查询表达式中处理Null值

　　此示例演示如何处理源集合中可能的 null 值。诸如 IEnumerable<T> 等对象集合可能包含值为 null 的元素。如果源集合为 null 或包含值为 null 的元素，并且查询未处理 null 值，当您执行查询时将会引发 NullReferenceException。

 var query1 =

                        from c in categories

                        where c != null

                        join p in products on c.ID equals

                            (p == null ? null : p.CategoryID)

                        select new { Category = c.Name, Name = p.Name };

　　在 join 子句中，只要其中一个比较键是可以为 null 的类型，您就可以在查询表达式中将另一个比较键强制转换成可以为 null 的类型。在下面的示例中，假定 EmployeeID 是一个列，其中包含类型为 int? 的值：

 　　　　　　     void TestMethod(Northwind db)

                {

                    var query =

                        from o in db.Orders

                        join e in db.Employees

                            on o.EmployeeID equals (int?)e.EmployeeID

                        select new { o.OrderID, e.FirstName };

                }

在查询表达式中处理异常

　　下面的示例演示如何将异常处理代码移至查询表达式外部。仅当该方法不依赖于查询的任何本地变量时，才能这样做。

   class ExceptionsOutsideQuery

    {

        static void Main()

        {

            // DO THIS with a datasource that might

            // throw an exception. It is easier to deal with

            // outside of the query expression.

            IEnumerable<int> dataSource;

            try

            {

                dataSource = GetData();

            }

            catch (InvalidOperationException)

            {

                // Handle (or don't handle) the exception

                // in the way that is appropriate for your application.

                Console.WriteLine("Invalid operation");

                goto Exit;

            }

            // If we get here, it is safe to proceed.

            var query = from i in dataSource

                        select i * i;

            foreach (var i in query)

                Console.WriteLine(i.ToString());

            //Keep the console window open in debug mode

            Exit:

            Console.WriteLine("Press any key to exit");

            Console.ReadKey();

        }

        // A data source that is very likely to throw an exception!

        static IEnumerable<int> GetData()

        {

            throw new InvalidOperationException();

        }

    }

　　在某些情况下，对在查询内引发的异常的最佳响应可能是立即停止执行查询。请注意，try 块将 foreach 循环而不是查询本身封闭起来。这是因为 foreach 循环是实际执行查询的场所。

class QueryThatThrows

    {

        static void Main()

        {

            // Data source.

            string[] files = { "fileA.txt", "fileB.txt", "fileC.txt" };

            // Demonstration query that throws.

            var exceptionDemoQuery =

                from file in files

                let n = SomeMethodThatMightThrow(file)

                select n;

            // Runtime exceptions are thrown when query is executed.

            // Therefore they must be handled in the foreach loop.

            try

            {

                foreach (var item in exceptionDemoQuery)

                {

                    Console.WriteLine("Processing {0}", item);

                }

            }

            // Catch whatever exception you expect to raise

            // and/or do any necessary cleanup in a finally block

            catch (InvalidOperationException e)

            {

                Console.WriteLine(e.Message);

            }

            //Keep the console window open in debug mode

            Console.WriteLine("Press any key to exit");

            Console.ReadKey();

        }

        // Not very useful as a general purpose method.

        static string SomeMethodThatMightThrow(string s)

        {

            if (s[4] == 'C')

                throw new InvalidOperationException();

            return @"C:\newFolder\" + s;

        }

    }

    /* Output:

        Processing C:\newFolder\fileA.txt

        Processing C:\newFolder\fileB.txt

        Operation is not valid due to the current state of the object.

     */

参考

　　[1] MSDN,查询表达式Null值处理

巴特西

LINQ查询表达式（5） - LINQ Null值处理&异常处理

查询表达式中处理Null值

在查询表达式中处理异常

参考

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