使用Expression動態創建lambda表達式

一、前言

　　根據前端傳來的數據條件進行字段過濾時，如果使用判斷字段名再手寫lambda的方式，在條件多的情況下會非常繁瑣。所以尋找可以動態創建lamada的方式，直接根據傳來的字段進行過過濾，后端不用關心傳來的是啥字段，實現前后端解耦。

二、代碼

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

namespace Do.Common.Helper
{
    public class LambdaHelper
    {
        /// <summary>
        /// 創建lambda表達式：p=>true
        /// </summary>
        /// <typeparam name="T"></typeparam>
        /// <returns></returns>
        public static Expression<Func<T, bool>> True<T>()
        {
            return p => true;
        }

        /// <summary>
        /// 創建lambda表達式：p=>false
        /// </summary>
        /// <typeparam name="T"></typeparam>
        /// <returns></returns>
        public static Expression<Func<T, bool>> False<T>()
        {
            return p => false;
        }

        /// <summary>
        /// 創建lambda表達式：p=>p.propertyName
        /// </summary>
        /// <typeparam name="T"></typeparam>
        /// <typeparam name="TKey"></typeparam>
        /// <param name="sort"></param>
        /// <returns></returns>
        public static Expression<Func<T, TKey>> GetOrderExpression<T, TKey>(string propertyName)
        {
            ParameterExpression parameter = Expression.Parameter(typeof(T), "p");
            return Expression.Lambda<Func<T, TKey>>(Expression.Property(parameter, propertyName), parameter);
        }

        /// <summary>
        /// 創建lambda表達式：p=>p.propertyName == propertyValue
        /// </summary>
        /// <typeparam name="T"></typeparam>
        /// <param name="column"></param>
        /// <param name="value"></param>
        /// <returns></returns>
        public static Expression<Func<T, bool>> CreateEqual<T>(string propertyName, string propertyValue)
        {
            ParameterExpression parameter = Expression.Parameter(typeof(T), "p");//創建參數p
            MemberExpression member = Expression.PropertyOrField(parameter, propertyName);
            ConstantExpression constant = Expression.Constant(propertyValue);//創建常數
            return Expression.Lambda<Func<T, bool>>(Expression.Equal(member, constant), parameter);
        }

        /// <summary>
        /// 創建lambda表達式：p=>p.propertyName != propertyValue
        /// </summary>
        /// <typeparam name="T"></typeparam>
        /// <param name="propertyName"></param>
        /// <param name="propertyValue"></param>
        /// <returns></returns>
        public static Expression<Func<T, bool>> CreateNotEqual<T>(string propertyName, string propertyValue)
        {
            ParameterExpression parameter = Expression.Parameter(typeof(T), "p");//創建參數p
            MemberExpression member = Expression.PropertyOrField(parameter, propertyName);
            ConstantExpression constant = Expression.Constant(propertyValue);//創建常數
            return Expression.Lambda<Func<T, bool>>(Expression.NotEqual(member, constant), parameter);
        }

        /// <summary>
        /// 創建lambda表達式：p=>p.propertyName > propertyValue
        /// </summary>
        /// <typeparam name="T"></typeparam>
        /// <param name="column"></param>
        /// <param name="value"></param>
        /// <returns></returns>
        public static Expression<Func<T, bool>> CreateGreaterThan<T>(string propertyName, string propertyValue)
        {
            ParameterExpression parameter = Expression.Parameter(typeof(T), "p");//創建參數p
            MemberExpression member = Expression.PropertyOrField(parameter, propertyName);
            ConstantExpression constant = Expression.Constant(propertyValue);//創建常數
            return Expression.Lambda<Func<T, bool>>(Expression.GreaterThan(member, constant), parameter);
        }

        /// <summary>
        /// 創建lambda表達式：p=>p.propertyName < propertyValue
        /// </summary>
        /// <typeparam name="T"></typeparam>
        /// <param name="column"></param>
        /// <param name="value"></param>
        /// <returns></returns>
        public static Expression<Func<T, bool>> CreateLessThan<T>(string propertyName, string propertyValue)
        {
            ParameterExpression parameter = Expression.Parameter(typeof(T), "p");//創建參數p
            MemberExpression member = Expression.PropertyOrField(parameter, propertyName);
            ConstantExpression constant = Expression.Constant(propertyValue);//創建常數
            return Expression.Lambda<Func<T, bool>>(Expression.LessThan(member, constant), parameter);
        }

        /// <summary>
        /// 創建lambda表達式：p=>p.propertyName >= propertyValue
        /// </summary>
        /// <typeparam name="T"></typeparam>
        /// <param name="column"></param>
        /// <param name="value"></param>
        /// <returns></returns>
        public static Expression<Func<T, bool>> CreateGreaterThanOrEqual<T>(string propertyName, string propertyValue)
        {
            ParameterExpression parameter = Expression.Parameter(typeof(T), "p");//創建參數p
            MemberExpression member = Expression.PropertyOrField(parameter, propertyName);
            ConstantExpression constant = Expression.Constant(propertyValue);//創建常數
            return Expression.Lambda<Func<T, bool>>(Expression.GreaterThanOrEqual(member, constant), parameter);
        }

        /// <summary>
        /// 創建lambda表達式：p=>p.propertyName <= propertyValue
        /// </summary>
        /// <typeparam name="T"></typeparam>
        /// <param name="column"></param>
        /// <param name="value"></param>
        /// <returns></returns>
        public static Expression<Func<T, bool>> CreateLessThanOrEqual<T>(string propertyName, string propertyValue)
        {
            ParameterExpression parameter = Expression.Parameter(typeof(T), "p");//創建參數p
            MemberExpression member = Expression.PropertyOrField(parameter, propertyName);
            ConstantExpression constant = Expression.Constant(propertyValue);//創建常數
            return Expression.Lambda<Func<T, bool>>(Expression.LessThanOrEqual(member, constant), parameter);
        }

        /// <summary>
        /// 創建lambda表達式：p=>p.propertyName.Contains(propertyValue)
        /// </summary>
        /// <typeparam name="T"></typeparam>
        /// <param name="column"></param>
        /// <param name="value"></param>
        /// <returns></returns>
        private static Expression<Func<T, bool>> GetContains<T>(string propertyName, string propertyValue)
        {
            ParameterExpression parameter = Expression.Parameter(typeof(T), "p");
            MemberExpression member = Expression.PropertyOrField(parameter, propertyName);
            MethodInfo method = typeof(string).GetMethod("Contains", new[] { typeof(string) });
            ConstantExpression constant = Expression.Constant(propertyValue, typeof(string));
            return Expression.Lambda<Func<T, bool>>(Expression.Call(member, method, constant), parameter);
        }

        /// <summary>
        /// 創建lambda表達式：!(p=>p.propertyName.Contains(propertyValue))
        /// </summary>
        /// <typeparam name="T"></typeparam>
        /// <param name="column"></param>
        /// <param name="value"></param>
        /// <returns></returns>
        private static Expression<Func<T, bool>> GetNotContains<T>(string propertyName, string propertyValue)
        {
            ParameterExpression parameter = Expression.Parameter(typeof(T), "p");
            MemberExpression member = Expression.PropertyOrField(parameter, propertyName);
            MethodInfo method = typeof(string).GetMethod("Contains", new[] { typeof(string) });
            ConstantExpression constant = Expression.Constant(propertyValue, typeof(string));
            return Expression.Lambda<Func<T, bool>>(Expression.Not(Expression.Call(member, method, constant)), parameter);
        }
    }
}

轉載請注明出處，原文鏈接：
https://blog.csdn.net/yl2isoft/article/details/53196092