Rory Primrose

Learn from my mistakes, you don't have time to make them yourself

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NSubstitute and FluentAssertions Redux

Posted on April 25, 2021

Way back in 2014 I posted about Bridging between NSubstitute and FluentAssertions. The code sample provides a way of using FluentAssertions checks in NSubstitute arguments. There have been some slight changes in these packages over the last seven years. Here is an updated version of how to get this to work with the current versions of those packages.

An additional change in this sample is a rename from Verify.That to Match.On which reads a little better.

using System;
using System.Diagnostics;
using System.Linq;
using FluentAssertions.Execution;
using NSubstitute.Core.Arguments;

public static class Match
    public static T On<T>(Action<T> action)
        return ArgumentMatcher.Enqueue(new AssertionMatcher<T>(action));

    private class AssertionMatcher<T> : IArgumentMatcher<T>
        private readonly Action<T> _assertion;

        public AssertionMatcher(Action<T> assertion)
            _assertion = assertion;

        public bool IsSatisfiedBy(T argument)
            using var scope = new AssertionScope();


            var failures = scope.Discard().ToList();

            if (failures.Count == 0)
                return true;

            failures.ForEach(x => Trace.WriteLine(x));

            return false;

As in the 2014 post, this can be used like the following:

            y => y.Expires.Should().BeCloseTo(DateTimeOffset.UtcNow.Add(verificationExpiry), 2000)));