1.Func<Student, bool>和Expression<Func<Student, bool>>的区别
class Program
{
static void Main(string[] args)
{
Func<Student, bool> func = t => t.Name == "";
//匿名函数
Expression<Func<Student, bool>> expression = t => t.Name == "";
//表达式树 有Body Right、Left,Right里面又有Right、Left，它们的类型都是继承自 Expression 。这种节点下面有节点，可以无限附加下去的数据结构我们称为树结构数据。也就是我们的表达式树。
Console.Read();
}
} public class Student
{
public string Name { get; set; }
public int Age { get; set; }
public string Address { get; set; }
public string Sex { get; set; }
}
2.解析表达式树
using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Linq.Expressions; namespace RedisTest
{
class Program
{
static void Main(string[] args)
{
Expression<Func<Student, bool>> expression = t => t.Name == "" && t.Name == "";
AnalysisExpression.VisitExpression(expression);
/*
运算符:AndAlso
运算符:Equal
字段名称:Name，类型:System.String
常量值:1111
运算符:Equal
字段名称:Name，类型:System.String
常量值:2222
*/
Console.Read();
}
} public class Student
{
public string Name { get; set; }
public int Age { get; set; }
public string Address { get; set; }
public string Sex { get; set; }
} public static class AnalysisExpression
{
public static void VisitExpression(Expression expression)
{
switch (expression.NodeType)
{
case ExpressionType.Call://执行方法
MethodCallExpression method = expression as MethodCallExpression;
Console.WriteLine("方法名:" + method.Method.Name);
for (int i = ; i < method.Arguments.Count; i++)
VisitExpression(method.Arguments[i]);
break;
case ExpressionType.Lambda://lambda表达式
LambdaExpression lambda = expression as LambdaExpression;
VisitExpression(lambda.Body);
break;
case ExpressionType.Equal://相等比较
case ExpressionType.AndAlso://and条件运算
BinaryExpression binary = expression as BinaryExpression;
Console.WriteLine("运算符:" + expression.NodeType.ToString());
VisitExpression(binary.Left);
VisitExpression(binary.Right);
break;
case ExpressionType.Constant://常量值
ConstantExpression constant = expression as ConstantExpression;
Console.WriteLine("常量值:" + constant.Value.ToString());
break;
case ExpressionType.MemberAccess:
MemberExpression Member = expression as MemberExpression;
Console.WriteLine("字段名称:{0}，类型:{1}", Member.Member.Name, Member.Type.ToString());
break;
default:
Console.Write("UnKnow");
break;
}
} }
}
3.认识IQueryable和IQueryProvider接口
ElementType 代表当然这个Query所对应的类型
Expression 包含了我们当然Query所执行的所有查询或者是其它的操作
IQueryProvider则是负责处理上面的Expression的实现
IQueryProvider只有两个操作，CreateQuery和Execute分别有泛型版本和非泛型版本。 CreatQuery用于构造一个IQueryable<T>的对象，这个类其实没有任何实现，只是继承了IQueryable和IEnumrable接口。主要用于计算指定表达式目录树所表示的查询，返回的结果是一个可枚举的类型。 而Execute会执行指定表达式目录树所表示的查询，返回指定的结果。
4.实现自己的IQueryable<T>、IQueryProvider
using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Linq.Expressions; namespace RedisTest
{
class Program
{
static void Main(string[] args)
{
var aa = new MyQueryable<Student>();
var bb = aa.Where(t => t.Name == "");
var cc = bb.Where(t => t.Sex == "男");
var dd = cc.AsEnumerable();
var ee = cc.ToList();
//每次在执行 Where 查询操作符的时候都会把lambda转化为express存放在MyQueryable的Expression里
//调用 AsEnumerable() 方法的时候并不会去实际取值（只是得到一个IEnumerable）[注意：在EF里面查询不要先取IEnumerable后滤筛，因为AsEnumerable()会生成查询全表的sql]
//执行 ToList() 方法时才去真正调用迭代器 GetEnumerator() 取值
//真正取值的时候，会去执行 IQueryProvider 中的 Execute 方法。（就是在调用这个方法的时候解析表达式数，然后执行取得结果）
//我们看到真正应该办实事的 Execute 我们却让他返回默认值了。
}
} public class Student
{
public string Name { get; set; }
public int Age { get; set; }
public string Address { get; set; }
public string Sex { get; set; }
} public class MyQueryProvider : IQueryProvider
{
public IQueryable<TElement> CreateQuery<TElement>(Expression expression)
{
return new MyQueryable<TElement>(expression);
} public IQueryable CreateQuery(Expression expression)
{
throw new NotImplementedException();
} public TResult Execute<TResult>(Expression expression)
{
return default(TResult);
} public object Execute(Expression expression)
{
return new List<object>();
}
}
public class MyQueryable<T> : IQueryable<T>
{
public MyQueryable()
{
_provider = new MyQueryProvider();
_expression = Expression.Constant(this);
} public MyQueryable(Expression expression)
{
_provider = new MyQueryProvider();
_expression = expression;
}
public Type ElementType
{
get { return typeof(T); }
} private Expression _expression;
public Expression Expression
{
get { return _expression; }
} private IQueryProvider _provider;
public IQueryProvider Provider
{
get { return _provider; }
} public IEnumerator GetEnumerator()
{
return (Provider.Execute(Expression) as IEnumerable).GetEnumerator();
} IEnumerator<T> IEnumerable<T>.GetEnumerator()
{
var result = _provider.Execute<List<T>>(_expression);
if (result == null)
yield break;
foreach (var item in result)
{
yield return item;
}
}
}
}
再一个例子
class Program
{
static void Main(string[] args)
{
var DB = new wolf_testEntities();
decimal aa = ;
ParameterExpression pExpression = Expression.Parameter(typeof(People));
var getPropertyValue = Expression.Property(pExpression, typeof(People).GetProperty("Num"));
ConstantExpression cExpression = Expression.Constant(aa);
BinaryExpression bExpression = Expression.MakeBinary(ExpressionType.GreaterThan, getPropertyValue, cExpression);
Expression<Func<People, bool>> lambda = Expression.Lambda<Func<People, bool>>(bExpression, pExpression);
//Expression Tree 的条件 x=>x.Num>0
var bbb = DB.People.Where(lambda).ToList();
Console.ReadLine();
}
}