How to use NEWID() in SQL Server User Defined Functions (UDF)

Using NEWID() in an SQL Server user defined function returns an error message:
Msg 443, Level 16, State 1, ...
Invalid use of a side-effecting operator 'newid' within a function.

Here is a trick to get round this. Create a simple view that returns one row containing one column:
CREATE VIEW dbo.vu_newid
AS
    SELECT    NEWID() as mynewid


Test this out to prove it is working and returns a different NEWID each time.

Then in your function, use the view to return a NEWID or set of NEWIDs as follows:

To populate a variable:
DECLARE @mynewid uniqueidentifier
...
SELECT @mynewid = mynewid FROM dbo.vu_newid

To use in ORDER BY:
SELECT mt.*, vid.mynewid as mynewid
FROM   mytable mt
  CROSS JOIN dbo.vu_newid vid
ORDER BY vid.mynewid

To use in ROW_NUMBER():
SELECT ROW_NUMBER() OVER(ORDER BY vid.mynewid) as rn,
       mt.*
FROM   mytable mt
  CROSS JOIN dbo.vu_newid vid

Happy coding!