rspec/rules/S4015/csharp/rule.adoc

== Why is this an issue?

Decreasing the https://learn.microsoft.com/en-us/dotnet/csharp/programming-guide/classes-and-structs/access-modifiers[accessibility level] of an inherited method that is not https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/override[overridable] to https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/private[private] will shadow the name of the base method and can lead to confusion.

[source,csharp]
----
public class Base
{
    public void SomeMethod(int count) { }
}
public class Derived : Base
{
    private void SomeMethod(int count) { } // Noncompliant
}

class Program
{
    public void DoWork()
    {
        var derived = new Derived();
        derived.SomeMethod(42); // Base.SomeMethod is accessed here
    }
}
----

Another potential problem is the case of a class deriving from `Derived` and accessing `SomeMethod`. In this scenario, the method accessed will instead be the `Base` implementation, which might not be what was expected.

[source,csharp]
----
public class Base
{
    public void SomeMethod(int count) { }
}
public class Derived : Base
{
    private void SomeMethod(int count) { } // Noncompliant
}

public class SecondDerived : Derived
{
    public void DoWork()
    {
        SomeMethod(42); // Base.SomeMethod is accessed here
    }
}
----

One way to mitigate this, is by sealing the `Derived` class by using the https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/sealed[sealed] modifier, thus preventing inheritance from this point on.

[source,csharp]
----
public class Base
{
    public void SomeMethod(int count) { }
}
public sealed class Derived : Base
{
    private void SomeMethod(int count) { } // Compliant: class is marked as sealed
}
----

Another way to mitigate this, is by having the `Derived` implementation match the https://learn.microsoft.com/en-us/dotnet/csharp/programming-guide/classes-and-structs/access-modifiers[accessibility] modifier of the `Base` implementation of `SomeMethod`. From a caller's perspective, this is closer to the expected behavior.

[source,csharp]
----
using System;

namespace MyLibrary
{
    public class Base
    {
        public void SomeMethod(int count) { }
    }
    public class Derived : Base
    {
        public void SomeMethod(int count) { } // Compliant: same accessibility as Base.SomeMethod
    }

    public class Program
    {
        public void DoWork()
        {
            var derived = new Derived();
            derived.SomeMethod(42); // Derived.SomeMethod is called 
        }
    }
}
----

Last but not least, consider using a different name for the `Derived` method, thus completely eliminating any confusion caused by the naming collision.

[source,csharp]
----
public class Base
{
    public void SomeMethod(int count) { }
}
public class Derived : Base
{
    private void SomeOtherMethod(int count) { } // Compliant 
}
----

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