Csharp中表达式树
Csharper中的表达式树这节课来了解一下表示式树是什么？
在C#中，表达式树是一种数据结构，它可以表示一些代码块，如Lambda表达式或查询表达式。表达式树使你能够查看和操作数据，就像你可以查看和操作代码一样。它们通常用于创建动态查询和解析表达式。
一、认识表达式树
为什么要这样说？它和委托有什么区别？
创建一个简单的表达式树和委托
public class ExpressionDemo
{
void Show()
{
Func<int, bool> fun1 = x => x > 10;
Expression<Func<int, bool>> expression1 = x => x > 10;
}
}然后f12转到定义
public sealed class Expression<TDelegate> : LambdaExpression尝试用大括号定义一个表达式树
debug运行后，用vs查看一下定义的表达式树对象.
发现表达式树一些特点:
[*]可以通过lambda表达式来声明
[*]是一个泛型类的接口，类型参数是一个委托
[*]Expression声明中，不能包含大括号.
[*]通过VS展开查看，包含body(lamubda的主体部分)，描述了参数的名称和类型，描述了返回值的名称和类型； 展开body, body包含 左边是什么，右边是什么，式子的操作类型是什么.
结论:
表达式树,是一个计算式的描述，按照常规的计算逻辑，通过类的属性来进行描述多个节点之间的关系； 形似于一个树形结构----二叉树； 二叉树不断地去分解，可以得到这个式子中的任何一个独立的元素；----是一个二叉树，是一个数据结构； 如果需要可以把这个结构不断的拆解；得到中间的最小元素；在需要的时候，也可以通过每个元素，组装起来；
委托是一个类，而表达式树是一个二叉树的数据结构。
为了更加深入的了解表达式树，这里也使用ilspy进行反编译，以便于更加了解表达式树的本质.
这里使用一个比较复杂的表达式树的语句来方便我们去理解
Expression<Func<int ,int ,int>> expression2= (x, y) => x *y+2+3;
优化一下这段代码
//定义2个变量
ParameterExpression parameterExpression = Expression.Parameter(typeof(int), "x");
ParameterExpression parameterExpression2 = Expression.Parameter(typeof(int), "y");
//定义常量
var contact1 = Expression.Constant(2, typeof(int));
var contact2= Expression.Constant(3, typeof(int));
//定义表达式 x*y
var MultiplyXy= Expression.Multiply(parameterExpression, parameterExpression2);
//定义表达式 x*y的结果+2
var add1 = Expression.Add(MultiplyXy, contact1);
//定义表达式 x*y+2的结果+3
var add2 = Expression.Add(add1, contact2);
//定义最终的lambda表达式
Expression<Func<int, int, int>> expression2 = Expression.Lambda<Func<int, int, int>>(add2, new ParameterExpression
{
parameterExpression,
parameterExpression2
});如图所示的解析:
已经将相应的代码粘贴到上方，就是类似二叉树结构的因式分解，转换成为最小的子问题，最后解决一个需要解决的大问题。
二、动态拼装Expression
我们自己去拼装一个表达式树去理解表达式树的秘密.
首先创建一个People类
public class People
{
public int Age { get; set; }
public string Name { get; set; }
public int Id;
}下面来拼接一个比较复杂的表达式
Expression<Func<People, bool>> predicate = c => c.Id == 10 && c.Name.ToString().Equals("张三");对应的表达式树的代码
//定义一个People类型的参数
ParameterExpression parameterExpression = Expression.Parameter(typeof(People), "c");
//获取People的Id属性
PropertyInfo? propertyId = typeof(People).GetProperty("Id");
//定义10这个常量
ConstantExpression constantExpression = Expression.Constant(10, typeof(int));
//定义c.Id>10这个表达式
BinaryExpression left =Expression.GreaterThan(Expression.Property(parameterExpression, propertyId), constantExpression);
//获取People的Name属性
PropertyInfo? propertyName = typeof(People).GetProperty("Name");
//c.Name
MemberExpression memName = Expression.Property(parameterExpression, propertyName);
//to string方法
MethodInfo? methodtostring=typeof(string).GetMethod("ToString",new Type);
//调用tostring方法
MethodCallExpression instance =Expression.Call(memName, methodtostring,Array.Empty<Expression>());
//获取equals方法
MethodInfo? methodEquals = typeof(string).GetMethod("Equals", new Type[] { typeof(string) });
//定义c.Name.ToString().Equals("张三")这个表达式
MethodCallExpression right = Expression.Call(instance, methodEquals, Expression.Constant("张三", typeof(string)));
//定义c.Age<25这个表达式
PropertyInfo? propertyAge = typeof(People).GetProperty("Age");
ConstantExpression constantExpression2 = Expression.Constant(25, typeof(int));
BinaryExpression right2 = Expression.LessThan(Expression.Property(parameterExpression, propertyAge), constantExpression2);
//定义c.Id>10 && c.Name.ToString().Equals("张三") && c.Age<25这个表达式
BinaryExpression and1 = Expression.AndAlso(left, right);
BinaryExpression and2 = Expression.AndAlso(and1, right2);
//定义最终的lambda表达式
Expression<Func<People, bool>> expression = Expression.Lambda<Func<People, bool>>(and2, new ParameterExpression
{
