Support for ```required``` properties

Problem Statement

Traditionally, Autofac creates objects by calling one of their constructors. The constructor accepts dependencies and then assigns/initializes values.

Starting in C#11, dependencies can also be expressed using the required keyword. When a property is required, if the constructor that was called does not have the [SetsRequiredMembers] attribute, the property must have a value set immediately following construction.

Currently AutoFac does not support this core enhancement to object construction.

Desired Solution

When Autofac constructs an object, the constructor should be examined for [SetsRequiredMembers]. If it is not found, all writable properties containing the RequiredMemberAttribute attribute should be injected.

    public class Foo1 { 
        //This should be injected
        public required Dependency? Dep { protected get; init; }
    }

    public class Foo2 {
        //This should not
        public required Dependency? Dep { protected get; init; }

        [SetsRequiredMembers]
        public Foo2() {
            this.Dep = null;
        }

    }

Additional Context

Because C#11 introduces this core-level enhancement to object construction, the above rules should always be enabled.

The following code will be helpful in implementing this enhancement:

    public class PS : IPropertySelector {
        public static IPropertySelector Instance { get; }

        private static string RequiredAttributeName { get; }
        private static string SetsRequiredMembersAttributeName { get; }

        static PS() {
            Instance = new PS();

            //The type names are stored as strings so that this can also be used with legacy .NET platforms
            RequiredAttributeName = "System.Runtime.CompilerServices.RequiredMemberAttribute";
            SetsRequiredMembersAttributeName = "System.Diagnostics.CodeAnalysis.SetsRequiredMembersAttribute";
        }

        public bool NeedsPropertyInjection(ConstructorInfo constructorInfo) {
            var SetsRequiredMembers = constructorInfo.GetCustomAttributesData().Any(x => string.Equals(x.AttributeType.FullName, SetsRequiredMembersAttributeName, StringComparison.InvariantCultureIgnoreCase));

            var ret = !SetsRequiredMembers;

            return ret;

        }

        public bool InjectProperty(PropertyInfo propertyInfo, object instance) {
            var ret = propertyInfo.CanWrite
                && propertyInfo.GetCustomAttributesData().Any(x => string.Equals(x.AttributeType.FullName, RequiredAttributeName, StringComparison.InvariantCultureIgnoreCase))
                ;

            return ret;

        }
    }

[alistairjevans]

@TonyValenti, required properties shipped: https://github.com/autofac/Autofac/releases/tag/v7.0.0

[tillig]

I don’t think we need to be sold on the value of the feature. At this point it’s more a question of how it should work and how we can remain compatible with M.E.DI. I’m almost more concerned about that compatibility issue because trying to retrofit compatibility has caused us a lot of trouble in the past. Folks, for whatever reason, choose Autofac as the underlying container and then complain when it isn’t identical to the Microsoft container.