Linq学习笔记

有如下一段代码,db.Employess 是Table类型,而Table类型继承了IQueryable<T> ,.net3.5中的扩展方法特性,在Queryable中定义了Select Where ....等一系列方法,这些方法跟linq查询中的关键字select where 等对应,

linq 查询语句,编译后最终形成对IQueryable<T>类型中,Select Where等形式函数调用

Queryable 与 Enumerable 类似,是linq 查询实现的基础, 在Enumerable里,一般将 lambda 表达公式编译成匿名委托调用,

而在Queryable类中则将lambda 表达公式,编译成lambda表达式树, 接着把这些表达树(类似解释型的编程语言),传递给相应的提供程序,如 sql2000 等,提供程序,根据 lambada树, 解释SQL语句, 接着通过ADO.NET 实现数据库的操作.

代码:    

 class Program
    {
        static void Main(string[] args)
        {
            DBNorthwindDataContext db=new DBNorthwindDataContext();
            var q1 = from emp in db.Employees
                     where emp.EmployeeID > 2
                     select new { emp.EmployeeID, emp.FirstName };
            var q2 = db.Employees.Where(emp => emp.EmployeeID > 2).Select(emp => new { emp.EmployeeID, emp.FirstName });
            Console.ReadLine();
        }
    }

--------------

编译后的结果

   // Methods
    private static void Main(string[] args)
    {
        ParameterExpression CS$0$0000;
        DBNorthwindDataContext db = new DBNorthwindDataContext();
        var q1 = db.Employees.Where<Employees>(Expression.Lambda<Func<Employees, bool>>(Expression.GreaterThan(Expression.Property(CS$0$0000 = Expression.Parameter(typeof(Employees), "emp"), (MethodInfo) methodof(Employees.get_EmployeeID)), Expression.Constant(2, typeof(int))), new ParameterExpression[] { CS$0$0000 })).Select(Expression.Lambda(Expression.New((ConstructorInfo) methodof(<>f__AnonymousType0<int, string>..ctor, <>f__AnonymousType0<int, string>), new Expression[] { Expression.Property(CS$0$0000 = Expression.Parameter(typeof(Employees), "emp"), (MethodInfo) methodof(Employees.get_EmployeeID)), Expression.Property(CS$0$0000, (MethodInfo) methodof(Employees.get_FirstName)) }, new MethodInfo[] { (MethodInfo) methodof(<>f__AnonymousType0<int, string>.get_EmployeeID, <>f__AnonymousType0<int, string>), (MethodInfo) methodof(<>f__AnonymousType0<int, string>.get_FirstName, <>f__AnonymousType0<int, string>) }), new ParameterExpression[] { CS$0$0000 }));
        var q2 = db.Employees.Where<Employees>(Expression.Lambda<Func<Employees, bool>>(Expression.GreaterThan(Expression.Property(CS$0$0000 = Expression.Parameter(typeof(Employees), "emp"), (MethodInfo) methodof(Employees.get_EmployeeID)), Expression.Constant(2, typeof(int))), new ParameterExpression[] { CS$0$0000 })).Select(Expression.Lambda(Expression.New((ConstructorInfo) methodof(<>f__AnonymousType0<int, string>..ctor, <>f__AnonymousType0<int, string>), new Expression[] { Expression.Property(CS$0$0000 = Expression.Parameter(typeof(Employees), "emp"), (MethodInfo) methodof(Employees.get_EmployeeID)), Expression.Property(CS$0$0000, (MethodInfo) methodof(Employees.get_FirstName)) }, new MethodInfo[] { (MethodInfo) methodof(<>f__AnonymousType0<int, string>.get_EmployeeID, <>f__AnonymousType0<int, string>), (MethodInfo) methodof(<>f__AnonymousType0<int, string>.get_FirstName, <>f__AnonymousType0<int, string>) }), new ParameterExpression[] { CS$0$0000 }));
        Console.ReadLine();
    }
可以发现q1,跟q2查询最终编译成函数形式,并且一模一样.