逆變不起作用

Question

public interface IMyControl<in T> where T : ICoreEntity 
{ 
    void SetEntity(T dataObject); 
} 

public class MyControl : UserControl, IMyControl<DataObject> // DataObject implements ICoreEntity 
{ 
    void SetEntity(T dataObject); 
} 

目前爲止還好，但爲什麼會創建null？

var control = LoadControl("~/Controls/MyControl.ascx"); // assume this line works 
IMyControl<ICoreEntity> myControl = control; 

myControl現在是空...

Answer 1

你不能有dataObject作爲參數，這個工作。方法只能返回它。

public interface ICoreEntity { } 
public class DataObject: ICoreEntity { } 

public interface IMyControl<out T> where T : ICoreEntity 
{ 
    T GetEntity(); 
} 

public class MyControl : IMyControl<DataObject> // DataObject implements ICoreEntity 
{ 
    public DataObject GetEntity() 
    { 
     throw new NotImplementedException(); 
    } 
} 

現在，您可以：

MyControl control = new MyControl(); 
IMyControl<ICoreEntity> myControl = control;