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WCF Service won't pass back inherited class

Posted on 2011-09-29
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Last Modified: 2012-05-12
Ok as we all know in OO programming one should program to the interface or to the abstract/base class.  I have created a service which has two methods one returning an instance of the base class and another returning an instance of a concrete class that inherits from the base class.  

To call these service methods, I created a client application.  The service method returning an instance of the base class works but the method returning an instance of the class which inherits from the base class doesn't (some sort of communication error or so it claims).  I tried using the KnownType Attribute to no avail.  What am I missing?  I have attached a zip file with the projects.  You will need to add a reference to the service since some filetypes are not allowed to be uploaded.

TIA,
Mo MyTest.zip
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Question by:modotx
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Accepted Solution

by:
theHollow earned 500 total points
ID: 36818415
Hello
I still believe the solution is in the KnownType attribute.

KnownTyoe usage:
Lets's say I have a base class "MyClassBase", which has some properties.

I also have two classes which inherit the "MyClassBase" class.
Let's call them "MyClass1" and "MyClass2".

I have some service methods.
They looks like this in the service interface:
[OperationContract]
MyClassBase SomeMethod1();

[OperationContract]
MyClassBase SomeMethod2();

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And it looks like this in my service class:
public MyClassBase SomeMethod1()
{
    return new MyClass1();
}

public MyClassBase SomeMethod2()
{
    return new MyClass2();
}

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This will fail unless you put some KnownType attributes.

The KnownType attribute should be put on the base class.
Like this:
[KnownType(typeof(MyClass1))]
[KnownType(typeof(MyClass2))]
public MyClassBase
{
    public String SomeProperty { get; set; }
}

public MyClass1 : MyClassBase
{
    public Int32 SomeOtherProperty { get; set; }
}

public MyClass2 : MyClassBase
{
    public Boolean SomeDifferentProperty { get; set; }
}

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Hope it helps :-)
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Author Comment

by:modotx
ID: 36891394
It more than helped, it worked!  :)

All of the examples I was looking at did not explain clearly where the KnownType attribute should be placed with the base class and that it only applied to return types of methods.  As a matter of fact, some said it wasn't necessary to to use the KnownType.

I wonder why the serializer can't or isn't built with enough smarts to deduce the inheritance tree.  Is it just about performance?

Thank you so much though.
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Author Closing Comment

by:modotx
ID: 36891396
Thanks again!
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LVL 4

Expert Comment

by:theHollow
ID: 36891648
Hehe
No problem :-) I have been fighting the same problem before.

So KnownType is highly necessary. It only works a little different way than one might think is logical at first.
I don't know why it's like that, but I bet it's about performance and to reduce the number of bytes to send over the internet.

Wishing a good week-end :-)
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