parameterExpression
});
//编译表达式
Func<People, bool> func = expression.Compile();
//调用表达式
People people = new People()
{
Id = 11,
Name = "张三",
Age = 20
};
Console.WriteLine(func(people));拼接后的结果
Expression<Func<People, bool>> expression2 = p => p.Id == 10 && p.Name.Equals("阳光下的微笑");例子2
//按关键字是否存在来拼装；
Expression<Func<People, bool>> exp = p=> true;
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);
exp= exp.And(c=>c.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;
exp = exp.And(p => p.Age > iAge);
}现在使用表达式树进行链接
Expression lambda1 = x => x.Age > 5; Expression lambda2 = x => x.Id > 5; //按关键字是否存在来拼装；
Expression<Func<People, bool>> exp = p=> true;
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);
exp= exp.And(c=>c.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;
exp = exp.And(p => p.Age > iAge);
}; Expression lambda3 = lambda1.And(lambda2); //且 两个都满足，通过&&链接 Expression lambda4 = lambda1.Or(lambda2);//或 两个只要有一个就可以通过或者来链接||Expression lambda5 = lambda1.Not();//非这里实现了常见的且、或、非逻辑运算符的表达式
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 两个都满足，通过&&链接
Expression<Func<People, bool>> lambda4 = lambda1.Or(lambda2);//或 两个只要有一个就可以通过或者来链接||
Expression<Func<People, bool>> lambda5 = lambda1.Not();//非现在有一个新的需求，需要把People拷贝到NewPeople这个新的类，来看下效率怎么样?
People和PeopleCopy类
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);
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;
var body = Expression.Not(expr.Body);
return Expression.Lambda<Func<T, bool>>(body, candidateExpr);
}
}
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;
}
}直接赋值的方式
public class People
{
public int Age { get; set; }
public string Name { get; set; }
public int Id;
}
/// <summary>
/// 实体类Target
/// PeopleDTO
/// </summary>
public class PeopleCopy
{
public int Age { get; set; }
public string Name { get; set; }
public int Id;
}反射赋值的方式
PeopleCopy peopleCopy1 = new PeopleCopy()
{
Id = people.Id,
Name = people.Name,
Age = people.Age
};json序列化的方式
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 propName = tIn.GetType().GetProperty(itemOut.Name);
itemOut.SetValue(tOut, propName.GetValue(tIn));
}
foreach (var itemOut in tOut.GetType().GetFields())
{
var fieldName = tIn.GetType().GetField(itemOut.Name);
itemOut.SetValue(tOut, fieldName.GetValue(tIn));
}
return tOut;
}
}
PeopleCopy peopleCopy2= ReflectionMapper.Trans<People, PeopleCopy>(people);表达式目录树的方式
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)
{
string strTin = JsonConvert.SerializeObject(tIn);
return JsonConvert.DeserializeObject<TOut>(strTin);
}
}
PeopleCopy peopleCopy3 = SerializeMapper.Trans<People, PeopleCopy>(people);表达式+反射+泛型类的方式
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))
{
#region 这里是拼装---赋属性值的代码
ParameterExpression parameterExpression = Expression.Parameter(typeof(TIn), "p");
//MemberBinding: 就是一个表达式目录树
List<MemberBinding> memberBindingList = new List<MemberBinding>();
foreach (var item in typeof(TOut).GetProperties()) //这里是处理属性的
{
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()) //处理字段的
{
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());//组装了一个转换的过程；
Expression<Func<TIn, TOut>> lambda = Expression.Lambda<Func<TIn, TOut>>(memberInitExpression, new ParameterExpression[]
{
parameterExpression
});
#endregion
Func<TIn, TOut> func = lambda.Compile();//拼装是一次性的
_Dic = func;
}
return ((Func<TIn, TOut>)_Dic).Invoke(tIn);
}
}
PeopleCopy peopleCopy4 = ExpressionMapper.Trans<People, PeopleCopy>(people);最后运行一百万次，来看一下效率。
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);
}
}
}
PeopleCopy peopleCopy5 = ExpressionGenericMapper<People, PeopleCopy>.Trans(people);看运行后的结果
核心：动态生成硬编码；----代码运行的时候生成了一段新的逻辑；
