Short Walks - Instantiating an Object Without calling the Constructor

October 21, 2017

One of the things that caught my attention at DDD North was the mention of a way to instantiate an object without calling its constructor.

Disclaimer

Typically, classes have code in their constructors that are necessary for their functionality, so you may find that doing this will cause your program to fall over.

System.Runtime.Serialization

The title of the namespace is probably the first thing that betrays the fact that you shouldn’t be doing this; but we’re already halfway down the rabbit hole!

Here’s some code that will create a class using reflection the normal way:



    static void Main(string[] args)
    {
        var test = Activator.CreateInstance<MyTestClass>();
        test.MyMethod();

        Console.WriteLine("Hello World!");
        Console.ReadLine();
    }

    public class MyTestClass
    {
        public MyTestClass()
        {
            Console.WriteLine("MyTestClass Initialise");
        }

        public string test1 { get; set; }

        public void MyMethod()
        {
            Console.WriteLine("Test MyMethod.");
        }
    }

The output is:

And here’s the code that circumvents the constructor:



        static void Main(string[] args)
        {
            var test2 = FormatterServices.GetUninitializedObject(typeof(MyTestClass)) as MyTestClass;
            test2.MyMethod();

            Console.WriteLine("Hello World!");
            Console.ReadLine();
        }

And we haven’t invoked the constructor: