[数据结构与算法]C# 表达式目录树Expression

表达式目录树

表达式目录树：语法树，或者说是一种数据结构
1.表达式目录树Expression：System.Linq.Expressions;
2.描述了多个变量或者和常量之间的关系，按照一定的规则进行组装！

• 可以向委托一样使用lambd表达式快捷声明；
• 不能有语句体，声明只能有一行代码；
• 可以通过Compile()，编译成一个委托；

Func<int, int, int> func = (m, n) =>
{
    int i = 0;
    return m * n + 2;
};  //委托  拉姆达表达式其实是作为委托的一个参数,本质是一个方法（匿名方法）
Expression<Func<int, int, int>> exp = (m, n) => m * n + 2; //数据结构--就像对一个计算做了一个精确的描述，展开之后发现，分为左边，右边，每个元素都可以把值都获取出来，二叉树
var erpPlu= exp.Compile();//表达式目录树可以通过compile 转换成一个委托

//表达式目录树：语法树，或者说是一种数据结构
int iResult1 = func.Invoke(12, 23);
int iResult2 = exp.Compile().Invoke(12, 23);

表达式目录树的拼装

手动拼装表达式目录树，不是用的lambda的快捷方式

//表达式目录树的拼装
Expression<Func<int>> expression = () => 123 + 234;  //两个常量相加-----表达式目录树的快捷声明

Expression constant123 = Expression.Constant(123);
Expression constant234 = Expression.Constant(234);
Expression expressionAdd = Expression.Add(constant123, constant234);
var exp = Expression.Lambda<Func<int>>(expressionAdd);
var func = exp.Compile();
int iResult = func.Invoke();

Expression<Func<int, int, int>> exp = (m, n) => m * n + m + n + 2; //快捷声明--其实编译器提供的便捷功能---语法糖--具体实现可通过反编译工具查看

//具体实现可通过反编译工具查看
ParameterExpression parameterExpression = Expression.Parameter(typeof(int), "m");
ParameterExpression parameterExpression2 = Expression.Parameter(typeof(int), "n");
Expression expContant2 = Expression.Constant(2, typeof(int));

Expression multipley = Expression.Multiply(parameterExpression, parameterExpression2);
Expression expAdd = Expression.Add(multipley, parameterExpression);

Expression expAdd1 = Expression.Add(expAdd, parameterExpression2);
Expression expAdd2 = Expression.Add(expAdd1, expContant2);
Expression<Func<int, int, int>> expression = Expression.Lambda<Func<int, int, int>>(expAdd2, new ParameterExpression[]
{
parameterExpression,
parameterExpression2
});
Func<int, int, int> fun = expression.Compile();
int iResult = fun.Invoke(10, 11);

var peopleQuery = new List<People>().AsQueryable();
Expression<Func<People, bool>> lambda = x => x.Id.ToString().Equals("5");
peopleQuery.Where(lambda);

ParameterExpression parameterExpression = Expression.Parameter(typeof(People), "x");
FieldInfo idfield = typeof(People).GetField("Id");
var idExp = Expression.Field(parameterExpression, idfield);

MethodInfo toString = typeof(int).GetMethod("ToString", new Type[0]);
var toStringExp = Expression.Call(idExp, toString, Array.Empty<Expression>()); 
var Equals = typeof(string).GetMethod("Equals", new Type[] { typeof(string) });
Expression expressionConstant5 = Expression.Constant("5", typeof(string));
var equalsExp = Expression.Call(toStringExp, Equals, new Expression[]
 {
    expressionConstant5
 });
Expression<Func<People, bool>> expression = Expression.Lambda<Func<People, bool>>(equalsExp, new ParameterExpression[]
{
 parameterExpression
});
Func<People, bool> func = expression.Compile();
var bResult = func.Invoke(new People()
{
    Id = 5,
    Name = "海贝"
});

new List<People>().AsQueryable().Where(expression);

应用

Linq to SQL

var dbSet = new List<People>().AsQueryable();//EF DbSet 
dbSet.Where(p => p.Age == 25 & p.Name.Contains("阳光下的微笑"));

