How do you convert number to string?

How do you convert number to string?

For my Products I would like to create a new SKU which consists of several Single Line fields and the Auto_Number field. 
When I try the following:

  1. input.SKU = input.Auto_Number;
I get the following error: 

"Variable 'Indd_File_Name' is already defined of datatype 'STRING' but trying to update 'BIGINT' datatype"

When I try 

  1. input.Indd_File_Name = input.Auto_Number.toString;
I get 

Invalid collection object found

But it also won't let me put String values into a Number field: E.g. w hen I try: 

  1. input.Number = "SPOT"+input.Auto_Number+input.Type;

In which input.Type is a String field I get the following error:

Variable 'Number' is already defined of datatype 'BIGINT' but trying to update 'STRING' datatype

Anybody having any suggestions how to convert Number Types to String Types?

Regards,

Lennert