四、表达式树和sql
为什么要使用表达式目录树来拼装解析呢？
可以提供重用性
如果封装好一个方法，接受一个表达式树，在解析的时候，其实就是不断的访问，访问的时候，会按照固定的规则，避免出错;
任何的一个表达式树都可以用一个通用的方法解析并且支持泛型,更加容易去封装;
例子:
需要的扩展类
{
People people = new People()
{
Id = 11,
Name = "Richard",
Age = 31
};
long common = 0;
long generic = 0;
long cache = 0;
long reflection = 0;
long serialize = 0;
{
Stopwatch watch = new Stopwatch();
watch.Start();
for (int i = 0; i < 1_000_000; i++)
{
PeopleCopy peopleCopy = new PeopleCopy()
{
Id = people.Id,
Name = people.Name,
Age = people.Age
};
}
watch.Stop();
common = watch.ElapsedMilliseconds;
}
{
Stopwatch watch = new Stopwatch();
watch.Start();
for (int i = 0; i < 1_000_000; i++)
{
PeopleCopy peopleCopy = ReflectionMapper.Trans<People, PeopleCopy>(people);
}
watch.Stop();
reflection = watch.ElapsedMilliseconds;
}
{
Stopwatch watch = new Stopwatch();
watch.Start();
for (int i = 0; i < 1_000_000; i++)
{
PeopleCopy peopleCopy = SerializeMapper.Trans<People, PeopleCopy>(people);
}
watch.Stop();
serialize = watch.ElapsedMilliseconds;
}
{
Stopwatch watch = new Stopwatch();
watch.Start();
for (int i = 0; i < 1_000_000; i++)
{
PeopleCopy peopleCopy = ExpressionMapper.Trans<People, PeopleCopy>(people);
}
watch.Stop();
cache = watch.ElapsedMilliseconds;
}
{
Stopwatch watch = new Stopwatch();
watch.Start();
for (int i = 0; i < 1_000_000; i++)
{
PeopleCopy peopleCopy = ExpressionGenericMapper<People, PeopleCopy>.Trans(people);
}
watch.Stop();
generic = watch.ElapsedMilliseconds;
}
Console.WriteLine($"common = {common} ms"); //性能最高，但是不能通用；
Console.WriteLine($"reflection = {reflection} ms");
Console.WriteLine($"serialize = {serialize} ms");
Console.WriteLine($"cache = {cache} ms");
Console.WriteLine($"generic = {generic} ms"); //性能好，而且扩展性也好===又要马儿跑，又要马儿不吃草。。。
}对应的表达式解析
public class OperationsVisitor : ExpressionVisitor
{
public Expression Modify(Expression expression)
{
Console.WriteLine(expression.ToString()) ;
//ExpressionVisitor:
//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
return this.Visit(expression);
}
/// <summary>
/// 覆写父类方法
/// </summary>
/// <param name="b"></param>
/// <returns></returns>
protected override Expression VisitBinary(BinaryExpression b)
{
if (b.NodeType == ExpressionType.Add)
{
Expression left = this.Visit(b.Left);
Expression right = this.Visit(b.Right);
return Expression.Subtract(left, right);
}
else if (b.NodeType==ExpressionType.Multiply) //如果是相乘
{
Expression left = this.Visit(b.Left);
Expression right = this.Visit(b.Right);
return Expression.Divide(left, right); //相除
}
return base.VisitBinary(b);
}
/// <summary>
/// 覆写父类方法
/// </summary>
/// <param name="node"></param>
/// <returns></returns>
protected override Expression VisitConstant(ConstantExpression node)
{
return base.VisitConstant(node);
}同时表达式树中已经通过使用观察者模式封装好了Visit方法.
[*]Visit方法--访问表达式树的入口---分辨是什么类型的表达式目录
[*]调度到更加专业的方法中进一步访问，访问一边以后，生成一个新的表达式目录. --- 有点像递归，不全是递归
[*]因为表达式目录树是一个二叉树，ExpreesionVistor一直往下访问，一直到叶子节点;通过二叉树的遍历就访问了所有的节点.
[*]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.
现在开始将表达式树跟sql语句进行连接
例子:
扩展类
Expression<Func<int, int, int>> exp = (m, n) => m * n + 2;
OperationsVisitor visitor = new OperationsVisitor();
//visitor.Visit(exp);
Expression expNew = visitor.Modify(exp);对应的sql语句的解析
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);
string right = this._StringStack.Pop();
string left = this._StringStack.Pop();
this._StringStack.Push(String.Format(format, left, right));
return m;
}
}在我自己的看法，使用表达式树而不是传统的方式去解析sql语句的优点
[*]通过二叉树的方式表达，更加的有条理性
[*]使用泛型等技术更方式实现一个通用的sql语句的解析。
[*]会有类型检查，出错后也能使用异常处理。
来源:https://www.cnblogs.com/wenlong-4613615/p/18084777
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