C Delegates 委托

C# Delegates 委托

声明委托

根据定义委托的可见性和委托的作用域，可以在委托的定义上应用任意常见的访问修饰符：public，private，protected。

使用委托

namespace Delegates { 
    class Program { 
    // 声明委托 private delegate string GetString(); static void Main(string[] args) { 
    // 定义一个委托的实例getString GetString getString = null; int i = 99; // 给委托赋值 getString = i.ToString; Console.WriteLine($"{getString()}"); } } } 

在给委托赋值时，这个方法必须匹配委托声明时的签名，在调用时使用了类似方法的getString()，C#编译器会用getString.Invoke()代替getString()。

Action< T >和Func< T >委托

 class Program { 
    private delegate double DoubleOp(double x); static void Main(string[] args) { 
    DoubleOp[] operations = { 
    MathOpertions.MuliplyByTwo,MathOpertions.Squate}; for(int i = 0; i < operations.Length; i++) { 
    ProcessAndDisplayNumble(operations[i], 2.0); ProcessAndDisplayNumble(operations[i], 7.94); ProcessAndDisplayNumble(operations[i], 1.414); } Console.WriteLine($""); } static void ProcessAndDisplayNumble(DoubleOp action, double value) { 
    double result = action(value); Console.WriteLine($"value={value},operation result ={result}"); } } class MathOpertions { 
    public static double MuliplyByTwo(double value) => value * 2; public static double Squate(double value) => value * value; } 

上面列举了一个普通的委托实例，需要先声明一个委托，下面用Fun< double T , double T >修改上面的代码，不需要声明委托。

 static void Main(string[] args) { 
    Func<double,double>[] operations = { 
    MathOpertions.MuliplyByTwo,MathOpertions.Squate}; for(int i = 0; i < operations.Length; i++) { 
    ProcessAndDisplayNumble(operations[i], 2.0); ProcessAndDisplayNumble(operations[i], 7.94); ProcessAndDisplayNumble(operations[i], 1.414); } Console.WriteLine($""); } static void ProcessAndDisplayNumble(Func<double, double> action, double value) { 
    double result = action(value); Console.WriteLine($"value={value},{action.ToString()} result ={result}"); } } class MathOpertions { 
    public static double MuliplyByTwo(double value) => value * 2; public static double Squate(double value) => value * value; } 

多播委托

以上每个委托都只包含一个方法调用，调用委托的次数和调用 方法的次数相同。委托可以包含多个方法，如果调用多播委托，就会按照顺序连续调用多播委托中的方法。多播委托可以识别‘+’、‘+=’、‘-’、‘-=’。

 class Program { 
    static void Main(string[] args) { 
    Action<double> operations = null; operations = MathOpertions.MuliplyByTwo; operations += MathOpertions.Square; double[] arr = new double[]{ 
    2.0, 7.94, 1.414 }; foreach(var item in arr) { 
    operations(item); } Console.WriteLine($"Hello World"); } } class MathOpertions { 
    public static void MuliplyByTwo(double value) { 
    double result = value * 2; Console.WriteLine($"MuliplyByTwo value={value} reult={result}"); } public static void Square(double value) { 
    double result = value * 2; Console.WriteLine($"Square value={value} reult={result}"); } } 

使用多播委托会有一个问题：通过委托调用的其中一个方法抛出异常，整个迭代就会停止。如下代码所示，在方法中假装抛出异常：

 class Program { 
    static void Main(string[] args) { 
    Action<double> operations = null; operations = MathOpertions.MuliplyByTwo; operations += MathOpertions.Square; double[] arr = new double[]{ 
    2.0, 7.94, 1.414 }; foreach(var item in arr) { 
    operations(item); } Console.WriteLine($"Hello World"); } } class MathOpertions { 
    public static void MuliplyByTwo(double value) { 
    double result = value * 2; Console.WriteLine($"MuliplyByTwo value={value} reult={result}"); throw new Exception("this is a Exception"); } public static void Square(double value) { 
    double result = value * 2; Console.WriteLine($"Square value={value} reult={result}"); } } 

解决这个问题：使用try…catch，抛出异常后循环继续

class Program { 
    static void Main(string[] args) { 
    Action<double> operations = null; operations = MathOpertions.MuliplyByTwo; operations += MathOpertions.Square; double[] arr = new double[]{ 
    2.0, 7.94, 1.414 }; foreach(var item in arr) { 
    try { 
    operations(item); }catch(Exception e) { 
    Console.WriteLine(e.Message); } } Console.WriteLine($"Hello World"); } } class MathOpertions { 
    public static void MuliplyByTwo(double value) { 
    double result = value * 2; Console.WriteLine($"MuliplyByTwo value={value} reult={result}"); throw new Exception("this is a Exception"); } public static void Square(double value) { 
    double result = value * 2; Console.WriteLine($"Square value={value} reult={result}"); } } 

虽然循环继续，但是每次只执行第一个方法，因为第一个方法抛出异常后委托的迭代会停止。解决这个问题,需要自己迭代方法列表：

 class Program { 
    static void Main(string[] args) { 
    Action<double> operations = null; operations = MathOpertions.MuliplyByTwo; operations += MathOpertions.Square; Delegate[] delegates = operations.GetInvocationList(); double[] arr = new double[]{ 
    2.0, 7.94, 1.414 }; foreach(var item in arr) { 
    foreach(Action<double> d in delegates) { 
    try { 
    d(item); } catch (Exception e) { 
    Console.WriteLine(e.Message); } } } Console.WriteLine($"Hello World"); } } class MathOpertions { 
    public static void MuliplyByTwo(double value) { 
    double result = value * 2; Console.WriteLine($"MuliplyByTwo value={value} reult={result}"); throw new Exception("this is a Exception"); } public static void Square(double value) { 
    double result = value * 2; Console.WriteLine($"Square value={value} reult={result}"); } }