== Why is this an issue?

While the properties of a https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/readonly[`readonly`] https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/reference-types[reference type] field can still be changed after initialization, those of a `readonly` https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/builtin-types/value-types[value type] field, such as a https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/builtin-types/struct[`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

[source,csharp,diff-id=1,diff-type=noncompliant]
----
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

[source,csharp,diff-id=1,diff-type=compliant]
----
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 - https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/readonly[readonly (C# Reference)] 
* Microsoft Learn - https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/reference-types[Reference types (C# reference)]
* Microsoft Learn - https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/builtin-types/value-types[Value types (C# reference)]
* Microsoft Learn - https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/builtin-types/struct[Structure types (C# reference)]


ifdef::env-github,rspecator-view[]

'''
== Implementation Specification
(visible only on this page)

=== Message

Restrict "x" to be a reference type or remove this assignment of "y"; it is useless if "x" is a value type.


'''
== Comments And Links
(visible only on this page)

=== on 13 May 2015, 20:13:12 Ann Campbell wrote:
\[~tamas.vajk]

* I've got the name of the equivalent R# rule, but I'm not sure how to find its key...
* I suspect I may have missed some of the subtleties intended in this rule. Feel free to start an edit iteration

=== on 18 May 2015, 09:15:33 Tamas Vajk wrote:
Updated the rule. Could you please check its language, and if it makes sense to you too?

=== on 21 May 2015, 14:34:22 Ann Campbell wrote:
see what you think [~tamas.vajk]

=== on 22 May 2015, 11:28:16 Tamas Vajk wrote:
Looks good, I adjusted some wording in the first sentence of the description

=== on 22 May 2015, 12:10:14 Ann Campbell wrote:
Thanks [~tamas.vajk]. Looks good

endif::env-github,rspecator-view[]