Static holder types should be sealed

A public or protected type contains only static members and is not declared with the sealed (C# Reference) (NotInheritable) modifier.

This rule assumes that a type that contains only static members is not designed to be inherited, because the type does not provide any functionality that can be overridden in a derived type. A type that is not meant to be inherited should be marked with the sealed modifier to prohibit its use as a base type.

To fix a violation of this rule, mark the type as sealed.

Exclude a warning from this rule only if the type is designed to be inherited. The absence of the sealed modifier suggests that the type is useful as a base type.

The following example shows a type that violates the rule.

Imports System

Namespace DesignLibrary

    Public Class StaticMembers
    
        Private Shared someField As Integer 

        Shared Property SomeProperty As Integer
            Get
                Return someField
            End Get
            Set
                someField = Value
            End Set
        End Property

        Private Sub New()
        End Sub

        Shared Sub SomeMethod()
        End Sub

    End Class

End Namespace

using System;

namespace DesignLibrary
{
    public class StaticMembers
    {
        static int someField;

        public static int SomeProperty
        {
            get
            {
                return someField;
            }
            set
            {
                someField = value;
            }
        }

        StaticMembers() {}

        public static void SomeMethod() {}
    }
}

using namespace System;

namespace DesignLibrary
{
    public ref class StaticMembers
    {
        static int someField;

        StaticMembers() {}

    public:
        static property int SomeProperty
        {
            int get()
            {
                return someField;
            }

            void set(int value)
            {
                someField = value;
            }
        }

        static void SomeMethod() {}
    };
}

Static holder types should not have constructors