Expression<Action<int>> actionExpression = n => Console.WriteLine(n);
 Expression<Func<int, bool>> funcExpression1 = (n) => n < 0;
 Expression<Func<int, int, bool>> funcExpression2 = (n, m) => n - m == 0;

//通過 Expression 類創(chuàng)建表達(dá)式樹
 // lambda：num => num == 0
 ParameterExpression pExpression = Expression.Parameter(typeof(int)); //參數(shù)：num
 ConstantExpression cExpression = Expression.Constant(0); //常量：0
 BinaryExpression bExpression = Expression.MakeBinary(ExpressionType.Equal, pExpression, cExpression); //表達(dá)式：num == 0
 Expression<Func<int, bool>> lambda = Expression.Lambda<Func<int, bool>>(bExpression, pExpression); //lambda 表達(dá)式：num => num == 0

Expression<Func<int, bool>> funcExpression = num => num == 0;
 //開始解析
 ParameterExpression pExpression = funcExpression.Parameters[0]; //lambda 表達(dá)式參數(shù)
 BinaryExpression body = (BinaryExpression)funcExpression.Body; //lambda 表達(dá)式主體：num == 0
 Console.WriteLine($"解析：{pExpression.Name} => {body.Left} {body.NodeType} {body.Right}");

//創(chuàng)建表達(dá)式樹
 Expression<Func<string, int>> funcExpression = msg => msg.Length;
 //表達(dá)式樹編譯成委托
 var lambda = funcExpression.Compile();
 //調(diào)用委托
 Console.WriteLine(lambda("Hello, World!"));
 //語法簡化
 Console.WriteLine(funcExpression.Compile()("Hello, World!"));

const int n = 1;
 const int m = 2;
 //待執(zhí)行的表達(dá)式樹
 BinaryExpression bExpression = Expression.Add(Expression.Constant(n), Expression.Constant(m));
 //創(chuàng)建 lambda 表達(dá)式
 Expression<Func<int>> funcExpression = Expression.Lambda<Func<int>>(bExpression);
 //編譯 lambda 表達(dá)式
 Func<int> func = funcExpression.Compile();
 //執(zhí)行 lambda 表達(dá)式
 Console.WriteLine($"{n} + {m} = {func()}");

internal class Program
 {
 private static void Main(string[] args)
 {
 Expression<Func<int, bool>> funcExpression = num => num == 0;
 Console.WriteLine($"Source: {funcExpression}");
 var visitor = new NotEqualExpressionVisitor();
 var expression = visitor.Visit(funcExpression);
 Console.WriteLine($"Modify: {expression}");
 Console.Read();
 }
 /// <summary>
 /// 不等表達(dá)式樹訪問器
 /// </summary>
 public class NotEqualExpressionVisitor : ExpressionVisitor
 {
 public Expression Visit(BinaryExpression node)
 {
 return VisitBinary(node);
 }
 protected override Expression VisitBinary(BinaryExpression node)
 {
 return node.NodeType == ExpressionType.Equal
  ? Expression.MakeBinary(ExpressionType.NotEqual, node.Left, node.Right) //重新弄個(gè)表達(dá)式：用 != 代替 ==
  : base.VisitBinary(node);
 }
 }
 }

 ParameterExpression pExpression1 = Expression.Parameter(typeof(string));
 ParameterExpression pExpression2 = Expression.Parameter(typeof(string), "msg");

 const int num1 = 250;
 const float num2 = 250;
 ConstantExpression cExpression1 = Expression.Constant(num1);
 ConstantExpression cExpression2 = Expression.Constant(num2);

 Expression lambda1 = Expression.Lambda<Func<int>>(Expression.Constant(250));
 Expression lambda2 = Expression.Lambda<Func<int>>(Expression.Constant(250), "CustomName", null);