WCF: Serializing and Deserializing generic collections

Question

I have a class Team that holds a generic list:

[DataContract(Name = \"TeamDTO\", IsReference = true)]
public class Team
{
    [DataMember]
    private IList&         


        

Answer 1

It sounds like your WCF service reference is creating a proxy class rather than using the existing type. Proxy classes can only use simple arrays and not any .NET specific types like the generic List.

To avoid this proxy class conversion, in the Add Service Reference screen, click the Advanced button, and then make sure "Reuse types in referenced assemblies" is checked. This will ensure that the existing class (with the generic List) is used when serializing and deserializing the object.

Answer 2

You've run into one of the DataContractSerializer gotchas.

Fix: Change your private member declaration to:

[DataMember]
private List<Person> members = new List<Person>();

OR change the property to:

[DataMember()]
public IList<Person> Feedback {
    get { return m_Feedback; }
    set {
        if ((value != null)) {
            m_Feedback = new List<Person>(value);

        } else {
            m_Feedback = new List<Person>();
        }
    }
}

And it will work. The Microsoft Connect bug is here

This problem occurs when you deserialize an object with an IList<T> DataMember and then try to serialize the same instance again.

If you want to see something cool:

using System;
using System.Collections.Generic;

class TestArrayAncestry
{
    static void Main(string[] args)
    {
        int[] values = new[] { 1, 2, 3 };        
        Console.WriteLine("int[] is IList<int>: {0}", values is IList<int>);
    }
}

It will print int[] is IList<int>: True.

I suspect this is possibly the reason you see it come back as an array after deserialization, but it is quite non-intuitive.

If you call the Add() method on the IList<int> of the array though, it throws NotSupportedException.

One of those .NET quirks.

Answer 3

Taken straight from my blog. i hope it will be helpful:

I recently ran into an issue where we were consuming a WCF service and using a custom model binder in our ASP.net MVC app. Everything worked fine excerpt when we were serializing our ILists. IList gets serialized to arrays by default. I ended up converting our the arrays back to ILists using Reflection and calling the following method in the custom model binder. Here is how method looks like:

public object ConvertArraysToIList(object returnObj)    
{

if (returnObj != null)
{
    var allProps = returnObj.GetType().GetProperties().Where(p => p.PropertyType.IsPublic 
        && p.PropertyType.IsGenericType 
        && p.PropertyType.Name==typeof(IList<>).Name).ToList();

    foreach (var prop in allProps)
    {
        var value = prop.GetValue(returnObj,null);
        //set the current property to a new instance of the IList<>
        var arr=(System.Array)value; 
        Type listType=null;

        if(arr!=null)
        {
            listType= arr.GetType().GetElementType();
        }

        //create an empty list of the specific type
        var listInstance = typeof(List<>)
          .MakeGenericType(listType)
          .GetConstructor(Type.EmptyTypes)
          .Invoke(null);

        foreach (var currentValue in arr)
        {
            listInstance.GetType().GetMethod("Add").Invoke(listInstance, new[] { currentValue });
        }

        prop.SetValue(returnObj, listInstance, null);
    }
}

return returnObj;
}

Answer 4

I got this error while transporting an IList read from a database via LINQ. The WCF was hosted in IIS 7 on a Windows Server 2008 x64.

The app pool crashed with no warnings.

[ServiceBehavior]
public class Webservice : IWebservice
{

    public IList<object> GetObjects()
    {
        return Database.Instance.GetObjects();
    }
}

Its not exactly the same problem but may have the same cause.

The resolution for was to install MS hotfix KB973110 http://support.microsoft.com/kb/971030/en-us