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  • 关于表达式树

    我这篇写的应该比脑袋好。

    class Program
    {
        static void Main(string[] args)
        {
            Test();
        }
    
        //简单类型 int
        public static void Test()
        {
            List<int> list = new List<int>() { 1, 2, 3, 4 };
    
            ParameterExpression parameter = Expression.Parameter(typeof(int), "p");
    
            ExpressionType op = ExpressionType.Equal;
            var para2 = Expression.Constant(1);
            var result2 = Expression.MakeBinary(op, parameter, para2);
    
            var expression = Expression.Lambda(result2, parameter) as Expression<Func<int, bool>>;
    
            var newList = list.Where(expression.Compile());
        }
    
        //复杂类型
        public static void Test1()
        {
            List<ClassA> list = new List<ClassA>();
            list.Add(new ClassA() { A1 = 1, A2 = "2" });
            list.Add(new ClassA() { A1 = 11, A2 = "22" });
    
            ParameterExpression parameter = Expression.Parameter(typeof(ClassA), "p");
    
            var left = Expression.PropertyOrField(parameter, "A1");
            var right = Expression.Constant(1);
            ExpressionType op = ExpressionType.GreaterThan;
    
            var result = Expression.MakeBinary(op, left, right);
    
            var expression = Expression.Lambda(result, parameter) as Expression<Func<ClassA, bool>>;
    
            var newList = list.Where(expression.Compile());
        }
    
        //拼接2个条件
        public static void Test2()
        {
            List<ClassA> list = new List<ClassA>();
            list.Add(new ClassA() { A1 = 1, A2 = "2" });
            list.Add(new ClassA() { A1 = 11, A2 = "22" });
            list.Add(new ClassA() { A1 = 122, A2 = "122" });
    
            ParameterExpression parameter = Expression.Parameter(typeof(ClassA), "p");
    
            var left = Expression.PropertyOrField(parameter, "A1");
            var right = Expression.Constant(1);
            ExpressionType op = ExpressionType.GreaterThan;
    
            var result = Expression.MakeBinary(op, left, right);
    
    
            right = Expression.Constant(100);
            op = ExpressionType.LessThan;
    
            var result2 = Expression.MakeBinary(op, left, right);
    
            ExpressionType et = ExpressionType.And;
            var newResult = Expression.MakeBinary(et, result, result2);
    
            var expression = Expression.Lambda(newResult, parameter) as Expression<Func<ClassA, bool>>;
    
            var newList = list.Where(expression.Compile());
        }
    
        //调用自定义方法
        public static void Test3()
        {
            List<ClassA> list = new List<ClassA>();
            list.Add(new ClassA() { A1 = 11, A2 = "2" });
            list.Add(new ClassA() { A1 = 11, A2 = "22" });
            list.Add(new ClassA() { A1 = 122, A2 = "122" });
    
            ParameterExpression parameter = Expression.Parameter(typeof(ClassA), "p");
    
            var para1 = Expression.PropertyOrField(parameter, "A2");
    
            MethodCallExpression expCall = Expression.Call(parameter,
                typeof(ClassA).GetMethod("TestMethod"), para1);
    
            ExpressionType op = ExpressionType.Equal;
            var para2 = Expression.Constant(true);
            var result2 = Expression.MakeBinary(op, expCall, para2);
    
            var expression = Expression.Lambda(result2, parameter) as Expression<Func<ClassA, bool>>;
    
            var newList = list.Where(expression.Compile());
        }
    }    
    
    public class ClassA
    {
        public int A1 { get; set; }
        public string A2 { get; set; }
    
        public bool TestMethod(string pa)
        {
            return pa == "22";
        }
    }
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  • 原文地址:https://www.cnblogs.com/Jax/p/1686594.html
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