Question

object o = new { foo = 1 };

Is there any other way to add properties to the object?

Neither o["foo"] = 1 nor o.foo = 1 seems to be working.

And if i type

var o = new { foo = 1 };

Why is the type of var an “AnonomuysType” and not object?

Answer 1

You can’t just add properties to compiled types. The only time that will work as properties (well, kind-of) is if you are using the dynamic API, and the object is something that supports dynamic properties, for example:

dynamic obj = new ExpandoObject();
obj.Foo = 123;
obj.Bar = "abc";

however, those values are not available via reflection. An easier approach may be something that behaves as a dictionary:

var obj = new Dictionary<string,object>();
obj["Foo"] = 123;
obj["Bar"] = "abc";

there are more discoverable, via the dictionary API. Re your final question: when you do:

var o = new { foo = 1 };

the new { foo = 1 } causes the compiler to generate an anonymous type, which is a regular C# class with read-only properties and an unpronounceable name. This can be referred to as object (it is a regular class, after all), but when you use var the compiler uses the known type (with the horrible name), which gives you access to the properties (.foo in this case).