Expression<Func<People, bool>> exp = null;
Console.WriteLine("用户输入个名称，为空就跳过");
string name = Console.ReadLine();
if (!string.IsNullOrWhiteSpace(name))
{
    exp = p => p.Name.Contains(name);
}
Console.WriteLine("用户输入个最小年纪，为空就跳过");
string age = Console.ReadLine();
if (!string.IsNullOrWhiteSpace(age) && int.TryParse(age, out int iAge))
{
    exp = p => p.Age > iAge;
}

上面的玩法是不是只有最后一个条件才生效？如果需要多个条件都满足；怎么办？ 当然是拼装啊；
拼装可以从最小粒度来组装表达式目录树；如果有一个封装，你把各种条件给我，我从最小粒度开始一个一个的拼装起来，不就是一个长的表达式目录树了吗？

解决方案：
调用方可以组装一个很长的表达式目录树传递过来；
表达式目录树传递过来以后，在这里应该做什么？应该解析；
所有信息都在表达式目录树里面，自然也可以把他解析（找出来）
解析就可以通过ExpressionVisitor解析----生成对应的Sql语句；

ExpressionVisitor

表达式目录树的访问者----访问者模式；
1.Visit方法–访问表达式目录树的入口—分辨是什么类型的表达式目录
2.调度到更加专业的方法中进一步访问,访问一遍之后，生成一个新的表达式目录 —有点像递归，不全是递归；
3.因为表达式目录树是个二叉树，ExpressionVisitor一直往下访问，一直到叶节点；那就访问了所有的节点；
4.在访问的任何一个环节，都可以拿到对应当前环节的内容(参数名称、参数值。。)，就可以进一步扩展；

为什么要使用表达式目录树来拼装解析呢：
1.可以提高重用性；
2.如果封装好一个方法，接受一个表达式目录树，在解析的时候，其实就是不断的访问，访问有规则；
3.任何一个表达式目录树都可以调用当前方法来解析；
4.表达式目录树可以支持泛型；

            {
                Expression<Func<People, bool>> lambda = x => x.Age > 5 && x.Id > 5
                                                         && x.Name.StartsWith("1") //  like '1%'
                                                         && x.Name.EndsWith("1") //  like '%1'
                                                         && x.Name.Contains("1");//  like '%1%'

                //string sql = string.Format("Delete From [{0}] WHERE [Age]>5 AND [ID] >5"
                    , typeof(People).Name
                    , " [Age]>5 AND [ID] >5" ); 

                ConditionBuilderVisitor vistor = new ConditionBuilderVisitor();
                vistor.Visit(lambda);
                Console.WriteLine(vistor.Condition());
            }
            { 
               // ((( [Age] > '5') AND( [Name] =  [name] )) OR( [Id] > '5' )) 
               string name = "AAA"; 
                Expression<Func<People, bool>> lambda = x => x.Age > 5 && x.Name == name || x.Id > 5;
                ConditionBuilderVisitor vistor = new ConditionBuilderVisitor();
                vistor.Visit(lambda);
                Console.WriteLine(vistor.Condition());
            }
            {
                Expression<Func<People, bool>> lambda = x => x.Age > 5 || (x.Name == "A" && x.Id > 5);
                ConditionBuilderVisitor vistor = new ConditionBuilderVisitor();
                vistor.Visit(lambda);
                Console.WriteLine(vistor.Condition());
            }
            {
                Expression<Func<People, bool>> lambda = x => (x.Age > 5 || x.Name == "A") && x.Id > 5;
                ConditionBuilderVisitor vistor = new ConditionBuilderVisitor();
                vistor.Visit(lambda);
                Console.WriteLine(vistor.Condition());
            }

自己封装的解析器，这就是EF6的底层原理，根据表达式树自动生成相应的sql语句。

    public class ConditionBuilderVisitor : ExpressionVisitor
    {
        private Stack<string> _StringStack = new Stack<string>();

        public string Condition()
        {
            string condition = string.Concat(this._StringStack.ToArray());
            this._StringStack.Clear();
            return condition;
        }

