What is Message Parameter Attribute?

The Message Parameter Attributes  also defined in the System.Service Model namespace. It describes, it controls how the names of any operation parameters and return values appear in the service description, that is how they are emitted as  part of the WSDL for the service. 

This attribute has only one property called  the Name property.

Example:

[OperationContractAttribute]

[(return: MessageParameter(Name= "responseString")

String SomeOp([MessageParameter(Name = "string")]string s);

In the above example, the Message Parameter Attribute is used with this parameter to control how both parameter and return values are serialized to XML request and response element at the transport layer.