如何告诉C＃在对象的基类中查找属性？

Question

I'm getting the error " T does not contain a definition for Id " below in the specified line, even though when I debug, I see that "item" does indeed have a property "Id" in its base class .

How do I specify here that I want C# to look in the item's base class for Id (and why doesn't it do this automatically?)?

//public abstract class Items<T> : ItemBase (causes same error)
public abstract class Items<T> where T : ItemBase
{
    public List<T> _collection = new List<T>();
    public List<T> Collection
    {
        get
        {
            return _collection;
        }
    }

    public int GetNextId()
    {
        int highestId = 0;
        foreach (T item in _collection)
        {
           //ERROR: "T does not contain a definition for Id
           if (item.Id > highestId) highestId = item.Id; 
        }

        return highestId;
    }

}

Here's how the classes are being defined:

public class SmartForm : Item
{
    public string IdCode { get; set; }
    public string Title { get; set; }
    public string Description { get; set; }
    public int LabelWidth { get; set; }
    public int FormWidth { get; set; }
    ...


public abstract class Item : ItemBase
{
    public int Id { get; set; }
    public DateTime WhenCreated { get; set; }
    public string ItemOwner { get; set; }
    public string PublishStatus { get; set; }
    public int CorrectionOfId { get; set; }
    ...

Answer 1

Your problem is that there is no constraint on T, so, at compile time, all the compiler knows is that T is some sort of object. If you know what type T will always inherit off, you can add a generic constraint to the class definition:

public abstract class Items<T> : ItemBase where T : Item
{
//....
}

When you're debugging, T is instantiated to an Item (or subclass of) that does have an Id property, but the compiler doesn't know that at compile time, hence the error.

Answer 2

You got your generic constraint wrong.

public abstract class Items<T> : ItemBase

should be:

public abstract class Items<T> where T : ItemBase

What happened is that while your class has a collection of items, your ItemBase is not associated with T

Answer 3

Because T could be absolutely anything, and C# is meant to be type-safe. Use a constraint on T to a type which has an id property, and you should be OK. See here for more info.

Answer 4

您必须使用where子句指定T始终从ItemBase派生。

Answer 5

You accidentally let Items inherit ItemBase, instead of T inheriting ItemBase. Just change

: ItemBase

to

where T : ItemBase

Answer 6

this might help :-)

   foreach (ItemBase item in _collection)
    {
       if (item.Id > highestId) highestId = (Item)item.Id; 
    }