        /// <summary>
        /// 如果是二元表达式
        /// </summary>
        /// <param name="node"></param>
        /// <returns></returns>
        protected override Expression VisitBinary(BinaryExpression node)
        {
            if (node == null) throw new ArgumentNullException("BinaryExpression");

            this._StringStack.Push(")");
            base.Visit(node.Right);//解析右边
            this._StringStack.Push(" " + node.NodeType.ToSqlOperator() + " ");
            base.Visit(node.Left);//解析左边
            this._StringStack.Push("(");

            return node;
        }

        /// <summary>
        /// 解析属性
        /// </summary>
        /// <param name="node"></param>
        /// <returns></returns>
        protected override Expression VisitMember(MemberExpression node)
        {
            if (node == null) throw new ArgumentNullException("MemberExpression");
            //this._StringStack.Push(" [" + node.Member.Name + "] ");
            return node; 
            if (node.Expression is ConstantExpression)
            {
                var value1 = this.InvokeValue(node);
                var value2 = this.ReflectionValue(node);
                //this.ConditionStack.Push($"'{value1}'");
                this._StringStack.Push("'" + value2 + "'");
            }
            else
            {
                this._StringStack.Push(" [" + node.Member.Name + "] ");
            }
            return node;
        }


        private object InvokeValue(MemberExpression member)
        {
            var objExp = Expression.Convert(member, typeof(object));//struct需要
            return Expression.Lambda<Func<object>>(objExp).Compile().Invoke();
        }

        private object ReflectionValue(MemberExpression member)
        {
            var obj = (member.Expression as ConstantExpression).Value;
            return (member.Member as FieldInfo).GetValue(obj);
        }

        /// <summary>
        /// 常量表达式
        /// </summary>
        /// <param name="node"></param>
        /// <returns></returns>
        protected override Expression VisitConstant(ConstantExpression node)
        {
            if (node == null) throw new ArgumentNullException("ConstantExpression");
            this._StringStack.Push(" '" + node.Value + "' ");
            return node;
        }
        /// <summary>
        /// 方法表达式
        /// </summary>
        /// <param name="m"></param>
        /// <returns></returns>
        protected override Expression VisitMethodCall(MethodCallExpression m)
        {
            if (m == null) throw new ArgumentNullException("MethodCallExpression");

            string format;
            switch (m.Method.Name)
            {
                case "StartsWith":
                    format = "({0} LIKE {1}+'%')";
                    break;

                case "Contains":
                    format = "({0} LIKE '%'+{1}+'%')";
                    break;

                case "EndsWith":
                    format = "({0} LIKE '%'+{1})";
                    break;

                default:
                    throw new NotSupportedException(m.NodeType + " is not supported!");
            }
            this.Visit(m.Object);
            this.Visit(m.Arguments[0]);
            string right = this._StringStack.Pop();
            string left = this._StringStack.Pop();
            this._StringStack.Push(String.Format(format, left, right));

            return m;
        }
    }

    internal static class SqlOperator
    {
        internal static string ToSqlOperator(this ExpressionType type)
        {
            switch (type)
            {
                case (ExpressionType.AndAlso):
                case (ExpressionType.And):
                    return "AND";
                case (ExpressionType.OrElse):
                case (ExpressionType.Or):
                    return "OR";
                case (ExpressionType.Not):
                    return "NOT";
                case (ExpressionType.NotEqual):
                    return "<>";
                case ExpressionType.GreaterThan:
                    return ">";
                case ExpressionType.GreaterThanOrEqual:
                    return ">=";
                case ExpressionType.LessThan:
                    return "<";
                case ExpressionType.LessThanOrEqual:
                    return "<=";
                case (ExpressionType.Equal):
                    return "=";
                default:
                    throw new Exception("不支持该方法");
            }

        }
    }

表达式目录扩展

表达式目录树动态拼接的实现方式：

    /// <summary>
    /// 合并表达式 And Or  Not扩展
    /// </summary>
    public static class ExpressionExtend
    {
        /// <summary>
        /// 合并表达式 expr1 AND expr2
        /// </summary>
        /// <typeparam name="T"></typeparam>
        /// <param name="expr1"></param>
        /// <param name="expr2"></param>
        /// <returns></returns>
        public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> expr1, Expression<Func<T, bool>> expr2)
        {
            //return Expression.Lambda<Func<T, bool>>(Expression.AndAlso(expr1.Body, expr2.Body), expr1.Parameters); 错误的写法，两个表达式不是同一个参数
			//将两个表达式的参数统一为参数c
            ParameterExpression newParameter = Expression.Parameter(typeof(T), "c");
            NewExpressionVisitor visitor = new NewExpressionVisitor(newParameter);
            var left = visitor.Replace(expr1.Body);
            var right = visitor.Replace(expr2.Body); //为了能够生成一个新的表达式目录树
            var body = Expression.And(left, right);
             return Expression.Lambda<Func<T, bool>>(body, newParameter);

