C # knowledge points-Delegates

First, what is a delegate

Delegates, like classes, are user-defined types;

A class represents a collection of data and methods, while a delegate holds one or more methods;

Ii. use of Delegates 1. Declaring delegate types

A delegate is a type, and as with a class, a delegate type must be declared before it is used to create the variable and the object of the type;

Attention:

Start with the delegate keyword;

no method body;

2. Create a Delegate object

A delegate is a reference type, so there are references and objects;

Method One: Use an object with the new operator to create an expression

Method Two: An implicit conversion between a shortcut, a method name, and its corresponding delegate type

3. Assigning a value to a delegate

Because the delegate is a reference type, the reference that is contained in the delegate variable is changed by assigning a value. The old delegate object will be reclaimed by the garbage collector

4. Combination of Delegates

Attention:

The delegate is constant, and the delegate object cannot be changed after it is created

5. Adding a method to a delegate

Note: To add the result of a method to a delegate,

In fact, since the delegate type is immutable, all the results of adding 3 methods to the invocation list of the delegate are actually variables that point to a completely new delegate

6. Removing a method from a delegate

Attention:

If there are multiple instances of a method in the invocation list, the-= operator will search from the last list and remove the first instance that matches the method;

Attempts to delete a method that does not exist in the delegate have no effect;

Attempting to delete an empty delegate throws an exception. We can judge whether the invocation list of a delegate is empty by a delegate and a null comparison. If the invocation list is empty, the delegate is null

7. Invoking delegates

You can invoke a delegate just as you call a method

Attention:

Invoking a delegate with a parameter invokes each member of its invocation list with the same parameter value;

If a method appears more than once in the invocation list, it is called once each time the method is encountered in the list when the delegate is called.

7. Calling a delegate with a return value

Analysis

Attention:

The value returned by the last method in the invocation list is the value returned by the delegate invocation;

The return value of all other methods in the invocation list is ignored

8. Calling a delegate with reference parameters

Attention:

If the delegate has a reference parameter, the parameter value changes based on the return value of one or more methods in the invocation list;

When the next method in the delegate list is called, the new value of the parameter is passed to the next method;

C # knowledge points-Delegates