A getter-only property can only be set on initialization of the class, so either with a fixed value or one from a constructor. That does make it the equivalent of a read-only field, but it's a lot easier to keep byte-level parity with future versions.

In its simplest form, it means you can do this:

    public property Foo { get; } = 5;

Your class now has a Foo property that nobody can set, not even methods in the class itself, but that can be publicly read.

These become a lot more useful with primary constructors (also a new C# 6 feature):

    public class MyImmutableClass(int usersFoo) {

        public property Foo { get; } = usersFoo;

    }

This will create a constructor on the class that takes the usersFoo argument, and immediately assign the value of that argument to your read-only automatic property. It makes immutable classes a lot easier to create, and guarantees that they'll stay immutable.

edit: Permit made it clear that primary contructors have been dropped from the C# 6 spec. You'll just have to assign your readonly properties from regular constructors then :)

Internally, .Net automatic properties are compiled with a backing field and a getter and setter method. In the case of a read-only auto property, they will omit the setter, so there is no public artifact that could be abused.

A bit messy, but an overview with code samples can be found at http://msdn.microsoft.com/en-us/magazine/dn683793.aspx