FluentAssertions自定义EquivalencyStep如何正确打印属性上下文

问题背景 #

自定义PaginatedList<T>继承自List<T>并实现IPaginatedList接口扩展分页属性，使用FluentAssertions的BeEquivalentTo()时，默认只对比集合内容忽略分页属性。因此编写了PaginatedListEquivalencyStep手动验证这些属性，但验证失败时错误消息中的属性名称显示混乱，仅在添加.WithTrace()后才正常显示正确路径。

原因分析 #

这不是FluentAssertions的Bug，而是自定义EquivalencyStep的用法存在疏漏：

• 手动拼接AssertionScope的描述文本时，没有利用FluentAssertions内置的路径管理机制，导致手动拼接的文本与上下文自动追踪的路径发生叠加，生成了混乱的错误消息。
• .WithTrace()启用了详细路径追踪，临时覆盖了手动拼接的错误，但这并非正确的解决方式。

解决方案 #

正确的做法是利用验证上下文的CreateChildNode方法生成子属性的路径节点，让FluentAssertions自动维护属性路径，避免手动拼接带来的混乱。

修改后的PaginatedListEquivalencyStep实现 #

public sealed class PaginatedListEquivalencyStep : EquivalencyStep<IPaginatedList>
{
    protected override EquivalencyResult OnHandle(Comparands comparands, IEquivalencyValidationContext context, IEquivalencyValidator nestedValidator)
    {
        if (comparands.Subject is not IPaginatedList subject)
            throw new NotSupportedException();

        if (comparands.Expectation is not IPaginatedList expectation)
            throw new NotSupportedException();

        // 为每个分页属性创建子节点，让FluentAssertions自动管理路径
        ValidatePaginatedProperty(subject.ItemsCount, expectation.ItemsCount, nameof(IPaginatedList.ItemsCount), context, nestedValidator);
        ValidatePaginatedProperty(subject.CurrentPage, expectation.CurrentPage, nameof(IPaginatedList.CurrentPage), context, nestedValidator);
        ValidatePaginatedProperty(subject.PageSize, expectation.PageSize, nameof(IPaginatedList.PageSize), context, nestedValidator);

        // 验证集合内容
        new EnumerableEquivalencyStep().Handle(comparands, context, nestedValidator);

        return EquivalencyResult.AssertionCompleted;
    }

    private static void ValidatePaginatedProperty(int subjectValue, int expectationValue, string propertyName, IEquivalencyValidationContext context, IEquivalencyValidator nestedValidator)
    {
        var propertyComparands = new Comparands(subjectValue, expectationValue, context.Options);
        var childNode = context.CreateChildNode(propertyName);
        nestedValidator.AssertEquality(propertyComparands, childNode);
    }
}

或者，若仍想使用AssertionScope，需通过CreateChildNode获取正确的属性路径：

protected override EquivalencyResult OnHandle(Comparands comparands, IEquivalencyValidationContext context, IEquivalencyValidator nestedValidator)
{
    if (comparands.Subject is not IPaginatedList subject)
        throw new NotSupportedException();

    if (comparands.Expectation is not IPaginatedList expectation)
        throw new NotSupportedException();

    var itemsCountNode = context.CreateChildNode(nameof(IPaginatedList.ItemsCount));
    using (new AssertionScope(itemsCountNode.Description))
    {
        subject.ItemsCount.Should().Be(expectation.ItemsCount);
    }

    var currentPageNode = context.CreateChildNode(nameof(IPaginatedList.CurrentPage));
    using (new AssertionScope(currentPageNode.Description))
    {
        subject.CurrentPage.Should().Be(expectation.CurrentPage);
    }

    var pageSizeNode = context.CreateChildNode(nameof(IPaginatedList.PageSize));
    using (new AssertionScope(pageSizeNode.Description))
    {
        subject.PageSize.Should().Be(expectation.PageSize);
    }

    new EnumerableEquivalencyStep().Handle(comparands, context, nestedValidator);

    return EquivalencyResult.AssertionCompleted;
}

验证效果 #

修改后运行测试代码，错误消息将正常显示：

Expected property result.ValueOrDefault.ItemsCount to be 10, but found 3 (difference of -7)

内容的提问来源于stack exchange，提问作者user26719255