FluentAssertions own extensions

FluentAssertionsContext.cs

namespace Gist
{
    using FluentAssertions.Execution;
    using Xunit;

    public class MyCustomAssertionsFacts
    {
        [Fact]
        public void DoesNotPrintContextNicely()
        {
            var testee = new MyCustom(0);

            testee.Should().ContainPositiveValue();
            // error message is: Expected object to contain a positive value, but found 0.
            // expected message is: Expected testee to contain a positive value, but found 0.
        }
    }

    public class MyCustomAssertions
    {
        private readonly MyCustom subject;

        public MyCustomAssertions(MyCustom subject)
        {
            this.subject = subject;
        }

        public void ContainPositiveValue()
        {
            Execute.Assertion.ForCondition(this.subject.Value > 0)
                .FailWith("Expected {context} to contain a positive value, but found {0}.", this.subject.Value);
        }
    }

    public static class MyCustomAssertionsExtensions
    {
        public static MyCustomAssertions Should(this MyCustom actualValue)
        {
            return new MyCustomAssertions(actualValue);
        }
    }

    public class MyCustom
    {
        public MyCustom(int value)
        {
            this.Value = value;
        }

        public int Value { get; }
    }
}

You need to add the [CustomAssertion] attribute. See https://fluentassertions.com/documentation/

@dennisdoomen Thank you, works now. As I understood the documentation, this appeared to be only necessary if using Should() inside of custom assertions.

Hmm, that was not the intention.