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

tip

You can suppress this inspection to ignore specific issues, change its severity level to make the issues less or more noticeable, or disable it altogether.

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.