Use Try Catch Block in UDFs in SQL Server

Question

I have following lines of code in my user-defined function :

declare @annualStr  varchar(30)
declare @annual float
    begin try
          set @annualStr = dbo.GetXMLValues(@businessId,@id)
          set @annual = convert(float,isnull(@annualStr,'0'))
    end try
    begin catch
          set @annual = 0
    end catch

The value comes from dbo.GetXMLValues may be a varchar value so I need to use try catch block to convert value into float ..

But it throws an error saying.. it is invalid to use try catch blocks in UDF

Why?? And what to do to convert varchar value to float when it is unknown

Answer 1

Updated based on comments.

SET @annualstr = dbo.GetXMLValues(@businessId,@id);

IF @annualstr NOT LIKE '%[^0-9.]%' AND  @annualstr NOT LIKE '%.%.%' 
  BEGIN
    SET @annual = Cast(@annualstr As float);
  END;

SET @annual = Coalesce(@annual, 0);

Answer 2

Incidentally, if you're using SQL 2012+ Then you can use the new Try_Cast() function

SET @annualStr = dbo.GetXMLValues(@businessId,@id);
SET @annual = Coalesce(Try_Cast(@annualStr As float), 0);