        }
        /// <summary>
        /// 合并表达式 expr1 or expr2
        /// </summary>
        /// <typeparam name="T"></typeparam>
        /// <param name="expr1"></param>
        /// <param name="expr2"></param>
        /// <returns></returns>
        public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> expr1, Expression<Func<T, bool>> expr2)
        {

            ParameterExpression newParameter = Expression.Parameter(typeof(T), "c");
            NewExpressionVisitor visitor = new NewExpressionVisitor(newParameter);

            var left = visitor.Replace(expr1.Body);
            var right = visitor.Replace(expr2.Body);
            var body = Expression.Or(left, right);
            return Expression.Lambda<Func<T, bool>>(body, newParameter);
        }
        public static Expression<Func<T, bool>> Not<T>(this Expression<Func<T, bool>> expr)
        {
            var candidateExpr = expr.Parameters[0];
            var body = Expression.Not(expr.Body);

            return Expression.Lambda<Func<T, bool>>(body, candidateExpr);
        }
    }

    /// <summary>
    /// 建立新表达式
    /// </summary>
    internal class NewExpressionVisitor : ExpressionVisitor
    {
        public ParameterExpression _NewParameter { get; private set; }
        public NewExpressionVisitor(ParameterExpression param)
        {
            this._NewParameter = param;
        }
        public Expression Replace(Expression exp)
        {
            return this.Visit(exp);
        }
        protected override Expression VisitParameter(ParameterExpression node)
        {
            return this._NewParameter;
        }
    }

调用方如下：

            {
                Expression<Func<People, bool>> lambda1 = x => x.Age > 5;  
                Expression<Func<People, bool>> lambda2 = x => x.Id > 5;

                //Expression<Func<People, bool>> newExpress = x => x.Age > 5 && x.Id > 5;

                Expression<Func<People, bool>> lambda3 = lambda1.And(lambda2); //且
                Expression<Func<People, bool>> lambda4 = lambda1.Or(lambda2);//或
                Expression<Func<People, bool>> lambda5 = lambda1.Not();//非
                Do1(lambda3);
                Do1(lambda4);
                Do1(lambda5);
            }
        
        private static void Do1(Expression<Func<People, bool>> func)
        {
            List<People> people = new List<People>()
            {
                new People(){Id=4,Name="123",Age=4},
                new People(){Id=5,Name="234",Age=5},
                new People(){Id=6,Name="345",Age=6},
            };

            List<People> peopleList = people.Where(func.Compile()).ToList();
        }

对象深拷贝

硬编码

 PeopleCopy peopleCopy = new PeopleCopy()
 {
     Id = people.Id,
     Name = people.Name,
     Age = people.Age
 };

通过反射实现

    public class ReflectionMapper
    {
        /// <summary>
        /// 反射
        /// </summary>
        /// <typeparam name="TIn"></typeparam>
        /// <typeparam name="TOut"></typeparam>
        /// <param name="tIn"></param>
        /// <returns></returns>
        public static TOut Trans<TIn, TOut>(TIn tIn)
        {
            TOut tOut = Activator.CreateInstance<TOut>();
            foreach (var itemOut in tOut.GetType().GetProperties())
            {
                var propIn = tIn.GetType().GetProperty(itemOut.Name);
                itemOut.SetValue(tOut, propIn.GetValue(tIn)); 
            }

            foreach (var itemOut in tOut.GetType().GetFields())
            {
                var fieldIn = tIn.GetType().GetField(itemOut.Name);
                itemOut.SetValue(tOut, fieldIn.GetValue(tIn)); 
            }
            return tOut;
        }
    }

通过序列化实现

    /// <summary>
    /// 使用第三方序列化反序列化工具
    /// 还有automapper
    /// </summary>
    public class SerializeMapper
    {
        /// <summary>
        /// 序列化反序列化方式
        /// </summary>
        /// <typeparam name="TIn"></typeparam>
        /// <typeparam name="TOut"></typeparam>
        public static TOut Trans<TIn, TOut>(TIn tIn)
        {
            return JsonConvert.DeserializeObject<TOut>(JsonConvert.SerializeObject(tIn));
        }
    }

