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