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.