using System;

using System.IO;

using System.Text;

using Newtonsoft.Json;

public class Program

{

    public static void Main()

    {

        var obj = new { key = "value" };

        StringBuilder sb = new StringBuilder();

        using (StringWriter sw = new StringWriter(sb))

        using (JsonTextWriter writer = new JsonTextWriter(sw))

        {

            writer.QuoteChar = '\'';

            JsonSerializer ser = new JsonSerializer();

            ser.Serialize(writer, obj);

        }

        Console.WriteLine(sb.ToString());

    }

}

​