Why is this an issue?

While the properties of a readonly reference type field can still be changed after initialization, those of a readonly value type field, such as a struct, cannot.

If the member could be either a class or a struct then assignment to its properties could be unreliable, working sometimes but not others.

How to fix it

There are two ways to fix this issue:

• Restrict the type of the field to a class
• Remove the assignment entirely, if it is not possible to restrict the type of the field

Code examples

Noncompliant code example

interface IPoint
{
    int X { get; set; }
    int Y { get; set; }
}

class PointManager<T1, T2>
    where T1 : IPoint
    where T2 : IPoint
{
    readonly T1 point1;  // this could be a struct
    readonly T2 point2;  // this could be a struct

    public PointManager(T1 point1, T2 point2)
    {
        this.point1 = point1;
        this.point2 = point2;
    }

    public void MovePoints(int newX)
    {
        point1.X = newX; //Noncompliant: if point is a struct, then nothing happened
        point2.X = newX; //Noncompliant: if point is a struct, then nothing happened
    }
}

Compliant solution

interface IPoint
{
    int X { get; set; }
    int Y { get; set; }
}

class PointManager<T1, T2>
    where T1 : IPoint
    where T2 : class, IPoint
{
    readonly T1 point1;  // this could be a struct
    readonly T2 point2;  // this is a class

    public PointManager(T1 point1, T2 point2)
    {
        this.point1 = point1;
        this.point2 = point2;
    }

    public void MovePoints(int newX) // assignment to point1 has been removed
    {
        point2.X = newX; // Compliant: point2 is a class
    }
}

Resources

Documentation

• Microsoft Learn - readonly (C# Reference)
• Microsoft Learn - Reference types (C# reference)
• Microsoft Learn - Value types (C# reference)
• Microsoft Learn - Structure types (C# reference)