Question

I’m having a class like the following:

using System;
using System.Collections.Generic;
using System.Runtime.Serialization;

[DataContract()]
public class TestCol : List<Test> { }

[DataContract()]
public class MainTest
{
    public TestCol Components { get; set; }
}

[DataContract()]
public class Test
{
    public Test() { }
    public String Name { get; set; }
}

And a webservice with the following webmethod like this:

[WebMethod]
public String Test(MainTest input)
{
    String rtrn = String.Empty;
    foreach (Test test in input.Components)
        rtrn += test.Name;
    return rtrn;
}

Which is called by AJAX with the following method:

var Test = {};
Test.Name = "Test";

var MainTest = {};
MainTest.Components = [];
MainTest.Components.push(Test);

$.ajax({
    type: "POST",
    url: "WebService/WSTest.asmx/Test",
    contentType: "application/json; charset=utf-8",
    dataType: "json",
    data: JSON.stringify({
        "input": MainTest 
    }),
    success: function(data, textStatus) {
        console.log("success");
    },
    error: function(XMLHttpRequest, textStatus, errorThrown) {
        window.console && console.log && console.log(XMLHttpRequest.responseText + " || " + textStatus + " || " + errorThrown);
    }
});

When executing the AJAX call, it will return errors. I found out that the error is with the typed class TestCol, which has no properties.
Now do I have found 2 solutions that require changes in the C# classes:

1. Remove the TestCol class and change the Components property to List<Test> datatype:

   [DataContract()]
   public class MainTest
   {
       public List<Test> Components { get; set; }
   }
   
   [DataContract()]
   public class Test
   {
       public Test() { }
       public String Name { get; set; }
   }
   

2. Or add an extra property to the TestCol class and change the webmethod:

   [DataContract()]
   public class TestCol : List<Test>
   {
       public List<Test> Components { get; set; }
   }
   
   [DataContract()]
   public class MainTest
   {
       public TestCol Components { get; set; }
   }
   
   [DataContract()]
   public class Test
   {
       public Test() { }
       public String Name { get; set; }
   }
   

   &

   [WebMethod]
   public String Test(MainTest input)
   {
       String rtrn = String.Empty;
       foreach (Test test in input.Components.Components)
               rtrn += test.Name;
       return rtrn;
   }
   

Both solutions require changes in the C# classes, which I prefer not to, as other code is depended on it. Does anyone know a solution for this problem?

Edit: I’ve uploaded a test solution, containing above code: http://jeroenvanwarmerdam.nl/content/temp/JSONtoClassWebservice.zip

Answer 1

JavaScriptSerializer serialization: IEnumerable -> JavaScript Array

When the JavaScriptSerializer is used, it automatically converts an IEnumerable (without IDictionary) type — that covers List<> or anything derived from it — into an array.

Deserialization: JavaScript Array -> IEnumerable -> Collection Object

Now, upon deserialization from JSON, the JavaScriptSerializer must take the array, create an IEnumerable, then create an object for the field by passing that IEnumerable into its constructor.

Constructing Collection object via Constructor

Now, for List<> you have a constructor overload that takes an IEnumerable. So if you put List<Test> as the type of your component it creates it fine.

Constructors not inherited

However, TestCol does NOT have such a constructor! The reason why it worked with List<Test> and not with TestCol (which derives from List<Test>) is that the only thing that is not inherited between classes are constructors!

Therefore, the JavaScriptSerializer does not have any way to construct a TestCol from an IEnumerable. So it fails silently.

Deserialize Array by Creating List, then Casting to Type

Now the JavaScriptSerializer may then attempt to create a List<Test> from this IEnumerable<Test>, and then try to cast it into a TestCol.

Possible Solution

Solution: Try putting in:

public TestCol () {}       // Need this when you have another constructor
public TestCol (IEnumerable<Test> list) : base(list) {}         // Constructor that takes an IEnumerable
public TestCol (IList<Test> list) : base(list) {}         // Constructor that takes an IList

as your TestCol’s constructors.

And if it still doesn’t work, implement an explicit type cast from List<Test> to TestCol.

public static explicit operator TestCol(IList<Test> list) { return new TestCol(list); }