c# - 这个字段怎么为空？

Question

我得到一个空异常，但该字段被初始化为一个空列表。那么它怎么可能为空呢？

错误发生在该方法的第二行(在 _hydredProperties 上):

protected virtual void NotifyPropertyChanged<T>(Expression<Func<T>> expression)
{
    string propertyName = GetPropertyName(expression);

    if (!this._hydratedProperties.Contains(propertyName)) { this._hydratedProperties.Add(propertyName); }
}

这是声明字段的方式:

public abstract class EntityBase<TSubclass> : INotifyPropertyChanged where TSubclass : class
{
    private List<string> _hydratedProperties = new List<string>();

它是这样设置的:

    public Eta Eta
    {
        get { return this._eta; }

        set
        {
            this._eta = value;
            NotifyPropertyChanged(() => this.Eta);
        }
    }

这是完整的类(class)(删除了评论和不相关的部分):

[DataContract]
public abstract class EntityBase<TSubclass> : INotifyPropertyChanged where TSubclass : class
{
    private List<string> _hydratedProperties = new List<string>();

    public bool IsPropertyHydrated(string propertyName)
    {
        return this._hydratedProperties.Contains(propertyName);
    }

    public event PropertyChangedEventHandler PropertyChanged;

    protected virtual void NotifyPropertyChanged<T>(Expression<Func<T>> expression)
    {
        string propertyName = GetPropertyName(expression);

        if (!this._hydratedProperties.Contains(propertyName)) { this._hydratedProperties.Add(propertyName); }

        PropertyChangedEventHandler handler = PropertyChanged;
        if (handler != null)
        {
            handler(this, new PropertyChangedEventArgs(propertyName));
        }
    }

    public string GetPropertyName<T>(Expression<Func<T>> expression)
    {
        MemberExpression memberExpression = (MemberExpression)expression.Body;
        return memberExpression.Member.Name;
    }
}

派生类:

[DataContract]
public class Bin : EntityBase<Bin>
{
    private Eta _eta;

    [DataMember]
    public Eta Eta
    {
        get { return this._eta; }

        set
        {
            this._eta = value;
            NotifyPropertyChanged(() => this.Eta);
        }
    }
}

Answer 1

这是线索:

[DataContract]

是的。 DataContractSerializer does not call any constructor .相反，它使用 FormatterServices.GetUninitializedObject 来创建将被反序列化的对象。这会绕过构造函数调用。

你的初始化器:

private List<string> _hydratedProperties = new List<string>();

由编译器转换为隐式默认构造函数。

作为解决方法，您可以使用 a deserialization callback with OnDeserializingAttribute :

[DataContract]
public abstract class EntityBase<TSubclass> : INotifyPropertyChanged
    where TSubclass : class
{
    private List<string> _hydratedProperties;

    protected EntityBase()
    {
        Init();
    }

    private void Init()
    {
        _hydratedProperties = new List<string>()
    }

    [OnDeserializing]
    private void OnDeserializing(StreamingContext context)
    {
        Init();
    }

    // ... rest of code here
}