[c#] Namespace for [DataContract]

I can't find the namespace to use for [DataContract] and [DataMember] elements. According to what I've found, it seems that adding the following should be enough, but in my case it is not.

using System;
using System.Runtime.Serialization;

Here is a snippet of my code:

using System;
using System.Runtime.Serialization;

namespace MyNamespace {

    [DataContract]
    public class Tuple<T1, T2> {
            // A custom implementation of a Tuple
            //...
            //...
        }
}

And the error I get:

The type or namespace name 'DataContract' could not be found (are you missing a using directive or an assembly reference?)

Am I not using the right namespaces?

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

First, I add the references to my Model, then I use them in my code. There are two references you should add:

using System.ServiceModel;
using System.Runtime.Serialization;

then, this problem was solved in my program. I hope this answer can help you. Thanks.

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.

[DataContract] and [DataMember] attribute are found in System.ServiceModel namespace which is in System.ServiceModel.dll .

System.ServiceModel uses the System and System.Runtime.Serialization namespaces to serialize the datamembers.

I solved this problem by adding C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework.NETFramework\v4.0\System.Runtime.Serialization.dll in the reference