Example for https://github.com/yuriylsh/LanguageExt.UnitTesting/issues/9

LanguageExtUnitTestingIssue9.cs

using System;
using System.Collections.Generic;
using System.Linq;
using LanguageExt;
using LanguageExt.UnitTesting;
using Microsoft.VisualStudio.TestTools.UnitTesting;

namespace Issue9
{
    [TestClass]
    public class LanguageExtUnitTestingIssue9
    {
        private static readonly DateTime _date = new DateTime(2020, 6, 15);

        [TestMethod]
        public void Investor_CurrentTerm_OneForTheCurrentDate() {
            Investor investor = new() {
                Terms = new List<Term> {
                    new() { Start = new(2020, 1, 1), End = new(2020, 12, 31) }
                }
            };
            Option<Term> term = investor.CurrentTerm(_date);
            term.ShouldBeSome(t => Assert.AreEqual(new DateTime(2020, 1, 1), t.Start));
        }
    }

    public record Investor {
        public IReadOnlyList<Term> Terms { get; init; }
        public Option<Term> CurrentTerm(DateTime currentDate) =>
            Terms.FirstOrDefault(x => x.Start <= currentDate && x.End > currentDate) is {} found ? found : Option<Term>.None;
    }
    public record Term {
        public DateTime Start { get; init; }
        public DateTime End { get; init; }
    }
}

LanguageExtUnitTestingIssue9.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <TargetFramework>net5.0</TargetFramework>
    <LangVersion>latest</LangVersion>
    <IsPackable>false</IsPackable>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="LanguageExt.Core" Version="3.4.15" />
    <PackageReference Include="LanguageExt.UnitTesting" Version="3.3.5" />
    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.8.3" />
    <PackageReference Include="MSTest.TestAdapter" Version="2.1.2" />
    <PackageReference Include="MSTest.TestFramework" Version="2.1.2" />
  </ItemGroup>

</Project>