If you are using ExpandoObject for your dynamic object, you can cast to IDictionary<string, object>
and use an indexer;
dynamic expando = new ExpandoObject();
var dict = (IDictonary<string, object>)expando;
dict["123"] = 2;
Many other dynamic object implementations (e. g. JObject in Json.NET) provide similar functionality.
Here's an example with JObject:
var json = JsonConvert.SerializeObject(new Dictionary<string, object> { { "123", 10 } });
var deserialized = JsonConvert.DeserializeObject<object>(json);
// using the IDictionary interface
var ten = ((IDictionary<string, JToken>)deserialized)["123"].Value<JValue>().Value;
Console.WriteLine(ten.GetType() + " " + ten); // System.Int64 10
// using dynamic
dynamic d = deserialized;
Console.WriteLine(d["123"].Value.GetType() + " " + d["123"].Value); // System.Int64 10
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