Example of Evil Pairing (borrowed from https://docs.microsoft.com/en-us/dotnet/core/testing/unit-testing-with-dotnet-test)

PrimeService.cs

using System;

namespace Prime.Services
{
    public class PrimeService
    {
        public bool IsPrime(int candidate) 
        {
            return candidate != 1;
        } 
    }
}

PrimeService_IsPrimeShould.cs

using Xunit;
using Prime.Services;

namespace Prime.UnitTests.Services
{
    public class PrimeService_IsPrimeShould
    {
        private readonly PrimeService _primeService;

        public PrimeService_IsPrimeShould()
        {
            _primeService = new PrimeService();
        }

        [Fact]
        public void ReturnFalseGivenValueOf1()
        {
            var result = _primeService.IsPrime(1);

            Assert.False(result, $"1 should not be prime");
        }
    }
}