Add property to anonymous type after creation

Solution 1:

The following extension class would get you what you need.

public static class ObjectExtensions
{
    public static IDictionary<string, object> AddProperty(this object obj, string name, object value)
    {
        var dictionary = obj.ToDictionary();
        dictionary.Add(name, value);
        return dictionary;
    }

    // helper
    public static IDictionary<string, object> ToDictionary(this object obj)
    {
        IDictionary<string, object> result = new Dictionary<string, object>();
        PropertyDescriptorCollection properties = TypeDescriptor.GetProperties(obj);
        foreach (PropertyDescriptor property in properties){
            result.Add(property.Name, property.GetValue(obj));
        }
        return result;
    }
}

Solution 2:

I assume you mean anonymous types here, e.g. new { Name1=value1, Name2=value2} etc. If so, you're out of luck - anonymous types are normal types in that they're fixed, compiled code. They just happen to be autogenerated.

What you could do is write new { old.Name1, old.Name2, ID=myId } but I don't know if that's really what you want. Some more details on the situation (including code samples) would be ideal.

Alternatively, you could create a container object which always had an ID and whatever other object contained the rest of the properties.