Try-Catch in User Defined Function?

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