如何避免C#代码中的NullReferenceException？排查空引用问题

需要解决代码中的NullReferenceException问题——当equipmentSpaceAssociations为可枚举空集合时，foreach循环会触发报错。

相关代码：

var spacesEquipmentAssociations = Spaces?.SelectMany(s => s.AirBalance.EquipmentExhaustAirflow.Equipment.Where(e => e.AirHandlerId != null))
                ?? Enumerable.Empty<EquipmentSpaceAssociation>();
IncludeEquipmentAirflowsSum(exhaustEquipmentById, spacesEquipmentAssociations, true);

对应的方法实现：

private void IncludeEquipmentAirflowsSum<T>(
        IDictionary<Guid, T> exhaustEquipmentById,
        IEnumerable<EquipmentSpaceAssociation> equipmentSpaceAssociations,
        bool isSubtraction = false)
        where T : class, IMinMaxAirflow
    {
        _ = equipmentSpaceAssociations ?? throw new ArgumentNullException(nameof(equipmentSpaceAssociations));

        double sign = isSubtraction ? -1D : 1D;
        foreach (var equipmentSpaceAssociation in equipmentSpaceAssociations) // 此处报错
        {
            if (exhaustEquipmentById.TryGetValue(equipmentSpaceAssociation.EquipmentId, out var equipment))
            {
                    var multiplier = equipmentSpaceAssociation.Quantity * sign;
                    EquipmentExhaustAirflow.MaxAirflow += equipment.MaxAirflow * equipmentSpaceAssociation.Diversity * multiplier;
                    EquipmentExhaustAirflow.MinAirflow += equipment.MinAirflow * multiplier;
            }
        }          
    }

尝试检查equipmentSpaceAssociations != null和equipmentSpaceAssociations?.Any()，但在equipmentSpaceAssociations?.Any()这一行也会出现相同的空引用错误。错误详情：

System.ArgumentNullException Message=Value cannot be null.
(Parameter 'source') Source=System.Linq StackTrace: at
System.Linq.ThrowHelper.ThrowArgumentNullException(ExceptionArgument
argument) at System.Linq.Enumerable.Where[TSource](IEnumerable1 source, Func2 predicate) at
Domain.Shared.Models.ExhaustAirHandler.<>c__351.<ComputePropertiesMultiSpace>b__35_5(Space s) in C:\issue-dev\design-hub-dotnet\Domain\Shared\Models\AirHandlers\ExhaustAirHandler.cs:line 59 at System.Linq.Enumerable.SelectManySingleSelectorIterator2.MoveNext()

at
Domain.Shared.Models.ExhaustAirHandler.IncludeEquipmentAirflowsSum[T](IDictionary2 exhaustEquipmentById, IEnumerable1 equipmentSpaceAssociations,
Boolean isSubtraction) in
C:\issue-dev\design-hub-dotnet\Domain\Shared\Models\AirHandlers\ExhaustAirHandler.cs:line 82 at
Domain.Shared.Models.ExhaustAirHandler.ComputePropertiesMultiSpace[T](IDictionary`2
exhaustEquipmentById) in
C:\issue-dev\design-hub-dotnet\Domain\Shared\Models\AirHandlers\ExhaustAirHandler.cs:line 61

解决方案 #

从栈追踪可以看出，错误并非来自foreach循环本身，而是Linq查询的延迟执行阶段：SelectMany委托中，某个Space对象的s.AirBalance、s.AirBalance.EquipmentExhaustAirflow或s.AirBalance.EquipmentExhaustAirflow.Equipment为null，导致Where方法接收了null作为源集合，触发ArgumentNullException。

修复方法是在Linq查询中逐层添加空值检查，并用空集合兜底：

var spacesEquipmentAssociations = Spaces?.SelectMany(s => 
    // 逐层检查空值，任意层级为null则返回空集合
    s.AirBalance?.EquipmentExhaustAirflow?.Equipment?.Where(e => e.AirHandlerId != null) 
    ?? Enumerable.Empty<EquipmentSpaceAssociation>())
    ?? Enumerable.Empty<EquipmentSpaceAssociation>();
IncludeEquipmentAirflowsSum(exhaustEquipmentById, spacesEquipmentAssociations, true);

这样修改后，只要Space对象的任意关联属性为null，都会返回空集合，避免Where方法处理null源，从而解决枚举时的空引用错误。

内容的提问来源于stack exchange，提问作者kumar425