A field marked ``++readonly++`` can only be assigned as part of its declaration or in a constructor. While ``++readonly++`` reference types (e.g. classes) can still have their state changed subsequently, the same is not true of value types such as ``++struct++`` s. Thus, calling a method that updates object state on a ``++readonly++`` value type field simply has no effect (but runs without error!). The result is code that probably doesn't do what you thought it did.


This rule raises an issue when a method that is not marked ``++[Pure]++`` is invoked on a ``++readonly++`` value type field.

== Noncompliant Code Example

----
public struct S1
{
    public int value;

    public void SetValue()
    {
        value = 10;
    }
}

class Test
{
    static readonly S1 first;
    static S1 second;

    static void Main()
    {
        first.SetValue();  // Noncompliant
        second.SetValue();
        Console.WriteLine(first.value);  // Surprise! This writes 0
        Console.WriteLine(second.value); // This writes 10
    }
}
----