HLQ008: Consider adding readonly.

Cause

An enumerable is a value type but not defined as readonly.

Severity

Info

Rule description

When the compiler doesn't know if a value type is immutable, it may create defensive copies when a member method is called. Adding the readonly modifier to the method definition, informs the compiler that the method does not change the internal state of the enumerable. In this case, the defensive copy is not required.

If the enumerable is immutable, it's better to add the readonly modifier to the type definition. It forces the fields to be readonly and sets all the methods are automatically set to readonly.

How to fix violations

If the enumerable is immutable, add the readonly modifier to its definition.

When to suppress warnings

When the enumerable is not immutable.

You should then add the readonly modifier the methods that don't change the internal state of the enumerable.

Example of a violation

struct ValueTypeEnumerable<T>
{
    public Enumerator GetEnumerator() => new Enumerator();

    public struct Enumerator
    {
        public T Current => default;

        public bool MoveNext() => false;
    }
}

Example of how to fix

readonly struct ValueTypeEnumerable<T>
{
    public Enumerator GetEnumerator() => new Enumerator();

    public struct Enumerator
    {
        public T Current => default;

        public bool MoveNext() => false;
    }
}