CExpression Trees 详解

摘要：( add, new ParameterExpression { x, y } ); // 编译并执行 var compiled = addExpr.Compile; int result = compiled(3, 4); Console.WriteLine

表达式树(Expression Trees)是一种以树形数据结构表示代码的方式，其中每个节点都是一个表达式。它允许我们在运行时检查、修改和执行代码。

using System.Linq.Expressions;namespace AppExpressionTrees{internal class Program{static void Main{Expressionbool>> lambda1 = num => num lambda2 = Expression.Lambda>( lessThan, new ParameterExpression { numParam } ); Console.WriteLine(lambda2.Compile.Invoke(4)); Console.ReadKey; } } }

以上两种方式是一回事。

using System.Linq.Expressions;namespace AppExpressionTrees{internal class Program{static void Main{ParameterExpression x = Expression.Parameter(typeof(int), "x");ParameterExpression y = Expression.Parameter(typeof(int), "y");BinaryExpression add = Expression.Add(x, y);Expression> addExpr = Expression.Lambda>( add, new ParameterExpression { x, y } ); // 编译并执行 var compiled = addExpr.Compile; int result = compiled(3, 4); Console.WriteLine(result); Console.ReadKey; } } }

using System.Linq.Expressions;namespace AppExpressionTrees{internal class Program{static void Main{// 1. 常量表达式ConstantExpression constant = Expression.Constant(42);Console.WriteLine(constant.Value);// 2. 参数表达式ParameterExpression param = Expression.Parameter(typeof(string), "str");Console.WriteLine(param.Name);// 3. 方法调用表达式MethodCallExpression methodCall = Expression.Call(typeof(Console).GetMethod("WriteLine", new { typeof(string) }),Expression.Constant("Hello"));var compiledLambda = Expression.Lambda(methodCall).Compile; compiledLambda.Invoke; // 4. 条件表达式 Expression condition = Expression.Condition( Expression.Constant(true), Expression.Constant("Yes"), Expression.Constant("No") ); Console.WriteLine(condition.Reduce.ToString); Console.ReadKey; } } } using System;using System.Linq;using System.Linq.Expressions;using System.Collections.Generic;namespace AppExpressionTrees{public class Product{public int Id { get; set; }public string Name { get; set; }public decimal Price { get; set; }}public class QueryBuilder{public static Expression( Listconditions, bool isAndOperator = true) { if (conditions == null || conditions.Count == 0) throw new ArgumentException("条件不能为NULL"); var parameter = Expression.Parameter(typeof(T), "x"); Expression combinedExpression = null; foreach (var condition in conditions) { var property = Expression.Property(parameter, condition.PropertyName); var constant = Expression.Constant(condition.Value); Expression comparison = condition.Operation.ToLower switch { "equals" => Expression.Equal(property, constant), "greaterthan" => Expression.GreaterThan(property, constant), "lessthan" => Expression.LessThan(property, constant), "contains" => CreateContainsExpression(property, constant), "startswith" => CreateStartsWithExpression(property, constant), "endswith" => CreateEndsWithExpression(property, constant), _ => throw new ArgumentException($"Invalid operation: {condition.Operation}") }; // 组合条件 if (combinedExpression == null) { combinedExpression = comparison; } else { combinedExpression = isAndOperator ? Expression.AndAlso(combinedExpression, comparison) : Expression.OrElse(combinedExpression, comparison); } } return Expression.Lambda>(combinedExpression, parameter); } // 处理字符串 Contains private static Expression CreateContainsExpression(Expression property, Expression constant) { var containsMethod = typeof(string).GetMethod("Contains", new { typeof(string) }); return Expression.Call(property, containsMethod, constant); } // 处理字符串 StartsWith private static Expression CreateStartsWithExpression(Expression property, Expression constant) { var startsWithMethod = typeof(string).GetMethod("StartsWith", new { typeof(string) }); return Expression.Call(property, startsWithMethod, constant); } // 处理字符串 EndsWith private static Expression CreateEndsWithExpression(Expression property, Expression constant) { var endsWithMethod = typeof(string).GetMethod("EndsWith", new { typeof(string) }); return Expression.Call(property, endsWithMethod, constant); } public static ListApplyFilter> filter) { return list.Where(filter.Compile).ToList; } } // 新增过滤条件类 public class FilterCondition { public string PropertyName { get; set; } public string Operation { get; set; } public object Value { get; set; } } internal class Program { static void Main { // 创建测试数据 var products = new List{ new Product { Id = 1, Name = "Laptop Product", Price = 120.50m }, new Product { Id = 2, Name = "Smartphone", Price = 80.00m }, new Product { Id = 3, Name = "Tablet Product", Price = 200.75m }, new Product { Id = 4, Name = "Headphones", Price = 50.25m }, new Product { Id = 5, Name = "Smart Watch Product", Price = 150.00m } }; // 单个条件 - 价格大于100 var priceFilter = QueryBuilder.BuildFilter(new List{ new FilterCondition { PropertyName = "Price", Operation = "greaterthan", Value = 100m } }); var expensiveProducts = QueryBuilder.ApplyFilter(products, priceFilter); Console.WriteLine("产品价格 > 100 的商品:"); PrintProducts(expensiveProducts); // 多个条件 AND 操作 - 价格大于100且名称包含"Product" var multiAndFilter = QueryBuilder.BuildFilter(new List{ new FilterCondition { PropertyName = "Price", Operation = "greaterthan", Value = 100m }, new FilterCondition { PropertyName = "Name", Operation = "contains", Value = "Product" } }); var expensiveProductsWithName = QueryBuilder.ApplyFilter(products, multiAndFilter); Console.WriteLine("\n价格 > 100 且名称包含 'Product' 的商品:"); PrintProducts(expensiveProductsWithName); // 多个条件 OR 操作 - 价格大于100或名称包含"Product" var multiOrFilter = QueryBuilder.BuildFilter(new List{ new FilterCondition { PropertyName = "Price", Operation = "greaterthan", Value = 100m }, new FilterCondition { PropertyName = "Name", Operation = "contains", Value = "Product" } }, isAndOperator: false); var productsMatchingEitherCondition = QueryBuilder.ApplyFilter(products, multiOrFilter); Console.WriteLine("\n价格 > 100 或名称包含 'Product' 的商品:"); PrintProducts(productsMatchingEitherCondition); Console.ReadKey; } // 打印产品列表 static void PrintProducts(List

products) { if (products == null || products.Count == 0) { Console.WriteLine("没有找到匹配的产品"); return; } foreach (var product in products) { Console.WriteLine($"ID: {product.Id}, 名称: {product.Name}, 价格: {product.Price:C}"); } } } }