We are having the same issue with nullable reference types. Fixing it via attributes isn't trivial as the Should().NotBeNull() construct makes it impossible to properly set the needed attributes.

My proposal is to create an extra extension method ShouldNotBeNull() and properly decorate it with the NotNullAttribute etc.

using System;
using System.Diagnostics.CodeAnalysis;
using FluentAssertions.Primitives;

#nullable enable

namespace FluentAssertions
{
#if NETCOREAPP3_0

    public static class AssertionNullableExtensions
    {
        /// <summary>
        /// Asserts that the current object has been initialized.
        /// </summary>
        /// <param name="because">
        /// A formatted phrase as is supported by <see cref="string.Format(string,object[])" /> explaining why the assertion
        /// is needed. If the phrase does not start with the word <i>because</i>, it is prepended automatically.
        /// </param>
        /// <param name="becauseArgs">
        /// Zero or more objects to format using the placeholders in <paramref name="because" />.
        /// </param>
        public static AndConstraint<ObjectAssertions> ShouldNotBeNull(
            [NotNull] this object? actualValue,
            string because = "",
            params object[] becauseArgs)
        {
            var result = actualValue.Should().NotBeNull(because, becauseArgs);

            if (actualValue == null)
            {
                throw new ArgumentNullException(nameof(actualValue)); // Will never be thrown, needed only to trick the compiler
            }

            return result;
        }
    }

#endif
}

This extension properly shutdowns the warning.

object? someObject = null;
someObject.ShouldNotBeNull();
var result = someObject.ToString(); // No warnings here

I know this isn't perfectly fitting into the API but seems like an acceptable compromise to me. WDYT?

Let me know if it sounds good and I'll drop a PR (code is ready).

EDIT: cc @jnyrup

Support c# 8 nullable reference #1115

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