Example1_GetProducts.cs

[Test]
public void Example1_GetProducts_WhenPassingAValidCategory_ThenShouldReturnProducts()
{
    // arrange act assert
    Assert.IsTrue(GetProducts_WhenPassingAValidCategoryAndTheRepositoryIsAbleToReturnProducts_ThenShouldReturnProducts());
}

Example2_GetProducts.cs

[Test]
public void Example2_GetProducts_WhenPassingAValidCategory_ThenShouldReturnProducts()
{
        // setup repository mock
        var products = new List<Product>
        {
                new Product { Id = 1, Name = "Product 1" },
                new Product { Id = 2, Name = "Product 2" }
        };

        IProductCatalogRepository repository = Substitute.For<IProductCatalogRepository>();
        repository.GetProducts(Arg.Is("ElectronicDevices")).Returns(products);

        // create object under test
        var testSubject = new ProductCatalogService(repository);

        // assert
        var result = testSubject.GetProducts(Category.ElectronicDevices);

        Assert.IsNotNull(result);
        Assert.IsTrue(result.ToList().Count != 0);
}        

Example3_GetProducts.cs

[Test]
public void Example3_GetProducts_WhenPassingAValidCategory_ThenShouldReturnProducts()
{
  Given.AConcreteProductCatalogService.WhenBasedOnScenario(
    Given.AProductCatalogRepository.WhenReturningProductsForCategory("ElectronicDevices"))
    .When().GetProducts(Category.ElectronicDevices)
    .Then().Should().NotBeNull().And.HaveCountGreaterThan(0);
}

FluentAssertions.cs

// using assert
var expected = true; 
Assert.AreEqual(expected, actual); 

// using Fluent Assertions
result.Should().BeTrue(); 

given.cs

public static class Given
{
  public static IProductCatalogRepository AProductCatalogRepository => Substitute.For<IProductCatalogRepository>();
  
  public static ProductCatalogService AConcreteProductCatalogService => new ProductCatalogService(null);
        
  public static List<Product> AConcreteProductList => new List<Product>();
}

nSubstitute.cs

//Create:
var calculator = Substitute.For<ICalculator>();

//Set a return value:
calculator.Add(1, 2).Returns(3);
Assert.AreEqual(3, calculator.Add(1, 2));

//Check received calls:
calculator.Received().Add(1, Arg.Any<int>());
calculator.DidNotReceive().Add(2, 2);

Sample.cs

[Test]
public void Example3_GetProducts_WhenPassingAValidCategory_ThenShouldReturnProducts()
{
  Given.AConcreteProductCatalogService.BasedOnScenario(
    Given.AProductCatalogRepository.ThatReturnsProductsForCategory("ElectronicDevices"))
    .When().GetProducts(Category.ElectronicDevices)
    .Then().TheResult().Should().NotBeNull().And.HaveCountGreaterThan(0);
}

ScenarioHelper.cs

public static class ScenarioHelper
{
  public static T When<T>(this T @this)
  {
  return @this;
  }
        
  public static T Then<T>(this T @this)
  {
    return @this;
  }
  //...
}

Scenarios.cs

    public static class Scenarios
    {
        public static IProductCatalogRepository ThatReturnsProductsForCategory(this IProductCatalogRepository @this, string category)
        {
            @this.GetProducts(Arg.Is(category)).Returns(Given.AConcreteProductList.ThatReturnsTwoProducts());
            
            return @this;
        }
        
        public static ProductCatalogService BasedOnScenario(this ProductCatalogService @this, IProductCatalogRepository repository)
        {
            return new ProductCatalogService(repository);
        }

        //...
    }