Namespace for [DataContract]

DataContractAttribute Class is in the System.Runtime.Serialization namespace.

You should add a reference to System.Runtime.Serialization.dll. That assembly isn't referenced by default though. To add the reference to your project you have to go to References -> Add Reference in the Solution Explorer and add an assembly reference manually.

http://msdn.microsoft.com/en-us/library/system.runtime.serialization.datacontractattribute.aspx

DataContractAttribute is in System.Runtime.Serialization namespace and you should reference System.Runtime.Serialization.dll. It's only available in .Net >= 3

In visual studio for .Net 4.0 framework,

1. Try to add new reference to project.
2. On .Net Tab, Search System.Runtime.Serialization.
3. Now, you can use using System.Runtime.Serialization. And the error will not be shown.