CommunicationException: Error while trying to serialize parameter

Question

I have a webservice with a method that takes a parameter of abstract base type Specification<TDomainModel> as parameter. I realize that WCF don't know how to de-/serialize this without specifying known types. But when I try to specify them, I still get this exception:

There was an error while trying to serialize parameter http://myproject.org:specification. The InnerException message was 'Type 'MyProject.DomainModel.WebSnapshot.WebSnapshotSpecification' with data contract name 'WebSnapshotSpecification:http://schemas.datacontract.org/2004/07/MyProject.DomainModel.WebSnapshot' is not expected. Add any types not known statically to the list of known types - for example, by using the KnownTypeAttribute attribute or by adding them to the list of known types passed to DataContractSerializer.

If anyone can see where I have missed something, it would be very much appreciated :) The classes looks as follows:

WebSnapshotRepositoryService:

[ServiceContract(Namespace = "http://myproject.org")]
[ServiceKnownType(typeof(Specification<WebSnapshot>))]
[ServiceKnownType(typeof(WebSnapshotSpecification))]
public class WebSnapshotRepositoryService
{
    [OperationContract]
    public IEnumerable<WebSnapshot> Get(Specification<WebSnapshot> specification)
    {
        // Code here.
    }
}

and WebSnapshotSpecification:

public class WebSnapshotSpecification : Specification<WebSnapshot>
{
    public override Expression<Func<WebSnapshot, bool>> IsSatisfiedByExpression()
    {
        return (t) => true;
    }
}

which is a specification of WebSnapshot:

[DataContract(IsReference = true, Name = "WebSnapshot", Namespace = "MyProject.DomainModel.WebSnapshot")]
public class WebSnapshot
{
    [DataMember(Order = 1)]
    public virtual string Name { get; set; }
    [DataMember(Order = 2)]
    public virtual string PictureFilePath { get; set; }
    [DataMember(Order = 3)]
    public virtual int PictureHeight { get; set; }
    [DataMember(Order = 4)]
    public virtual int PictureWidth { get; set; }
}

finally, here is Specification<TDomainModel>:

public abstract class Specification<TDomainModel>
{
    public virtual bool IsSatisfiedBy(TDomainModel domainObject)
    {
        return IsSatisfiedByExpression().Compile().Invoke(domainObject);
    }

    public virtual TDomainModel AssembleObject()
    {
        return Activator.CreateInstance<TDomainModel>();
    }

    public abstract Expression<Func<TDomainModel, bool>> IsSatisfiedByExpression();
}

Sixto Saez Sixto Saez · Accepted Answer · 2011-07-29T12:38:39

It seems like you are trying to send a WebSnapshotSpecification class instance to the service and are expecting WCF to serialize/deserialize the IsSatisfiedByExpression method it contains. That scenario is not supported in WCF. If the WebSnapshotSpecification class had some properties then those will be serialized/deserialized but methods are never part of a WCF implicit or explicit DataContract definition.

CommunicationException: Error while trying to serialize parameter

1 Answers