使用NSubstitute模拟MongoDB Filter Definition时单元测试失败

我在编写单元测试验证MongoDB仓储的Get文档方法，测试代码如下：

[Fact]
public async Task GivenExistingDocumentId_WhenGetAsync_ThenDummyProjectionReturnedSuccessfully()
{
    // Arrange
    var documentId = "123idempotentkey";
    var sourceCollection = "testcollection";

    var dummyProjectionDto = new DummyProjectionDto
    {
        IdempotencyKey = "123idempotentkey",
        Description = "test-description"
    };

    // Ensure the filter is properly initialized
    var filter = Builders<DummyProjectionDto>.Filter.Eq("_id", documentId);
    Assert.NotNull(filter);

    var cursor = Substitute.For<IAsyncCursor<DummyProjectionDto>>();
    cursor.Current.Returns(new List<DummyProjectionDto> { dummyProjectionDto });

    // Ensure filter parameter is not null when calling FindAsync
    _mongoCollectionMock.FindAsync(Arg.Is(filter), null, Arg.Any<CancellationToken>()).Returns(cursor);

    // Act
    var result = await _repository.GetAsync(documentId, sourceCollection, default);

    // Assert
    result.Should().NotBeNull();
    result.IdempotencyKey.Should().Be(dummyProjectionDto.IdempotencyKey);
    result.Description.Should().Be(dummyProjectionDto.Description);
}

运行测试时抛出以下错误：

Message:  System.ArgumentNullException : Value cannot be null.
(Parameter 'filter')

Stack Trace:  Ensure.IsNotNull[T](T value, String paramName)
IMongoCollectionExtensions.FindAsync[TDocument](IMongoCollection1 collection, FilterDefinition1 filter, FindOptions`2 options,
CancellationToken cancellationToken)
DummyProjectionRepositoryTests.GivenExistingDocumentId_WhenGetAsync_ThenDummyProjectionReturnedSuccessfully() line 98
--- End of stack trace from previous location ---

我已经确认测试代码里的filter不为空（断言通过），仓储的GetAsync实现代码如下：

public async Task<DummyProjection> GetAsync(string documentId, string sourceCollection, CancellationToken ct = default)
{
    var filter = Builders<DummyProjectionDto>.Filter.Eq("_id", documentId);

    var result = await _database
        .GetCollection<DummyProjectionDto>(GetCollectionName(sourceCollection))
        .FindAsync(filter, null, ct);

    var projectionDto = await result.SingleOrDefaultAsync(ct);

    if (projectionDto == null)
    {
        throw new MongoException($"Document with ID {documentId} not found.");
    }
    else
    {
        var projectionDocument = new DummyProjection()
        {
            IdempotencyKey = projectionDto.IdempotencyKey,
            Description = projectionDto.Description,
        };

        return projectionDocument;
    }
}

明明filter不为空，测试为何仍抛出针对'filter'的ArgumentNullException？

• 核心原因：参数匹配不生效
  测试中用Arg.Is(filter)匹配FindAsync的filter参数，但FilterDefinition<T>的默认相等性判断不会认为测试里创建的filter和仓储方法内部创建的filter是同一个对象。NSubstitute找不到匹配的调用时会返回null，而MongoDB的FindAsync扩展方法会强制检查filter非空，因此抛出异常。

• 修改参数匹配逻辑
  放弃直接匹配filter对象，改用任意参数匹配或验证filter的逻辑内容：

  // 方式1：匹配任意FilterDefinition，简单直接
  _mongoCollectionMock.FindAsync(Arg.Any<FilterDefinition<DummyProjectionDto>>(), null, Arg.Any<CancellationToken>()).Returns(cursor);
  
  // 方式2：验证filter的实际内容，更严谨
  _mongoCollectionMock.FindAsync(
      Arg.Is<FilterDefinition<DummyProjectionDto>>(f => f.ToString() == Builders<DummyProjectionDto>.Filter.Eq("_id", documentId).ToString()),
      null,
      Arg.Any<CancellationToken>()
  ).Returns(cursor);
  

• 额外检查：确保集合模拟正确
  要确认_database.GetCollection<DummyProjectionDto>(...)返回的是你模拟的_mongoCollectionMock，如果返回未模拟的实例或null，同样会触发参数检查错误。可以在测试的Arrange阶段添加：

  _database.GetCollection<DummyProjectionDto>(Arg.Is(sourceCollection)).Returns(_mongoCollectionMock);
  

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