FluentAssertions：如何在每对元素上使用自定义比较来比较两个集合？答案

【问题标题】：FluentAssertions: How to compare two collections using a custom comparison on each pair of elements?FluentAssertions：如何在每对元素上使用自定义比较来比较两个集合？
【发布时间】：2019-09-17 12:33:19
【问题描述】：

给定以下输入：

var customers = new[] {
    new Customer { Name = "John", Age = 42 },
    new Customer { Name = "Mary", Age = 43 }
};
var employees = new[] {
    new Employee { FirstName = "John", Age = 42 },
    new Employee { FirstName = "Mary", Age = 43 }
};

使用 FluentAssertions 比较这些列表的最佳方法是什么？

我目前唯一的方法是这样的——非常类似于Enumerable.SequenceEqual：

using (var customerEnumerator = customers.GetEnumerator())
using (var employeeEnumerator = employees.GetEnumerator())
{
    while (customerEnumerator.MoveNext())
    {
        employeeEnumerator.MoveNext().Should().BeTrue();
        var (customer, employee) = (customerEnumerator.Current, employee.Current);

        customer.Name.Should().BeEquivalentTo(employee.FirstName);
        customer.Age.Should().Be(employee.Age);
    }
    employeeEnumerator.MoveNext().Should().BeFalse();
}

当然，这既不易于阅读，也不提供 FA 通常质量的诊断输出。是否有任何 FluentAssertions 内置方法来进行此断言？

【问题讨论】：

标签： c# unit-testing xunit assertion fluent-assertions

【解决方案1】：

改进断言的一种方法是将比较提取到自定义IEquivalencyStep 中，以指导如何比较Customer 和Employee。

它由两部分组成：

CanHandle 确定此比较何时适用，并且
Handle 执行自定义比较。

public class CustomerEmployeeComparer : IEquivalencyStep
{
    public bool CanHandle(IEquivalencyValidationContext context,
        IEquivalencyAssertionOptions config)
    {
        return context.Subject is Customer
            && context.Expectation is Employee;
    }

    public bool Handle(IEquivalencyValidationContext context, IEquivalencyValidator
        parent, IEquivalencyAssertionOptions config)
    {
        var customer = (Customer)context.Subject;
        var employee = (Employee)context.Expectation;

        customer.Name.Should().Be(employee.FirstName, context.Because, context.BecauseArgs);
        customer.Age.Should().Be(employee.Age, context.Because, context.BecauseArgs);

        return true;
    }
}

要在断言中使用CustomerEmployeeComparer，请通过在BeEquivalentTo 的EquivalencyAssertionOptions config 参数上调用Using(new CustomerEmployeeComparer()) 来添加它。

注意：由于您的示例需要按顺序比较两个列表，因此我在下面的示例中添加了WithStrictOrdering()。

[TestMethod]
public void CompareCustomersAndEmployeesWithCustomEquivalencyStep()
{
    // Arrange
    var customers = new[] {
        new Customer { Name = "John", Age = 42 },
        new Customer { Name = "Mary", Age = 43 }
    };

    var employees = new[] {
        new Employee { FirstName = "John", Age = 42 },
        new Employee { FirstName = "Mary", Age = 43 }
    };

    // Act / Assert
    customers.Should().BeEquivalentTo(employees, opt => opt
        .Using(new CustomerEmployeeComparer())
        .WithStrictOrdering());
}

public class Employee
{
    public string FirstName { get; set; }
    public int Age { get; set; }
}

public class Customer
{
    public string Name { get; set; }
    public int Age { get; set; }
}

将第一个 Employee 的名称更改为 Jonathan，现在给出以下失败消息：

Message: Expected item[0] to be "Jonathan" with a length of 8, but "John" has a length of 4, differs near "hn" (index 2).

With configuration:
- Use declared types and members
- Compare enums by value
- Include all non-private properties
- Include all non-private fields
- Match member by name (or throw)
- Without automatic conversion.
- UnitTestProject15.CustomerEmployeeComparer
- Without automatic conversion.
- Always be strict about the collection order

对于任何感兴趣的人，有一个相关的未解决问题是关于覆盖要比较的属性。 https://github.com/fluentassertions/fluentassertions/issues/535

【讨论】：