外部服务方法Mock失效，调用后返回Null问题求助

问题背景 #

测试中对ISupplyReservesClient的AssignOrderReserve方法做了Mock配置，但运行测试时Mock始终失效，返回Null。相关代码如下：

业务代码中调用的方法

AssignOrderReserveCommandResponseDto response = await _supplyReservesClient.AssignOrderReserve(
    masterReserveId,
    inboundReserveId,
    deliveryDate,
    orderInfos,
    cancellationToken)

当前Mock实现

public static readonly ISupplyReservesClient SupplyReservesClient = Substitute.For<ISupplyReservesClient>();
public static AssignOrderReserveCommandResponseDto assignOrderReserveCommandResponse;

static SupplyReservesClientMock()
{
    SupplyReservesClient
         .AssignOrderReserve(Arg.Any<Guid>(), Arg.Any<Guid>(),
             Arg.Any<DateTimeOffset>(), Arg.Any<IReadOnlyCollection<AssignOrderInfo>>(),
             Arg.Any<CancellationToken>())
         .Returns(_ => assignOrderReserveCommandResponse);
}

public static void SetAssignOrderReserveCommandResponseDto(long bidId, long otherBidId)
{
    Fixture fixture = new Fixture();
    assignOrderReserveCommandResponse = fixture.Build<AssignOrderReserveCommandResponseDto>()
        .With(x => x.OrderInfos, new List<AssignOrderReserveCommandResponseDto.OrderInfo>()
        {
            new(bidId, "1", DateTime.UtcNow.AddDays(3)),
            new(otherBidId, "1", DateTime.UtcNow.AddDays(3))
        })
        .With(x => x.InboundReserveId, fixture.Create<Guid>())
        .Create();
}

待Mock的目标方法内部逻辑

public async Task<AssignOrderReserveCommandResponseDto> AssignOrderReserve(
    Guid? masterReserveId,
    Guid? inboundReserveId,
    DateTimeOffset deliveryDate,
    IReadOnlyCollection<AssignOrderInfo> orderInfos,
    CancellationToken cancellationToken)
{
    AssignOrderReserveCommand command = AssignOrderReserveConverter
        .Convert(masterReserveId, inboundReserveId, deliveryDate, orderInfos);

    AssignOrderReserveCommandResponse response = await _supplyReservesClient
        .AssignOrderReserveAsync(command, cancellationToken: cancellationToken);

    return AssignOrderReserveConverter.Convert(response);
}

核心失效原因 #

1. Mock的方法与实际执行的方法不匹配

从目标方法的内部逻辑能明确看到：上层代码调用的AssignOrderReserve方法，内部实际转调了AssignOrderReserveAsync方法，而非直接调用外部服务的AssignOrderReserve。但你的Mock仅针对AssignOrderReserve做了配置，完全没处理AssignOrderReserveAsync，导致这个未被Mock的方法返回默认值Null，最终整个流程返回Null。

2. 参数类型匹配错误（次要但需修正）

目标方法中masterReserveId和inboundReserveId的类型是Guid?（可空Guid），但Mock配置中用的是Arg.Any<Guid>()（非空Guid）。如果业务代码中传入的是null值，参数匹配条件会不满足，同样会导致Mock失效返回Null。

修正后的Mock实现示例 #

public static readonly ISupplyReservesClient SupplyReservesClient = Substitute.For<ISupplyReservesClient>();
public static AssignOrderReserveCommandResponse assignOrderReserveCommandResponse; // 类型改为CommandResponse

static SupplyReservesClientMock()
{
    // 修正1：Mock实际调用的AssignOrderReserveAsync方法
    SupplyReservesClient
         .AssignOrderReserveAsync(Arg.Any<AssignOrderReserveCommand>(), Arg.Any<CancellationToken>())
         .Returns(_ => assignOrderReserveCommandResponse);

    // 可选：保留原AssignOrderReserve的Mock，修正参数类型为可空Guid
    SupplyReservesClient
         .AssignOrderReserve(Arg.Any<Guid?>(), Arg.Any<Guid?>(),
             Arg.Any<DateTimeOffset>(), Arg.Any<IReadOnlyCollection<AssignOrderInfo>>(),
             Arg.Any<CancellationToken>())
         .Returns(async (Guid? m, Guid? i, DateTimeOffset d, IReadOnlyCollection<AssignOrderInfo> o, CancellationToken ct) => 
         {
             var command = AssignOrderReserveConverter.Convert(m, i, d, o);
             var resp = await SupplyReservesClient.AssignOrderReserveAsync(command, ct);
             return AssignOrderReserveConverter.Convert(resp);
         });
}

public static void SetAssignOrderReserveCommandResponse(long bidId, long otherBidId)
{
    Fixture fixture = new Fixture();
    // 修正：构造AssignOrderReserveCommandResponse实例（而非Dto）
    assignOrderReserveCommandResponse = fixture.Build<AssignOrderReserveCommandResponse>()
        .With(x => x.OrderInfos, new List<AssignOrderReserveCommandResponse.OrderInfo>()
        {
            new(bidId, "1", DateTime.UtcNow.AddDays(3)),
            new(otherBidId, "1", DateTime.UtcNow.AddDays(3))
        })
        .With(x => x.InboundReserveId, fixture.Create<Guid>())
        .Create();
}

内容的提问来源于stack exchange，提问作者Tanya