Is it possible to use the C#11 'required' modifier with .NET Framework 4.8 and .Net Standard 2.0?

Question

I'd like to be able to use the new C#11 required modifier with .NET Framework 4.8 and .Net Standard 2.0.

I'm using Visual Studio 2022 version 17.4. Is this possible?

Answer 1

This is possible, but you will have to provide some types that the compiler needs in order to support it. You will also need to compile using Visual Studio 2022 version 17.4 or later, along with C# 11.

The necessary types are defined in .NET 7.0, but they don't all exist in earlier versions. These types are as follows:

• static class IsExternalInit - Introduced with C# 9 and .NET 5.
• class RequiredMemberAttribute - Introduced with C# 11 and .NET 7.
• class CompilerFeatureRequiredAttribute - Introduced with C# 11 and .NET 7.

These types must all be defined within the System.Runtime.CompilerServices namespace. They should also be declared as internal - if they are public and defined within a class library that is referenced by a .NET 7 project, you'll get multiply-defined errors.

You can declare them as follows - this must be included in every .NET 4.8 or .NET Standard 2.0 assembly that uses the required modifier:

using System.ComponentModel;

namespace System.Runtime.CompilerServices
{
#if !NET5_0_OR_GREATER

    [EditorBrowsable(EditorBrowsableState.Never)]
    internal static class IsExternalInit {}

#endif // !NET5_0_OR_GREATER

#if !NET7_0_OR_GREATER

    [AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct | AttributeTargets.Field | AttributeTargets.Property, AllowMultiple = false, Inherited = false)]
    internal sealed class RequiredMemberAttribute : Attribute {}

    [AttributeUsage(AttributeTargets.All, AllowMultiple = true, Inherited = false)]
    internal sealed class CompilerFeatureRequiredAttribute : Attribute
    {
        public CompilerFeatureRequiredAttribute(string featureName)
        {
            FeatureName = featureName;
        }

        public string FeatureName { get; }
        public bool   IsOptional  { get; init; }

        public const string RefStructs      = nameof(RefStructs);
        public const string RequiredMembers = nameof(RequiredMembers);
    }

#endif // !NET7_0_OR_GREATER
}

namespace System.Diagnostics.CodeAnalysis
{
#if !NET7_0_OR_GREATER
    [AttributeUsage(AttributeTargets.Constructor, AllowMultiple = false, Inherited = false)]
    internal sealed class SetsRequiredMembersAttribute : Attribute {}
#endif
}

Note the use of the #if directives to support different targets.

(Incidentally, the declaration of internal static class IsExternalInit {} also enables the use of the record type and the init only property setter feature that was introduced in C# 9.)

Once you've added the definitions above to a .NET Framework 4.8 or .NET Standard 2.0 assembly, you can use the required modifier in that assembly even if the assembly is referenced by a different .NET Framework 4.8 or .NET Standard 2.0 assembly that doesn't provide those definitions (although you'd still need to provide the definitions in that other assembly if you want to use the required modifier for classes defined within that assembly).

Caveats:

• All the .NET Framework 4.8 or .NET Standard 2.0 assemblies that use the required modifier must be compiled with Visual Studio 2022 version 17.4 or later.
• Those projects will need to specify the language version as 11 or latest using <LangVersion>11</LangVersion> or <LangVersion>latest</LangVersion>.
• You should not expose any public types that use the required modifier via a NuGet package.
• This approach is NOT supported, and it could stop working with a future release (I think that's pretty unlikely, but it's not possible to guarantee that it will always work).

References:

• C# 9 Records and Init Only Settings Without .NET 5 - This is a good blog article which goes into more detail on the caveats for exposing assemblies via NuGet. The article is about C# 9 records, but it is applicable here too.
• Required modifer reference - The Microsoft documentation for required.