WCF chokes on properties with no "set ". Any workaround?

Question

I have some class that I'm passing as a result of a service method, and that class has a get-only property:

[DataContract]
public class ErrorBase
{
  [DataMember]
  public virtual string Message { get { return ""; } }
}

I'm getting an exception on service side:

System.Runtime.Serialization.InvalidDataContractException: No set method for property 'Message' in type 'MyNamespace.ErrorBase'.

I have to have this property as only getter, I can't allow users to assign it a value. Any workaround I could use? Or am I missing some additional attribute?

Howitzer

Don't mark a property that cannot be deserialized with [DataMember]. Adding the attribute will simply cause DataContractSerializer to throw an exception instead of checking if the property can be serialized or deserialized. – Geilich

Answer 1

Give Message a public getter but protected setter, so that only subclasses (and the DataContractSerializer, because it cheats :) may modify the value.

Drisko

Thanks, glad it was useful! This actually is just one of several uses for this trick. Since getters and setters are technically functions, you can also use this same technique to provide custom serialization of primitive types (perhaps a custom time format in XML) without needing to wield the intimidating IDataContractSurrogate. – Drisko

You could even make it private. The Serializer doesn't mind whether it's private, public, protected, internal or protected internal. – Distressed

and make the setter throw new NotSupportedException() – Restitution

Dont' forget the [DataMember] attribute :) – Maladjustment

Having a private set; works if you use [DataContract] and [DataMember]. If you omit them, you need public set;. – Evesham

Answer 2

Even if you dont need to update the value, the setter is used by the WCFSerializer to deserialize the object (and re-set the value).

This SO is what you are after: WCF DataContracts

Argon

So the only way for me to overcome the problem is to make it a method instead of property? Again, I can't allow "set" on this property – Howitzer

You could make it a method (eg GetMessage() { return ""; }) alternatively, I am pretty sure you could tell the WCF Serializer to ignore it. I'll see what I can find and let you know., – Argon

This stackoverflow question hits the nail on the head: https://mcmap.net/q/211936/-wcf-datacontracts – Argon

Answer 3

[DataMember(Name = "PropertyName")]
public string PropertyName
{
    get
    {
        return "";
    }
    private set
    { }
}

Holinshed

Answer 4

If you only have a getter, why do you need to serialize the property at all. It seems like you could remove the DataMember attribute for the read-only property, and the serializer would just ignore the property.

Radionuclide

It does indeed not make sense to serialize a derived property (e.g., an URL property that is computed from an ID property) to a persistent storage (e.g., a database) - upvote for that - but it does make sense to serialize it to a representation (e.g., JSON or XML) that is returned by an API request. – Persuasion

Answer 5

Couldn't you just have a "do-nothing" setter??

[DataContract]
public class ErrorBase
{
  [DataMember]
  public virtual string Message 
  {
      get { return ""; } 
      set { }
  }
}

Or does the DataContract serializer barf at that, too??

Nankeen

It doesn't barf, I just didn't want to let the developers using the client API think that they can assign stuff to the property. – Howitzer

Answer 6

Properties with DataMember attribute always requires set. You should re write simmilar object on the client application since DataContract members can always be assigned values.

Bevon

Answer 7

I had this problem with ASP.NET MVC and me wanting to use DataContractSerializer in order to be able to control the names on the items in the JSON output. Eventually I switched serializer to JSON.NET, which supports properties without setters (which DataContractSerializer doesn't) and property name control (which the built-in JSON serializer in ASP.NET MVC doesn't) via [JsonProperty(PropertyName = "myName")].

Agonic

Answer 8

If it's a viable option, then instead of having ErrorBase as the base class, define it as follows:

    public interface IError
    {
        string Message
        {
            [OperationContract]
            get;

            // leave unattributed
            set;
        }
    }

Now, even though a setter exists, it's inaccessible to the client via WCF channel, so it's as if it were private.

Liam

Answer 9

If your serializer is of type DataContractJsonSerializer (or any DataContractSerializer) you can also use DataContractSerializerSettings in the constructor, with the SerializeReadOnlyTypes property set to true.

Reeves

Thanks! I did not know that option existed. – Geilich