I am using DTOs in my code to transfer data between layers.
I have several extension methods to map them to my BL.
Now I want to use those DTOs as a result of my WCF APIs.
Is that a good approach I am not sure will those extension methods would be also exposed with the DTO classes or not , if yes how I can avoid it?
Your extension methods will not be exposed as a part of your wcf service contract. If your DTOs contain “normal” methods they also will not be exposed. Only DataContract of those DTOs will be shown in your service contract.