Understanding C# field initialization requirements

Field initialization come before base class constructor call, so it is not a valid object. Any method call with this as argument at this point leads to unverifiable code and throws a VerificationException if unverifiable code is not allowed. For example: in security transparent code.

• 10.11.2 Instance variable initializers
  When an instance constructor has no constructor initializer, or it has a constructor initializer of the form base(...), that constructor implicitly performs the initializations specified by the variable-initializers of the instance fields declared in its class. This corresponds to a sequence of assignments that are executed immediately upon entry to the constructor and before the implicit invocation of the direct base class constructor. The variable initializers are executed in the textual order in which they appear in the class declaration.
• 10.11.3 Constructor execution
  Variable initializers are transformed into assignment statements, and these assignment statements are executed before the invocation of the base class instance constructor. This ordering ensures that all instance fields are initialized by their variable initializers before any statements that have access to that instance are executed.

Everything in my answer is just my thoughts on 'why it would be dangerous to allow that kind of access'. I don't know if that's the real reason why it was restricted.

C# spec says, that field initialization happens in the order fields are declared in the class:

10.5.5.2. Instance field initialization

The variable initializers are executed in the textual order in which they appear in the class declaration.

Now, let's say the code you've mentioned is possible - you can call instance method from field initialization. It would make following code possible:

public class Progressor
{
    private string _first = "something";
    private string _second = GetMyString();

    private string GetMyString()
    {
        return "this is really important string";
    }
}

So far so good. But let's abuse that power a little bit:

public class Progressor
{
    private string _first = "something";
    private string _second = GetMyString();
    private string _third = "hey!";

    private string GetMyString()
    {
        _third = "not hey!";
        return "this is really important string";
    }
}

So, _second get's initialized before _third. GetMyString runs, _third get's "not hey!" value assigned, but later on it's own field initialization runs, and it's being set to `"hey!". Not really useful nor readable, right?

You could also use _third within GetMyString method:

public class Progressor
{
    private string _first = "something";
    private string _second = GetMyString();
    private string _third = "hey!";

    private string GetMyString()
    {
        return _third.Substring(0, 1);
    }
}

What would you expect to be value of _second? Well, before field initialization runs all the fields get default values. For string it would be null, so you'll get unexpected NullReferenceException.

So imo, designers decided it's just easier to prevent people from making that kind of mistakes at all.

You could say, OK let's disallow accessing properties and calling methods, but let's allow using fields that were declared above the one you want to access it from. Something like:

public class Progressor
{
    private string _first = "something";
    private string _second = _first.ToUpperInvariant();
}

but not

public class Progressor
{
    private string _first = "something";
    private string _second = _third.ToUpperInvariant();
    private string _third = "another";
}

That's seems useful and safe. But there is still a way to abuse it!

public class Progressor
{
    private Lazy<string> _first = new Lazy<string>(GetMyString);
    private string _second = _first.Value;

    private string GetMyString()
    {
        // pick one from above examples
    }
}

And all the problems with methods happen to come back again.

Section 10.5.5.2: Instance field initialization describes this behavior:

A variable initializer for an instance field cannot reference the instance being created. Thus, it is a compile-time error to reference this in a variable initializer, as it is a compile-time error for a variable initializer to reference any instance member through a simple-name

This behavior applies to your code because OnProgress is an implicit reference to the instance being created.