反射和序列化两种实现方式性能不太好；

通过表达式目录树实现

通过表达式目录树动态的生成硬编码

Func<People, PeopleCopy> func = p => new PeopleCopy()
{
    Id = p.Id,
    Name = p.Name,
    Age = p.Age
};
PeopleCopy peopleCopy3 = func.Invoke(people);

方法一：普通缓存

    /// <summary>
    /// 生成表达式目录树 缓存
    /// </summary>
    public class ExpressionMapper
    {
        /// <summary>
        /// 字典缓存--hash分布
        /// </summary>
        private static Dictionary<string, object> _Dic = new Dictionary<string, object>();

        /// <summary>
        /// 字典缓存表达式树
        /// </summary>
        /// <typeparam name="TIn"></typeparam>
        /// <typeparam name="TOut"></typeparam>
        /// <param name="tIn"></param>
        /// <returns></returns>
        public static TOut Trans<TIn, TOut>(TIn tIn)
        {
            string key = string.Format("funckey_{0}_{1}", typeof(TIn).FullName, typeof(TOut).FullName);
            if (!_Dic.ContainsKey(key))
            {
                ParameterExpression parameterExpression = Expression.Parameter(typeof(TIn), "p");
                List<MemberBinding> memberBindingList = new List<MemberBinding>();
                foreach (var item in typeof(TOut).GetProperties())
                {
                    MemberExpression property = Expression.Property(parameterExpression, typeof(TIn).GetProperty(item.Name));
                    MemberBinding memberBinding = Expression.Bind(item, property);
                    memberBindingList.Add(memberBinding);
                }
                foreach (var item in typeof(TOut).GetFields())
                {
                    MemberExpression property = Expression.Field(parameterExpression, typeof(TIn).GetField(item.Name));
                    MemberBinding memberBinding = Expression.Bind(item, property);
                    memberBindingList.Add(memberBinding);
                }
                MemberInitExpression memberInitExpression = Expression.MemberInit(Expression.New(typeof(TOut)), memberBindingList.ToArray());
                Expression<Func<TIn, TOut>> lambda = Expression.Lambda<Func<TIn, TOut>>(memberInitExpression, new ParameterExpression[]
                {
                    parameterExpression
                });
                Func<TIn, TOut> func = lambda.Compile();//拼装是一次性的
                _Dic[key] = func;
            }
            return ((Func<TIn, TOut>)_Dic[key]).Invoke(tIn);
        }
    }

方法二：泛型缓存，性能较高

    /// <summary>
    /// 生成表达式目录树  泛型缓存
    /// </summary>
    /// <typeparam name="TIn"></typeparam>
    /// <typeparam name="TOut"></typeparam>
    public class ExpressionGenericMapper<TIn, TOut>//Mapper`2
    {
        private static Func<TIn, TOut> _FUNC = null;
        static ExpressionGenericMapper()
        {
            ParameterExpression parameterExpression = Expression.Parameter(typeof(TIn), "p");
            List<MemberBinding> memberBindingList = new List<MemberBinding>();
            foreach (var item in typeof(TOut).GetProperties())
            {
                MemberExpression property = Expression.Property(parameterExpression, typeof(TIn).GetProperty(item.Name));
                MemberBinding memberBinding = Expression.Bind(item, property);
                memberBindingList.Add(memberBinding);
            }
            foreach (var item in typeof(TOut).GetFields())
            {
                MemberExpression property = Expression.Field(parameterExpression, typeof(TIn).GetField(item.Name));
                MemberBinding memberBinding = Expression.Bind(item, property);
                memberBindingList.Add(memberBinding);
            }
            MemberInitExpression memberInitExpression = Expression.MemberInit(Expression.New(typeof(TOut)), memberBindingList.ToArray());
            Expression<Func<TIn, TOut>> lambda = Expression.Lambda<Func<TIn, TOut>>(memberInitExpression, new ParameterExpression[]
            {
                    parameterExpression
            });
            _FUNC = lambda.Compile();//拼装是一次性的
        }
        public static TOut Trans(TIn t)
        {
            return _FUNC(t);
        }
    }