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逆变不起作用

contravariance asp.net c#

2

Paul · 技术社区 · 15 年前

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);
}

到目前为止一切正常,但为什么会创建null?

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

1 回复 | 直到 15 年前

1

2

Darin Dimitrov 15 年前

你不能有 dataObject

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();
    }
}

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