CompilerServices and Diagnostics.CodeAnalysis Attribute polyfills for the required keyword in .NetStandard2, .Net5, .Net6

CSharpKeywordRequiredAttributes.cs

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
//
// License statement and more attributes source code at
// https://github.com/dotnet/runtime/tree/main/src/libraries/System.Private.CoreLib/src/System/Runtime/CompilerServices
//
// ReSharper disable CheckNamespace
using System.ComponentModel;

namespace System.Runtime.CompilerServices
{
    /// <summary>
    /// Indicates that compiler support for a particular feature is required for the location where this attribute is applied.
    /// </summary>
    [AttributeUsage(AttributeTargets.All, AllowMultiple = true, Inherited = false)]
    public sealed class CompilerFeatureRequiredAttribute : Attribute
    {
        public CompilerFeatureRequiredAttribute(string featureName)
        {
            FeatureName = featureName;
        }

        /// <summary>
        /// The name of the compiler feature.
        /// </summary>
        public string FeatureName { get; }

        /// <summary>
        /// If true, the compiler can choose to allow access to the location where this attribute is applied if it does not understand <see cref="FeatureName"/>.
        /// </summary>
        public bool IsOptional { get; init; }

        /// <summary>
        /// The <see cref="FeatureName"/> used for the ref structs C# feature.
        /// </summary>
        public const string RefStructs = nameof(RefStructs);

        /// <summary>
        /// The <see cref="FeatureName"/> used for the required members C# feature.
        /// </summary>
        public const string RequiredMembers = nameof(RequiredMembers);
    }

    /// <summary>Specifies that a type has required members or that a member is required.</summary>
    [AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct | AttributeTargets.Field | AttributeTargets.Property, AllowMultiple = false, Inherited = false)]
    [EditorBrowsable(EditorBrowsableState.Never)]
    #if SYSTEM_PRIVATE_CORELIB
        public
    #else
        internal
    #endif
    sealed class RequiredMemberAttribute : Attribute { }

    /// <summary>
    /// Reserved to be used by the compiler for tracking metadata.
    /// This class should not be used by developers in source code.
    /// </summary>
    [EditorBrowsable(EditorBrowsableState.Never)]
    internal static class IsExternalInit { }

}

namespace System.Diagnostics.CodeAnalysis
{
    /// <summary>
    /// Specifies that this constructor sets all required members for the current type, and callers
    /// do not need to set any required members themselves.
    /// </summary>
    [AttributeUsage(AttributeTargets.Constructor, AllowMultiple = false, Inherited = false)]
    #if SYSTEM_PRIVATE_CORELIB
        public
    #else
        internal
    #endif
    sealed class SetsRequiredMembersAttribute : Attribute { }
}

In the .csproj file:

  <PropertyGroup>
    <LangVersion>12.0</LangVersion>
  </PropertyGroup>