Question

I found a topic on MSDN that talks that yes, this is possible.

I did a test that seems to break this statement:

using System;

namespace Test
{
    class Program
    {
        static void Main(string[] args)
        {
            Foo f = new Foo("1");
            Console.WriteLine(f.Bar); // prints 1
            f.Test("2");
            Console.WriteLine(f.Bar);// successfully prints 2
        }
    }

    class Foo
    {
        public Foo(string b)
        {
            this.Bar = b;
        }

        public string Bar { get; private set; }

        public void Test(string b)
        {
            // this would be impossible for readonly field!
            // next error would be occur: CS0191 or CS0191
            // A readonly field cannot be assigned to (except in a constructor or a variable initializer)
            this.Bar = b; 
        }
    }
}

Where am I wrong?

Answer 1

The answer below was written back in 2010. In C# 6 (released in 2015) you can write read-only automatically-implemented properties:

// This can only be assigned to in a constructor
public int Foo { get; }

---

You’re absolutely right. Properly read-only automatically implemented properties are currently impossible. Making the setter private isn’t the same thing, regardless of what some books and MSDN might say 🙂

If I ruled the world, this would not be the case. When I see some of the language designers at NDC 2010 in June (please come along!) I intend to try to persuade, bribe, cajole and generally make a nuisance of myself until they agree. It’s just one wafer-thin feature, after all.

Looking at that MSDN article, the text itself doesn’t say that it creates a read-only automatic property. It creates an immutable type using an automatic property, and that’s correct. The only problematic bits are the comments saying

// Read-only properties.

… which are definitely wrong. The framework agrees with us:

var prop = typeof(Contact).GetProperty("Name");
Console.WriteLine(prop.CanWrite); // Prints True