MssqlCastToUserDefinedType.sql

-- In the year 2020, we have self-driving cars, but MSSQL doesn't have the ability to cast to user-defined types.
-- This is an extremely hacky, slow, unsafe, untested, and completely unrecommended workaround.
declare @SystemTypeId tinyint,
        @MaxLength    smallint

select @SystemTypeId = system_type_id, @MaxLength = max_length from sys.types where name = 'your_user_defined_type_name';

declare @TypeFullName sysname = type_name(@SystemTypeId) + (case
                                                                -- char, varchar
                                                                when @SystemTypeId in (167, 175) then '(' + cast(@MaxLength as varchar) + ')'
                                                                -- nchar, nvarchar
                                                                when @SystemTypeId in (239, 231) then '(' + cast((@MaxLength / 2) as varchar) + ')'
                                                                else ''
                                                            end);

exec('select cast(something_to_be_cast as ' + @TypeFullName + ');');