Code Inspection: Assignment to a property of a readonly field can be useless

Consider the following situation - say you have an interface I and a struct that implements this interface:

interface I
{
    int Y { get; set; }
}
struct S : I
{
    public int Y { get; set; }
}
    

Now, say you have a class that has a readonly field of a type that implements I and a static method which assigns a readonly variable, like so:

class A<T> where T : I
{
    readonly T X;
    public static void Foo()
    {
        var a = new A<T>();
        a.X.Y = 7; // here
        Console.WriteLine(a.X.Y);
    }
}
    

If you now call the method with A<S>.Foo(), you’d expect it to print the value 7 to the console. In actual fact, it prints 0 (zero).

The main reason for this is that readonly fields can only be initialized in the constructor. Since this does not happen, the value of X is essentially the value of default(T). In the case of reference types, this will be null and will throw a NullReferenceException were you to run the code above. However, with value types, this presents a hidden error, as default(T) will yield a default-initialized S structure with its Y property predictably set to 0.

Last modified: 08 March 2021