所以我有一个委托定义为:
public delegate void MyDelegate<T>(T myParameter);
Resharper建议我按照以下方式制作T
逆变器:
public delegate void MyDelegate<in T>(T myParameter);
现在,我很难理解这有什么用处?我知道它阻止我使T
成为一种返回类型,但除此之外,通过使T
逆变而得到什么有用的约束?也就是说,当需要将委托与实例一起使用时,我可以使用
public delegate void MyDelegate<T>(T myParameter);
我无法使用
创建public delegate void MyDelegate<in T>(T myParameter);
答案 0 :(得分:6)
以下是一个实例,如果删除逆变in
标记,它将无法编译:
delegate void Callback<in T>(T t);
public Form1()
{
InitializeComponent();
Callback<Control> showText = control => MessageBox.Show(control.Text);
var button = new Button();
AddButtonClickCallback(button, showText);
var label = new Label();
AddLabelClickCallback(label, showText);
}
static void AddButtonClickCallback(Button button, Callback<Button> callback)
{
button.Click += delegate { callback(button); };
}
static void AddLabelClickCallback(Label label, Callback<Label> callback)
{
label.Click += delegate { callback(label); };
}
有点做作,当然,但至少应该让你知道没有它你不能做的事情。
特别要考虑AddLabelClickCallback
和AddButtonClickCallback
是否为库函数,Callback
是否为库委托。如果它是在没有逆变的情况下定义的,那么你必须定义不同的代理showButtonText
和showLabelText
,即使你只是希望他们做同样的事情。
答案 1 :(得分:4)
泛型上的in
关键字允许进行隐式转换。基本上,您可以为委托分配较少的派生委托类型...并不总是有用,请阅读此处以获取更多信息。
http://msdn.microsoft.com/en-us/library/dd469484.aspx
示例,来自MSDN文章:
// Contravariant delegate.
public delegate void DContravariant<in A>(A argument);
// Methods that match the delegate signature.
public static void SampleControl(Control control)
{ }
public static void SampleButton(Button button)
{ }
public void Test()
{
// Instantiating the delegates with the methods.
DContravariant<Control> dControl = SampleControl;
DContravariant<Button> dButton = SampleButton;
// You can assign dControl to dButton
// because the DContravariant delegate is contravariant.
dButton = dControl;
// Invoke the delegate.
dButton(new Button());
}
在该示例中,Control
将隐式转换为Button
类型,但如果Button
中未定义隐式转换为Button
类型,则可能会发生异常}}允许Control
成为Button
。
有关隐式转化定义的更多信息:http://msdn.microsoft.com/en-us/library/z5z9kes2.aspx
答案 2 :(得分:1)
在stackoverflow上已经多次询问过这个问题: Covariance and contravariance in programming languages
Covariance and Contravariance in C#, Part One
Covariance and Contravariance in C#, Part Two: Array Covariance
Covariance and Contravariance in C#, Part Three: Method Group Conversion Variance
Covariance and Contravariance in C#, Part Four: Real Delegate Variance
Covariance and Contravariance In C#, Part Five: Higher Order Functions Hurt My Brain
Covariance and Contravariance in C#, Part Six: Interface Variance
Covariance and Contravariance in C# Part Seven: Why Do We Need A Syntax At All?
Covariance and Contravariance in C#, Part Eight: Syntax Options
Covariance and Contravariance in C#, Part Nine: Breaking Changes
Covariance and Contravariance in C#, Part Ten: Dealing With Ambiguity