Linq Expression

        static Expression<Func<T, bool>> Lambda<T>() where T : class
        {
            //添加表达式参数, (p) =>  p.ParentId == 0 表达式中的参数 p, p 为可自定义参数名称
            ParameterExpression[] parameters = { Expression.Parameter(typeof(T), "p") }; 
            //定义表达式左侧内容, (p) =>  p.ParentId == 0 表达式中 p.ParentId, 
            //传入表达式参数, 反射拿到的属性, typeof(T).GetProperty("ParentId").GetGetMethod() 可直接写 "ParentId"
            var member = Expression.Property(parameters[0], typeof(T).GetProperty("ParentId").GetGetMethod());
            //定义表达式左侧内容, (p) =>  p.ParentId == 0 表达式中 0,  常量内容
            ConstantExpression constant = Expression.Constant(0, typeof(int));
            //Expression.Equal 定义等号, parameters传入表达式参数 p 
            return Expression.Lambda<Func<T, bool>>(Expression.Equal(member, constant), parameters);
      }

//调用lambda查询
itemList.Where(Lambda<T>().Compile()).FirstOrDefault();

另一个调用方法的例子:

using System.Collections.Generic;
using System.Linq;
using System.Linq.Expressions;
using System.Reflection;
using System.Text;
using System.Threading.Tasks;

namespace ExpressionLambda
{
    class Program
    {
        static void Main(string[] args)
        {
            Exec1();
            Exec2();
            Console.Read();
        }

        static void Exec1()
        {
            var a = Expression.Constant(1);
            var b = Expression.Constant(2);

            var add = Expression.Add(a, b);

            //创建 lambda 表达式
            Expression<Func<int>> funcExpression = Expression.Lambda<Func<int>>(add);
            //编译 lambda 表达式
            Func<int> func = funcExpression.Compile();

            Console.WriteLine(func());
        }

        static void Exec2()
        {
            var b = Expression.Constant(2);

            var intObj = new IntObj()
            {
                Value = 10
            };
            ParameterExpression expA = Expression.Parameter(typeof(IntObj), "a"); //参数a
            MethodCallExpression expCall = Expression.Call(null,
                typeof(Program).GetMethod("GetIntObjValue", BindingFlags.Static | BindingFlags.Public),
                expA); //Math.Sin(a)


            //Expression<Func<IntObj, int>> exp = Expression.Lambda<Func<IntObj, int>>(expCall, expA);
            var add = Expression.Add(expCall, b);

            //创建 lambda 表达式
            Expression<Func<IntObj,int>> funcExpression = Expression.Lambda<Func<IntObj,int>>(add,expA);
            //编译 lambda 表达式
            Func<IntObj,int> func = funcExpression.Compile();


            Console.WriteLine(func(intObj));
        }

        public static int GetIntObjValue(IntObj intObj)
        {
            return intObj.Value;
        }
    }

    public class IntObj
    {
        public int Value { get; set; }
    }


}