Compiler generating error when using C# required keyword

Question

I have the most recent visual studio (17.3.1) and I am trying to use the new required keyword on properties of my record. <LangVersion> is set to preview in my project file to get this functionality.

The compiler is throwing 3 errors when I use the required keyword

CS0656: Missing compiler required member 'System.Runtime.CompilerServices.RequiredMemberAttribute..ctor'
CS0656: Missing compiler required member 'System.Runtime.CompilerServices.CompilerFeatureRequiredAttribute..ctor'
CS0656: Missing compiler required member 'System.Runtime.CompilerServices.SetsRequiredMembersAttribute..ctor'

I understand this is preview functionality and not officially supported, but I was hoping someone else may have run into this and found a workaround so I can check out this feature.

Answer 1

You need to install .NET 7 SDK or above, BUT if you are working with multi-targeting environment, for example targeting both .NET 7 and NetStandard2.1, then you can just add those attributes in your csproj:

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);
}

namespace System.Runtime.CompilerServices;

/// <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)]
#if SYSTEM_PRIVATE_CORELIB
    public
#else
internal
#endif
    sealed class RequiredMemberAttribute : Attribute
{

}

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
{ }

Answer 2

Installing .NET 7.0 SDK fixed the issue.