C# Data Contracts

A data contract is a formal agreement between a service and a client that abstractly describes the data to be exchanged. That is, to communicate, the client and the service do not have to share the same types, only the same data contracts. A data contract precisely defines, for each parameter or return type, what data is serialized (turned into XML) to be exchanged.

Data Contract Basics

Windows Communication Foundation (WCF) uses a serialization engine called the Data Contract Serializer by default to serialize and deserialize data (convert it to and from XML). All .NET Framework primitive types, such as integers and strings, as well as certain types treated as primitives, such as DateTime and XmlElement, can be serialized with no other preparation and are considered as having default data contracts. Many .NET Framework types also have existing data contracts. For a full list of serializable types, see Types Supported by the Data Contract Serializer.

New complex types that you create must have a data contract defined for them to be serializable. By default, the DataContractSerializer infers the data contract and serializes all publicly visible types. All public read/write properties and fields of the type are serialized. You can opt out members from serialization by using the IgnoreDataMemberAttribute. You can also explicitly create a data contract by using DataContractAttribute and DataMemberAttribute attributes. This is normally done by applying the DataContractAttribute attribute to the type. This attribute can be applied to classes, structures, and enumerations. The DataMemberAttribute attribute must then be applied to each member of the data contract type to indicate that it is a data member, that is, it should be serialized. For more information, see Serializable Types.

Examples:

[ServiceContract]
public interface ISampleInterface
{
    // No data contract is required since both the parameter 
    // and return types are primitive types.
    [OperationContract]
    double SquareRoot(int root);

    // No Data Contract required because both parameter and return 
    // types are marked with the SerializableAttribute attribute.
    [OperationContract]
    System.Drawing.Bitmap GetPicture(System.Uri pictureUri);

    // The MyTypes.PurchaseOrder is a complex type, and thus 
    // requires a data contract.
    [OperationContract]
    bool ApprovePurchaseOrder(MyTypes.PurchaseOrder po);
}

namespace MyTypes
{
    [DataContract]
    public class PurchaseOrder
    {
        private int poId_value;

        // Apply the DataMemberAttribute to the property.
        [DataMember]
        public int PurchaseOrderId
        {

            get { return poId_value; }
            set { poId_value = value; }
        }
    }
}

Other Examples:

[ServiceContract]
public interface IMyService 
{
        [OperationContract]
        PurchaseOrderHistoryDC[] GetPurchaseOrderHistory(long PurchaseOrderID);

        [OperationContract]
        PurchaseOrderDC GetCurrentPurchaseOrder(long AccountID);

        [OperationContract]
        List<PurchaseOrderDC> GetPurchaseOrderList(long AccountID);

        [OperationContract]
        long UpdatePurchaseOrder(long PurchaseOrderID, decimal? Total, DateTime? PoDate, int UserID);
}

using System.Runtime.Serialization;

namespace MyApp.DataContract
{
    [DataContract]
    public class AccountDC
    {
       	[DataMember]
		public long AccountID { get; set; }

		[DataMember]
		public string FirstName { get; set; }

		[DataMember]
		public string LastName { get; set; }

    }
}

Sources:

https://docs.microsoft.com/en-us/dotnet/framework/wcf/feature-details/using-data-contracts