C#表达式树的基本用法讲解

Expression<Func<int, bool>> lambda = num => num < 5; 

ParameterExpression numPara = Expression.Parameter(typeof(int), "num");//参数num
ConstantExpression five = Expression.Constant(5, typeof(int));//常数5

BinaryExpression numLessThanFive = Expression.LessThan(numPara, five);
Expression<Func<int, bool>> lambda1 = Expression.Lambda<Func<int, bool>>(numLessThanFive, new ParameterExpression[] { numPara });

//参数value
ParameterExpression value = Expression.Parameter(typeof(int), "value");
//本地变量
ParameterExpression result = Expression.Parameter(typeof(int), "result");
//标签，用来跳出循环
LabelTarget label = Expression.Label(typeof(int));
//创建表达式块
BlockExpression block = Expression.Block(
 //添加本地参数result
 new[] { result },
 //result=1 赋值
 Expression.Assign(result, Expression.Constant(1)),
 //循环
 Expression.Loop(
  //循环条件
  Expression.IfThenElse(
   //如果 value>1
   Expression.GreaterThan(value, Expression.Constant(1)),
   //则 result*=value--;
   Expression.MultiplyAssign(result, Expression.PostDecrementAssign(value)),
   //否则跳出loop循环。跳到label的语句执行
   Expression.Break(label, result)
   ),
   label
  )
);
//编译表达式树
Func<int, int> factor = Expression.Lambda<Func<int, int>>(block, value).Compile();
//执行，输出结果120
Console.WriteLine(factor(5));

Expression<Func<int, bool>> expreTree = num => num < 5;

ParameterExpression param = (ParameterExpression)expreTree.Parameters[0];//num
BinaryExpression operation = (BinaryExpression)expreTree.Body;//<
ParameterExpression left = (ParameterExpression)operation.Left;//num
ConstantExpression right = (ConstantExpression)operation.Right;//5
                //output Decomposed expression: num => num LessThan 5 
Console.WriteLine("Decomposed expression:{0} = > {1} {2} {3}", param.Name, left.Name, operation.NodeType, right.Value);

// 创建表达式树
Expression<Func<int, bool>> expr = num => num < 5;
// 将表达式树编译成对应委托 
Func<int, bool> result = expr.Compile();
//调用委托方法，输出True
Console.WriteLine(result(4));
//也可以直接编译后调用，输出True
Console.WriteLine(expr.Compile()(4));

//创建表达式树的执行逻辑
BinaryExpression be = Expression.Power(Expression.Constant(2D), Expression.Constant(3D));
//创建表达式树
Expression<Func<double>> le = Expression.Lambda<Func<double>>(be);
//编译表达式树
Func<double> compileExpression = le.Compile();
//执行lambda表达式,获得结果8
double result = compileExpression();
Console.WriteLine(result);