Lambda 表达式的深化及使用
1.1 Lambda 表达式
1.1.1 创建委托或表达式树
“Lambda 表达式”是一个匿名函数，它可以包含表达式和语句，并且可用于创建委托或表达式树类型。
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class Program { static void Main(string[] args) { int j = myDelegate(5); //得到j = 25 Console.WriteLine(j); Console.Read(); } } |
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class Program { static void Main(string[] args) { Expression<del> myET = x => x * x; Console.WriteLine(myET);//得到x => ( x * x ) Console.Read(); } } delegate int del(int i); |
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class Program { static void Main(string[] args) { del myET = new del((x, y) => x + y); Console.WriteLine(myET(7,8));//15 Console.Read(); } } delegate int del(int i,int y); |
1.2 带有标准查询运算符的 Lambda
许多标准查询运算符都具有输入参数，其类型是泛型委托的 Func<T, TResult> 系列的其中之一。 Func<T, TResult> 委托使用类型参数定义输入参数的数目和类型，以及委托的返回类型。
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Func<int, bool> myFunc = x => x == 5; bool result = myFunc(6); //得到false Console.WriteLine(result); |
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int[] numbers = { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 }; int oddNumbers = numbers.Count(n => n % 2 == 1);//得到5(除2取余等于1的数字个数) Console.WriteLine(oddNumbers); |
1.3 Lambda 表达式创建表达式树
在将 lambda 表达式分配给 Expression<TDelegate> 类型的变量时，编译器将发出代码以生成一个表示 lambda 表达式的表达式树。
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Expression<Func<int, bool>> lambda = num => num < 5; Console.WriteLine(lambda);//得到num =>( num < 5 ) |
1.4 API 创建表达式树
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ParameterExpression numParam = Expression.Parameter(typeof(int), "num"); ConstantExpression five = Expression.Constant(5, typeof(int)); BinaryExpression numLessThanFive = Expression.LessThan(numParam, five); Expression<Func<int, bool>> lambda1 = Expression.Lambda<Func<int, bool>>( numLessThanFive, new ParameterExpression[] { numParam }); Console.WriteLine(numParam);//得到num Console.WriteLine(five);//5 Console.WriteLine(numLessThanFive);//得到( num < 5 ) |
1.5 分析表达式树
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Expression<Func<int, bool>> exprTree = num => num < 5; ParameterExpression param = (ParameterExpression)exprTree.Parameters[0]; BinaryExpression operation = (BinaryExpression)exprTree.Body; ParameterExpression left = (ParameterExpression)operation.Left; ConstantExpression right = (ConstantExpression)operation.Right; Console.WriteLine("Decomposed expression: {0} => {1} {2} {3}", param.Name, left.Name, operation.NodeType, right.Value); //得到Decomposed expression:num => num LessThan 5 |
1.6 编译表达式树
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Expression<Func<int, bool>> expr = num => num < 5; Func<int, bool> result = expr.Compile(); Console.WriteLine(result(4));//True Console.WriteLine(expr.Compile()(5));//False |
1.7 执行表达式
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BinaryExpression be = Expression.Power(Expression.Constant(2D), Expression.Constant(3D)); Expression<Func<double>> le = Expression.Lambda<Func<double>>(be); Func<double> compiledExpression = le.Compile(); double result = compiledExpression(); Console.WriteLine(result);//得到8 |
