有如下一段代码,db.Employess 是Table类型,而Table类型继承了IQueryable ,.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 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(); } }
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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 (Expression.Lambda >(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 ..ctor, <>f__AnonymousType0 ), 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 .get_EmployeeID, <>f__AnonymousType0 ), (MethodInfo) methodof(<>f__AnonymousType0 .get_FirstName, <>f__AnonymousType0 ) }), new ParameterExpression[] { CS$0$0000 })); var q2 = db.Employees.Where (Expression.Lambda >(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 ..ctor, <>f__AnonymousType0 ), 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 .get_EmployeeID, <>f__AnonymousType0 ), (MethodInfo) methodof(<>f__AnonymousType0 .get_FirstName, <>f__AnonymousType0 ) }), 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.
