协方差分配无效

Question

我试图通过IObservable<T>来概括对象属性更改的通知，但这不是问题的重点。问题的要点是：我做的以下作业报告了一个无效的协方差误差：

protected virtual void OnPropertyChanged<T>(
                      string propertyName, 
                      T oldValue, T newValue)
{
    IPropertyChangedNotification<Student, object> notification = 
        new PropertyChangedNotification<Student, T>(this,
        propertyName, oldValue, newValue);
    ...
}

此报告：

无法隐式转换类型PropertyChangedNotification<UsingSubjectToWatchProperty.Student, T>至CCD_ 3。存在显式转换（是否缺少强制转换？）

这是完整的代码：

class Student
{
    private ISubject<IPropertyChangedNotification<Student, object>> _subject = 
       new Subject<IPropertyChangedNotification<Student, object>>();;
    private string _name = null;
    public string Name
    {
        get
        {
            return _name;
        }
        set
        {
            var oldValue = _name;
            _name = value;
            OnPropertyChanged<string>("Name", oldValue, _name);
        }
    }
    protected virtual void OnPropertyChanged<T>(string propertyName, T oldValue, T newValue)
    {
        IPropertyChangedNotification<Student, object> notification = 
            new PropertyChangedNotification<Student, T>(this,
            propertyName, oldValue, newValue);
        _subject.OnNext(notification);
    }
}
public class PropertyChangedNotification<TDeclaringType, TPropertyType>
    : IPropertyChangedNotification<TDeclaringType, TPropertyType>
{
    public PropertyChangedNotification(TDeclaringType declaringObject, 
        string propertyName, 
        TPropertyType oldValue, 
        TPropertyType newValue)
    {
        DeclaringObject = declaringObject;
        PropertyName = propertyName;
        OldValue = oldValue;
        NewValue = newValue;
    }
    public TDeclaringType DeclaringObject { get; set; }
    public string PropertyName { get; set; }
    public TPropertyType OldValue { get; protected set; }
    public TPropertyType NewValue { get; protected set; }
}
public interface IPropertyChangedNotification<TDeclaringType, out TPropertyType>
{
    TDeclaringType DeclaringObject { get; set; }
    string PropertyName { get; set; }
    TPropertyType OldValue { get; }
    TPropertyType NewValue { get; }
}

PS:这不是生产代码。只是练习而已。

Answer 1

只有引用类型才支持协方差和逆变差（要将值类型转换为Object，需要将其装箱）。

因此，您需要将T约束为class:

void OnPropertyChanged<T>(string propertyName, T oldValue, T newValue)
    where T : class { ... }

或者，您可以只使用new PropertyChangedNotification<Student, object>()。

第三种选择是有一个没有TProperty:的接口

public interface IPropertyChangedNotification<TDeclaringType>
{
    TDeclaringType DeclaringObject { get; set; }
    string PropertyName { get; set; }
}
public interface IPropertyChangedNotification<TDeclaringType, out TPropertyType>
    :  IPropertyChangedNotification<TDeclaringType>
{
    TPropertyType OldValue { get; }
    TPropertyType NewValue { get; }
}

然后在Subject中使用它（因为在订阅时无论如何都必须将它强制转换为具体类型）：

ISubject<IPropertyChangedNotification<Student>>