web service cannot invoke method

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C#

Mobile

Android

Xamarin

hi,
i am working on a dynamic application with a webservice that serves information into the web, window and mobile application. The window and web application consumes the weservice without errors while the window application will not invoke the methods webservice.
Also, i tried to use xamarin but i have an error [system.Serializableattribute] not found in any namespace in the project. i will be glad to have your ideas.