c# - 如何在 C# 中声明委托实例？

Question

private delegate void stopMachineryDelegate();
public stopMachineryDelegate StopMachinery;

this.StopMachinery += new stopMachineryDelegate(painting);

在上面第二行的例子中，我们是创建一个委托实例还是一个委托变量？如果第二行创建了 stopMachineryDelegate 类型的委托实例，那么在第三行中我们在做什么？

Answer 1

On the second line you are declaring a variable of type stopMachineryDelegate. But at this point the variable is still unassigned having holding a default value of null.

On the third line you are assigning a value to this variable by creating a new instance of the delegate pointing to the painting function.

So once this variable is assigned you could use it to invoke the function it is pointing to:

this.StopMachinery();

which will basically invoke the painting method in the same class.

Answer 2

First line is used to define your delegate type (stopMachineryDelegate), as a delegate accepting no parameters and returning no values (void).

Second line is declaring a field of that type, named StopMachinery. At that point, StopMachinery is null.

The third line has some syntactic sugar behind it. If StopMachinery is null at that point, it will create a new instance of that MulticastDelegate, and then add the painting method delegate to its invocation list.

If you only wanted to assign a single delegate to that field, you could have simply written:

 // implicitly wrap the `painting` method into a new delegate and 
 // assign to the `StopMachinery` field
 this.StopMachinery = painting;  

On the other hand, using += allows you to specify a list of delegates to be invoked when you invoke StopMachinery:

 this.StopMachinery += painting;  
 this.StopMachinery += someOtherMethod;  
 this.StopMachinery += yetAnotherMethod;  

In the latter case, invocation of the StopMachinery delegate will invoke each of the methods in the invocation list, synchronously, in the order they were specified.