Using Data Contracts

Article
08/07/2009

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.

Example

The following example shows a service contract (an interface) to which the ServiceContractAttribute and OperationContractAttribute attributes have been explicitly applied. The example shows that primitive types do not require a data contract, while a complex type does.

<ServiceContract()>  _
Public Interface ISampleInterface
    ' No data contract is requred since both the parameter and return 
    ' types are both primitive types.
    <OperationContract()>  _
    Function SquareRoot(ByVal root As Integer) As Double 
    
    ' No Data Contract required because both parameter and return 
    ' types are marked with the SerializableAttribute attribute.
    <OperationContract()>  _
    Function GetPicture(ByVal pictureUri As System.Uri) As System.Drawing.Bitmap 
    
    ' The MyTypes.PurchaseOrder is a complex type, and thus 
    ' requires a data contract.
    <OperationContract()>  _
    Function ApprovePurchaseOrder(ByVal po As MyTypes.PurchaseOrder) As Boolean 
End Interface

[ServiceContract]
public interface ISampleInterface
{
    // No data contract is requred 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);
}

The following example shows how a data contract for the MyTypes.PurchaseOrder type is created by applying the DataContractAttribute and DataMemberAttribute attributes to the class and its members.

Namespace MyTypes
    <System.Runtime.Serialization.DataContractAttribute()>  _
    Public Class PurchaseOrder
        Private poId_value As Integer
        
        ' Apply the DataMemberAttribute to the property.
        
        <DataMember()>  _
        Public Property PurchaseOrderId() As Integer 
            
            Get
                Return poId_value
            End Get
            Set
                poId_value = value
            End Set
        End Property
    End Class 
End Namespace

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; }
        }
    }
}

Notes

The following notes provide items to consider when creating data contracts:

The IgnoreDataMemberAttribute attribute is only honored when used with unmarked types. This includes types that are not marked with one of the DataContractAttribute, SerializableAttribute, CollectionDataContractAttribute, or EnumMemberAttribute attributes, or marked as serializable by any other means (such as IXmlSerializable).
You can apply the DataMemberAttribute attribute to fields, and properties.
Member accessibility levels (internal, private, protected, or public) do not affect the data contract in any way.
The DataMemberAttribute attribute is ignored if it is applied to static members.
During serialization, property-get code is called for property data members to get the value of the properties to be serialized.
During deserialization, an uninitialized object is first created, without calling any constructors on the type. Then all data members are deserialized.
During deserialization, property-set code is called for property data members to set the properties to the value being deserialized.
For a data contract to be valid, it must be possible to serialize all of its data members. For a full list of serializable types, see Types Supported by the Data Contract Serializer.

Generic types are handled in exactly the same way as non-generic types. There are no special requirements for generic parameters. For example, consider the following type.

<DataContract()>  _
Public Class MyGenericType1 (Of T)
    ' Code not shown.
End Class

[DataContract]
public class MyGenericType1<T>
{
    // Code not shown.
}

This type is serializable whether the type used for the generic type parameter (T) is serializable or not. Because it must be possible to serialize all data members, the following type is serializable only if the generic type parameter is also serializable, as shown in the following code.

<DataContract()> Public Class MyGenericType2(Of T)

    <DataMember()> Dim theData As T

End Class

[DataContract]
public class MyGenericType2<T>
{
    [DataMember]
    T theData;
}

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Using Data Contracts

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Example

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