Try-Catch in User Defined Function? - tsql

Question

I'm trying to write a UDF to translate a string that is either a guid or a project code associated with that guid into the guid:
CREATE FUNCTION fn_user_GetProjectID
(
#Project nvarchar(50)
)
RETURNS uniqueidentifier
AS
BEGIN
declare #ProjectID uniqueidentifier
BEGIN TRY
set #ProjectID = cast(#Project as uniqueidentifier)
END TRY
BEGIN CATCH
set #ProjectID = null
END CATCH
if(#ProjectID is null)
BEGIN
select #ProjectID = ProjectID from Project where projectcode = #Project
END
return #ProjectID
END
This works fine if the above code is embedded in my Stored Procedures, but I'd like to make a function out of it so that I follow DRY.
When I try to create the Function, I get errors like this:
Msg 443, Level 16, State 14, Procedure fn_user_GetProjectID, Line 16
Invalid use of side-effecting or time-dependent operator in 'BEGIN TRY' within a function.
Does anyone have an idea how I can get around this error?
Edit: I know I can't use Try-Catch in a Function, I guess a simplified questions would be, is there a way to do a cast that will just return NULL if the cast fails, instead of an error?

Answer 1

Apparently you can't use TRY-CATCH in a UDF.
According to this bug-reporting page for SQL Server:
Books Online documents this behaviour,
in topic "CREATE FUNCTION
(Transact-SQL)": "The following
statements are valid in a function:
[...] Control-of-Flow statements
except TRY...CATCH statements. [...]"
But they were giving hope for the future back in 2006:
However, this is a severe limitation
that should be removed in a future
release. You should post a suggestion
in this regard and I will
wholeheartedly vote for it.

Answer 2

From MSDN:
A column or local variable of
uniqueidentifier data type can be
initialized to a value in the
following ways:
By using the NEWID function.
By converting from a string constant
in the form
xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx,
in which each x is a hexadecimal digit
in the range 0-9 or a-f.
For example,
6F9619FF-8B86-D011-B42D-00C04FC964FF
is a valid uniqueidentifier value.
You can use pattern matching to verify the string. Note that this won't work for specific encoding that reduces the size of the GUID:
declare #Project nvarchar(50)
declare #ProjectID uniqueidentifier
declare #HexPattern nvarchar(268)
set #HexPattern =
'[A-F0-9][A-F0-9][A-F0-9][A-F0-9][A-F0-9][A-F0-9][A-F0-9][A-F0-9]' +
'[A-F0-9][A-F0-9][A-F0-9][A-F0-9][A-F0-9][A-F0-9][A-F0-9][A-F0-9]' +
'[A-F0-9][A-F0-9][A-F0-9][A-F0-9][A-F0-9][A-F0-9][A-F0-9][A-F0-9]' +
'[A-F0-9][A-F0-9][A-F0-9][A-F0-9][A-F0-9][A-F0-9][A-F0-9][A-F0-9]'
/* Take into account GUID can have curly-brackets or be missing dashes */
/* Note: this will not work for GUIDs that have been specially encoded */
set #Project = '{' + CAST(NEWID() AS VARCHAR(36)) + '}'
select #Project
set #Project = REPLACE(REPLACE(REPLACE(#Project,'{',''),'}',''),'-','')
/* Cast as uniqueid if pattern matches, otherwise return null */
if #Project LIKE #HexPattern
select #ProjectID = CAST(
SUBSTRING(#Project,1,8) + '-' +
SUBSTRING(#Project,9,4) + '-' +
SUBSTRING(#Project,13,4) + '-' +
SUBSTRING(#Project,17,4) + '-' +
SUBSTRING(#Project,21,LEN(#Project)-20)
AS uniqueidentifier)
select #ProjectID

Answer 3

I know I can't use Try-Catch in a Function, I guess a simplified questions would be, is there a way to do a cast that will just return NULL if the cast fails, instead of an error?
Starting from SQL Server 2012 you could use TRY_CAST/TRY_CONVERT functions:
Returns a value cast to the specified data type if the cast succeeds; otherwise, returns null.
CREATE FUNCTION fn_user_GetProjectID(#Project nvarchar(50))
RETURNS uniqueidentifier
AS
BEGIN
declare #ProjectID uniqueidentifier = TRY_CAST(#Project as uniqueidentifier);
IF(#ProjectID is null)
BEGIN
select #ProjectID = ProjectID from Project where projectcode = #Project;
END
return #ProjectID;
END

Answer 4

Not sure, but why not flip it around... at first glance I would simplify it like this:
select #ProjectID =
ISNULL((select ProjectID from Project where
projectcode = #Project)
,(cast #Project as uniqueidentifier))
If this doesn't provide enough error handling, I'm sure there's a better way to pre-check that the cast can work without using try/catch...

Answer 5

My brute force method was to create my own ToGuid() function that verifies it can be converted to a GUID first, if not, it returns null. It may not be very fast but it does the job, and it is probably faster to convert the guid if it is one than to try to look it up in the table. EDIT: I meant to give credit to this blog, where I got the basis of my code for this function: http://jesschadwick.blogspot.com/2007/11/safe-handling-of-uniqueidentifier-in.html
CREATE FUNCTION [dbo].[ToGuid]
(
#input NVARCHAR(MAX)
)
RETURNS uniqueidentifier
AS
BEGIN
DECLARE #isValidGuid BIT;
DECLARE #temp NVARCHAR(MAX);
SET #isValidGuid = 1;
SET #temp = UPPER(LTRIM(RTRIM(REPLACE(REPLACE(REPLACE(#input, '-', ''), '{', ''), '}', ''))));
IF(#temp IS NOT NULL AND LEN(#temp) = 32)
BEGIN
DECLARE #index INT;
SET #index = 1
WHILE (#index <= 32)
BEGIN
IF (SUBSTRING(#temp, #index, 1) IN ('0', '1', '2', '3', '4', '5', '6', '7', '8', '9', '0', 'A', 'B', 'C', 'D', 'E', 'F'))
BEGIN
SET #index = #index + 1
END
ELSE
BEGIN
SET #isValidGuid = 0
BREAK;
END
END
END
ELSE
BEGIN
SET #isValidGuid = 0
END
DECLARE #ret UNIQUEIDENTIFIER
IF(#isValidGuid = 1)
set #ret = cast(#input AS UNIQUEIDENTIFIER)
ELSE
set #ret = NULL
RETURN #ret
END
I'm still very interested if there is a better answer than this.

Answer 6

Verify if #Project is a number using the ISNUMERIC function.
your code should looks like that:
declare #ProjectID uniqueidentifier
set #ProjectID = null
IF ISNUMERIC(#Project) > 0
BEGIN
set #ProjectID = cast(#Project as uniqueidentifier)
END
if(#ProjectID is null)
BEGIN
select #ProjectID = ProjectID from Project where projectcode = #Project
END
return #ProjectID