Moq调用Verify校验入参时抛出NullReferenceException异常

问题背景 #

使用Moq框架对同步流程做单元测试时，业务逻辑执行正常，但在调用Mock对象的Verify方法做调用校验时触发空引用异常。

待测试业务代码 #

（注：原贴代码存在几处语法笔误，包括构造函数参数缺逗号、elseif应为else if、Ilogger拼写错误，实际运行时需修正）

public class SyncManager
{
    private IPubHttpClient _pubHttpClient;
    private ILogService _logService;
    private ILogger<SyncManager> _logger;

    public SyncManager(IPubHttpClient pubClient, ILogService logService, ILogger<SyncManager> logger)
    {
        _pubHttpClient = pubClient;
        _logService = logService;
        _logger = logger;
    }

    public async Task Sync()
    {
        var syncStatus = SyncStatus.Error;

        // get logs
        var logs = await _logService.GetLogs();

        foreach (var log in logs)
        {
            if (!string.IsNullOrEmpty(log.CostCode))
               syncStatus = await GetAndSendCost(log);
            else
               syncStatus = await GetAndSendSort(log);         
        }
    }

    private async Task<SyncStatus> GetAndSendCost(Log log)
    {
        var cost = new Cost
        {
            CostCode = log.CostCode,
            CostName = log.Description,
            Active = log.Active
        };

        await _pubHttpClient.Push(new EventModel { Cost = cost, MessageType = log.Type.GetDescription() });

        return SyncStatus.Success;
    }

    private async Task<SyncStatus> GetAndSendSort(Log log)
    {
        var sort = new Sort
        {
            SortCode = log.SortCode,
            SortName = log.Description,
            Active = log.Active
        };

        await _pubHttpClient.Push(new EventModel { Sort = sort, MessageType = log.Type.GetDescription() });

        return SyncStatus.Success;
    }
}

public class Log
{
    public long Id { get; set; }
    public string SortCode { get; set; }
    public string CostCode { get; set; }
    public string Description { get; set; }
    public string Active { get; set; }
    public AuditType Type { get; set; }
}

public class EventModel 
{
    public Cost Cost { get; set; }
    public Sort Sort { get; set; }
    public string MessageType { get; set; }
}

public enum AuditType
{
    [Description("CREATE")]
    Create = 0,
    [Description("UPDATE")]
    Update = 1,
    [Description("DELETE")]
    Delete = 2
}

public static class EnumExtensions
{
    public static string GetDescription(this Enum enumValue)
    {
        return enumValue.GetType()
                   .GetMember(enumValue.ToString())
                   .First()
                   .GetCustomAttribute<DescriptionAttribute>()?
                   .Description ?? string.Empty;
    }
}

原单元测试代码 #

public class SyncManagerTests
{
    public readonly Mock<IPubHttpClient> _pubClientMock = new();
    public readonly Mock<ILogService> _logServiceMock = new();

    [Fact]
    public async Task Should_Sync()
    {
        var mockedCost = new Cost { Active = Status.Active, CostCode = "0000", CostName = "UNIT TEST" };
        var mockedSort = new Sort { Active = Status.Active, SortCode = "0001", SortName = "UNIT TEST" };
        var mockedLogs = new List<Log> { 
            new Log { CostCode = mockedCost.CostCode, Description = mockedCost.CostName, Active = mockedCost.Active, Id = 1 },
            new Log { SortCode = mockedSort.SortCode, Description = mockedSort.CostName, Active = mockedSort.Active, Id = 2 },
        };

        _logServiceMock.Setup(s => s.GetLogs()).ReturnsAsync(mockedLogs).Verifiable();
        _pubClientMock.Setup(p => p.Push(It.Is<EventModel>(x => x.Cost == mockedCost && x.MessageType == "CREATE"))).Returns(Task.CompletedTask).Verifiable();

        var syncManager = new SyncManager(_pubClientMock.Object, _logServiceMock.Object, Mock.Of<ILogger<SyncManager>>());

        await syncManager.Sync();

        _pubClientMock.Verify(p => p.Push(It.Is<EventModel>(
            x => x.Cost.CostName == mockedCost.CostName
            && x.Cost.CostCode == mockedCost.CostCode
            && x.Cost.Active == mockedCost.Active
            && x.MessageType == "CREATE")));
    }
}

异常表现 #

• 逐行调试Sync()方法，业务执行逻辑完全符合预期，运行过程中生成的EventModel实例属性值全部正确
• 执行_pubClientMock.Verify()时抛出System.NullReferenceException，提示校验表达式中x.Cost为NULL

根本原因 #

Moq执行Verify时，会遍历所有被拦截到的Push方法调用，逐个执行传入的匹配表达式判断是否符合校验规则。
测试用例中mock了2条日志数据：

1. 第一条日志带CostCode，会调用GetAndSendCost，传入Push的EventModel实例带Cost属性值，Sort属性为null
2. 第二条日志不带CostCode，会调用GetAndSendSort，传入Push的EventModel实例带Sort属性值，Cost属性为null

当Moq遍历到第二条调用记录时，会执行匹配表达式x => x.Cost.CostName == ...，此时x.Cost就是null，直接访问CostName属性自然会抛出空引用异常。
业务代码调试时不会触发该问题，是因为业务逻辑本身不会在Sort分支访问Cost属性，但Moq的校验逻辑会把所有调用都传入匹配表达式执行，没有区分调用场景。

另外原测试代码还存在两个隐藏问题：

• Setup方法中用x.Cost == mockedCost做匹配是引用相等判断，业务逻辑中每次都会new一个新的Cost实例，永远不会和测试中定义的mockedCost是同一个对象，该Setup规则永远不会命中
• mock的第二条日志数据没有显式赋值Type属性，枚举默认值为0即Create，和测试预期的MessageType="CREATE"存在逻辑偏差

修复方案 #

1. 给Verify的匹配表达式增加空值判断，先判断x.Cost不为空再访问子属性，避免遍历到Sort分支的调用时触发空引用，同时可以显式指定校验调用次数为1次，确保Cost分支确实被调用：

   _pubClientMock.Verify(p => p.Push(It.Is<EventModel>(
       x => x.Cost != null
       && x.Cost.CostName == mockedCost.CostName
       && x.Cost.CostCode == mockedCost.CostCode
       && x.Cost.Active == mockedCost.Active
       && x.MessageType == "CREATE")), Times.Once);
   

2. 修正Setup方法的匹配规则，去掉引用相等判断，改用属性值匹配，和Verify逻辑保持一致，确保Setup能正确命中。
3. 补全mock日志数据的Type属性显式赋值，避免枚举默认值导致测试数据和预期不符。
4. 如果需要校验Sort分支的调用逻辑，新增对应的Verify语句时，同样需要先判断x.Sort不为空再访问子属性。

内容的提问来源于stack exchange，提问作者Schoof