using System.Collections.Generic;
using System.Dynamic;

class Program
{
    static void Main()
    {
        // ExpandoObject doesn't have public Add method. Its Add is explicit implementation of IDictionary.
        IDictionary<string, object> obj = new ExpandoObject();
        obj.Add("string", "v");
        obj.Add("int", 1);
        obj.Add("object", new { X=1, Y=2 });
    }
}

IDictionary<string, object> obj = new ExpandoObject
{
    // Error CS1061 'ExpandoObject' does not contain a definition for 'Add'
    { "string", "v" },
    { "int", 1 },
    { "object", new { X = 1, Y = 2 } }
};