InvalidOperationException: A circular reference was detected while serializing an object of type ...

You are trying to use a WebMethod or a web service ScriptMethod in Javascript and you get an InvalidOperationException saying something about a circular reference. It happened to me when trying to read a DataTable in Javascript.

Why. The Javascript serialization of DataSets, DataTables and DataRows was available once in the ASP.Net Ajax web extensions. That's why you probably found a lot of Google results with people that could either only serialize DataSets, but not DataTables, or people that made it work by magic by adding some lines in the converters section of web.config, things that can't possibly work with your setup. Then, the option was removed in the final version of ASP.Net Ajax, only to be readded in the ASP.Net Futures, which is a test playground for future features of the platform.

What. There are several options, one being to reference an older version of ASP.Net Ajax and uses the converters there. But why bother? It's unlikely you use the DataTable or some other object in Javascript with all the options of the C# object. You probably just want to itterate through rows and read properties. So build your own converter.

How. Create a new library project. Add a class named DataTableConverter that inherits from JavaScriptConverter, and implement: IEnumerable<Type> SupportedTypes, IDictionary<string, object> Serialize(object obj, JavaScriptSerializer serializer) and object Deserialize(IDictionary<string, object> dictionary, Type type, JavaScriptSerializer serializer).

You probably won't need to Deserialize anything, you can leave that unimplemented. The list of convertible types is easy enough, all you are left with is the Serialize code, which is actually very easy, too. Then all you need to do is add this in the web.config file:

<system.web.extensions>
<scripting>
<webServices>
<jsonSerialization >
<converters>
<add name="DataTableAjaxFix" type="AjaxTypeConverters.DataTableConverter"/>
</converters>
</jsonSerialization>

And here is the complete C# code of my DataTableConverter, but you can easily adapt it to anything:

using System;
using System.Collections;
using System.Collections.Generic;
using System.Data;
using System.Web.Script.Serialization;

namespace AjaxTypeConverters
{
    public class DataTableConverter : JavaScriptConverter
    {
        public override IEnumerable<Type> SupportedTypes
        {
            get { return new Type[] {typeof (DataTable)}; }
        }

        public override object Deserialize(IDictionary<string, object> dictionary, Type type,JavaScriptSerializer serializer)
        {
            throw new NotImplementedException();
        }

        public override IDictionary<string, object> Serialize(object obj, JavaScriptSerializer serializer)
        {
            DataTable listType = obj as DataTable;

            if (listType != null)
            {
                // Create the representation.
                Dictionary<string, object> result = new Dictionary<string, object>();
                ArrayList itemsList = new ArrayList();
                foreach (DataRow row in listType.Rows)
                {
                    //Add each entry to the dictionary.
                    Dictionary<string, object> listDict = new Dictionary<string, object>();
                    foreach (DataColumn dc in listType.Columns)
                    {
                        listDict.Add(dc.ColumnName, row[dc]);
                    }
                    itemsList.Add(listDict);
                }
                result["Rows"] = itemsList;

                return result;
            }
            return new Dictionary<string, object>();
        }
    }
}