c# Delegate、Predicate、Func、Action、Expression 笔记

public void Test() {
    //泛型静态成员共享,aa.Count=bb.Count
    var aa = new MyList<int>();
    var bb = new MyList<int>();
    var cc = new MyList<string>();

    //返回int
    Func<int, int, int> add = Add;
    //无返回
    Action<string> print = Print;
    print(add(1, 2).ToString());

    //Predicate,返回bool型的泛型委托
    Point[] points = { new Point(100, 200), new Point(150, 250), new Point(250, 375), new Point(275, 395), new Point(295, 450) };
    Point first = Array.Find(points, ProductGT10);

    //委托1
    var add2 = new Func<int, int, int>(delegate (int x, int y) { return x + y; });
    var print2 = new Action<string>(delegate (string msg) { Console.WriteLine(msg); });
    print2(add2(22, 33).ToString());

    //委托2
    Func<int, int, int> add3 = delegate (int x, int y) { return x + y; };
    Action<string> print3 = delegate (string msg) { Console.WriteLine(msg); };
    print3(add3(33, 44).ToString());

    //Lambda表达式
    Func<int, int, int> add4 = (x, y) => { return x + y; };
    Action<string> print4 = (msg) => { Console.WriteLine(msg); };
    Action<string> print5 = msg => { Console.WriteLine(msg); };//单个参数可以省略"()"
    print4(add4(44, 55).ToString());
    print5(add4(55, 66).ToString());

    var students = new List<Student>();
    var name = "";
    //第一种
    var newaa = students.Find(new Predicate<Student>(delegate (Student student) {
        if (student.Name == name) return true;
        return false;
    }));
    //第二种
    newaa = students.Find(new Predicate<Student>((student) => {
        if (student.Name == name) return true;
        return false;
    }));
    //第三种
    newaa = students.Find((student) => {
        if (student.Name == name) return true;
        return false;
    });
    //第四种
    newaa = students.Find((student) => student.Name == name);

    //表达式树
    var para1 = Expression.Parameter(typeof(int), "x");
    var para2 = Expression.Parameter(typeof(int), "y");
    var paras = new ParameterExpression[] { para1, para2 };
    var body = Expression.Add(para1, para2);
    var expression = Expression.Lambda(body, paras);
    var func = expression.Compile() as Func<int, int, int>;
    var res = func(1, 2);
}

public static int Add(int i, int j) {
    return i + j;
}

public static void Print(string msg) {
    Console.WriteLine(msg);
}

private static bool ProductGT10(Point p) {
    if (p.X * p.Y > 100000) {
        return true;
    } else {
        return false;
    }
}

public class MyList<T> {
    T t = default(T);

    public static int Count;
    public MyList() {
        Count++;
    }
}