGIN TRY
SET #Output = CONVERT(Int, #Value)
END TRY
BEGIN CATCH
SET #Output = 0
END CATCH
RETURN #Output
END
I got error:
Msg 443, Level 16, State 14, Procedure u_TryCastInt, Line 10
Invalid use of a side-effecting operator 'BEGIN TRY' within a function.
Msg 443, Level 16, State 14, Procedure u_TryCastInt, Line 12
Invalid use of a side-effecting operator 'END TRY' within a function.
Msg 443, Level 16, State 14, Procedure u_TryCastInt, Line 13
Invalid use of a side-effecting operator 'BEGIN CATCH' within a function.
Msg 443, Level 16, State 14, Procedure u_TryCastInt, Line 15
Invalid use of a side-effecting operator 'END CATCH' within a function.