Setter赋值可空字段却触发Possible Null Reference警告，原因何在？

当Setter赋值给可空值、Getter返回非空值时，为何会收到Null Reference警告？

我的LocationSensor类代码如下：

public class LocationSensor
{
    private string? _contextualUnits;
    public string? UniqueUnits { get; set; }
    public string GeneralUnits { get; set; }
    /// <summary>
    /// This field is meant to embody the units based on the user viewing them. If this sensor
    /// has a unique unit, that is always taken. If not, we check to see if the application has
    /// specified a contextual unit based on the users chosen measurement system. If that is not specified,
    /// we default to the general units (usually the metric system).
    /// </summary>
    public string ContextualUnits
    {
        get { return UniqueUnits ?? _contextualUnits ?? GeneralUnits; }
        set { _contextualUnits = value; }
    }
    public string? ImperialUnits { get; set; }
}

赋值代码如下：

public async Task<LocationSensor> GetLocationSensorAsync(int sensorId, int locationId, bool? inMetric = null)
{
    LocationSensor locationSensor = await _locationSensorRepository.GetLocationSensorAsync(sensorId, locationId);
    if (inMetric != null)
    {
        locationSensor.ContextualUnits = inMetric.Value ? locationSensor.GeneralUnits : locationSensor.ImperialUnits;
    }
    return locationSensor;
}

明明是把可能为空的值赋值给可空私有字段_contextualUnits，为什么在locationSensor.ContextualUnits = inMetric.Value ? locationSensor.GeneralUnits : locationSensor.ImperialUnits;这一行会收到“possible null reference assignment”警告？

解答 #

这个警告的核心原因是C#的空引用分析只关注属性的公开声明类型，不会解析Setter内部的实现逻辑。

• ContextualUnits属性声明为string（非可空类型），而非string?。尽管Setter实际是将值存入可空的_contextualUnits字段，但编译器仅依据属性的公开类型判断——它认为你在给一个非可空的string属性赋值，而右侧的locationSensor.ImperialUnits是string?类型，存在为空的可能性，因此触发警告。

解决办法 #

1. 给空值兜底：如果ImperialUnits为空，自动回退到GeneralUnits，确保赋值的右侧始终是非空值：

   locationSensor.ContextualUnits = inMetric.Value 
       ? locationSensor.GeneralUnits 
       : locationSensor.ImperialUnits ?? locationSensor.GeneralUnits;
   

2. 强制告知编译器值非空：仅当你能确保当前场景下ImperialUnits一定不为空时使用，用!标记消除警告：

   locationSensor.ContextualUnits = inMetric.Value 
       ? locationSensor.GeneralUnits 
       : locationSensor.ImperialUnits!;
   

3. 用特性标记Setter允许空输入：使用[AllowNull]特性告诉编译器，尽管属性本身是非可空的，但Setter接受空值输入，既保留Getter的非空语义，又消除警告：

   [AllowNull]
   public string ContextualUnits
   {
       get { return UniqueUnits ?? _contextualUnits ?? GeneralUnits; }
       set { _contextualUnits = value; }
   }
   

内容的提问来源于stack exchange，提问作者kj49