Passing an instance of anonymous type over WCF

Question

I have a WCF service method that expects an object and then retrieves its properties using reflection.

On the client side I create an anonymous type object

var obj = new {FirstName="John", LastName="Doe"}

and pass it to the method. I'm getting an exception:

Type '<>f__AnonymousType0`2[System.String,System.String]' cannot be serialized. 
Consider marking it with the DataContractAttribute attribute, and marking all 
of its members you want serialized with the DataMemberAttribute attribute.  
See the Microsoft .NET Framework documentation for other supported types.

I can't mark the type or its members with serialization attributes because there is really no type nor properties declared explicitly. Is there a way to overcome that problem?

Answer 1

You've already got the answer. It can't be done.

In fact, you can't pass an instance of an anonymous type from one method to another within your program. You certainly can't pass them between programs.

Answer 2

No, there is not. While there are tricks and techniques to return objects of an anonymous type from a method, you cannot send them over WCF.

WCF needs to know all of its concrete types that will be sent across, since you're not really just calling a .NET method. Instead, the message call gets converted into a serialized message, and therefore, any "thing" being passed over a WCF call must be serializable - no exceptions.

Answer 3

The answers suggesting that you can't use an anonymous type over WCF are wrong.

If you are using the default DataContractSerializer to serialize your types on the channel, then yes, the answers are correct. This is because the DataContractSerializer supports the following scenarios:

1. Types serialized using the Serializable attribute
2. Types serialized using XML Serialization
3. Types serialized using DataContract
4. Plain-Old-C#-Object (POCO) Serialization

Respectively, they fail with anonymous types because of the following:

1. You can't apply attributes to anonymous types.
2. XML Serialization requires a default parameterless constructor, which anonymous types do not have.
3. Same as 1.
4. Same as 2.

However, you are not forced to use the DataContractSerializer to serialize your messages in WCF. You can create a custom message formatter which you can use to perform the serialization yourself.

You have a problem if the types you are sending out as the results of requests are anonymous types. When you get the results back, it's going to have a definite name within a namespace (not in a .NET sense, but in a SOA sense) and you are going to have to handle the mapping of that concrete type back to the anonymous type. However, because you don't have access to the actual anonymous type or ways of constructing it in code (at least in a dynamic way), you have no choice but to pass it along as an object if it's passed back to you, which makes it kind of worthless, since everyone will have to use bad practices such as dynamic (not a bad practice in itself, but to get around these limitations in this case, yes), or cast-by-example.

So in the end I will say that while it certainly is possible to serialize anonymous types and send them over the wire, the work invovled is usually not worth it.

Answer 4

As said before, the objects must be deserializable and so you will have to define the structure beforehand. However, you can use inheritance to defining them, and hence lower the pain. WCF provides the KnownType attribute to allow a Service Operation to receive an object of the base class and deserialize it into an object of a derived class. So you will still only have one (or a few) Service Operations that can handle all your scenarios.

Answer 5

Don't do this.

It's an attempt to be clever. Don't. Just declare the datatype you need, and use it. If you need a more loosely-defined datatype, just use a key-value mapping of some sort.

It will take you 5 minutes to write something that can handle this for good. Using any technique like this will cost you hours of debugging at some future point down the road.