1.OfType :
即接受基于IEnumerable<T> 接口的信息源,也接受那些在 .NET Framework 1.0 中出现的非参数化的 IEnumerable 接口(non-parameterized IEnumerable interface)。OfType 操作符允许用户在标准的 .NET collections 类(classic .NET collections)上应用标准查询操作符,就像下面的程序:
代码:
IEnumerable classic = new OlderCollectionType();
IEnumerable<object> modern = classic.OfType<object>();
在这个例子中,变量 modern 产生了与变量 classic 一样的顺序的数据列表(same sequence of values),但是不同的是它的类型兼容最新的 IEnumerable<T> 代码(modern IEnumerable<T> code),包括标准查询操作符。
OfType 操作符对新的信息源也是有用的,因为它允许从基于类型的信息源(source based on type)中过滤出数据(filtering values from)。当要制作新的顺序的时候(producing the new sequence),OfType 简单的忽略(omits)原有序列的成员(members of the original sequence)就可以了,这是与类型实参(type argument)不相符的。分析下面的程序,目标是将 string 字符串数据从一个有不同种类数据的数组(heterogeneous array)中分解出来:
代码:
object[] vals = { 1, "Hello", true, "World", 9.1 };
IEnumerable<string> justStrings = vals.OfType<string>();
当在 foreach 语句中列举(enumerate)变量 justStrings 中的数据时,将得到两个依次为“Hello”和“World”的 string 字符串序列(a sequence of two strings )。
2.ToList,ToArray:
对应用程序想缓存查询赋值的结果,ToList 和 ToArray 这两个操作符提供用来强制查询的直接赋值(force the immediate evaluation),以返回一个含有查询赋值的结果的 List<T> 或者Array 数组。
为了解延迟查询赋值是怎么工作的,请考察下面一段简单地查询一个数组的程序:
代码:
// declare a variable containing some strings
string[] names = { "Allen", "Arthur", "Bennett" };
// declare a variable that represents a query
IEnumerable<string> ayes = names.Where(s => s[0] == 'A');
// evaluate the query
foreach (string item in ayes)
Console.WriteLine(item);
// modify the original information source
names[0] = "Bob";
// evaluate the query again, this time no "Allen"
foreach (string item in ayes)
Console.WriteLine(item);
查询在每次变量 ayes 迭代结束(iterated over)时赋值。为了显示需要对结果做一份缓存的copy,我们可以简单给这个查询附加上一个 ToList 或 一个 ToArray 操作符,如下所示:
代码:
// declare a variable containing some strings
string[] names = { "Allen", "Arthur", "Bennett" };
// declare a variable that represents the result
// of an immediate query evaluation
string[] ayes = names.Where(s => s[0] == 'A').ToArray();
// iterate over the cached query results
foreach (string item in ayes)
Console.WriteLine(item);
// modifying the original source has no effect on ayes
names[0] = "Bob";
// iterate over result again, which still contains "Allen"
foreach (string item in ayes)
Console.WriteLine(item);
ToList 和 ToArray 都强制查询的赋值,就像执行一个标准查询操作符(如 First, ElementAt, Sum, Average, All, 等)返回一个单独的值(singleton values)一样。
public void ToArray() {
double[] doubles = { 1.7, 2.3, 1.9, 4.1, 2.9 };
var sortedDoubles =
from d in doubles
orderby d descending
select d;
var doublesArray = sortedDoubles.ToArray();
Console.WriteLine("Every other double from highest to lowest:");
for (int d = 0; d < doublesArray.Length; d += 2) {
Console.WriteLine(doublesArray[d]);
}
}
public void ToList() {
string[] words = { "cherry", "apple", "blueberry" };
var sortedWords =
from w in words
orderby w
select w;
var wordList = sortedWords.ToList();
Console.WriteLine("The sorted word list:");
foreach (var w in wordList) {
Console.WriteLine(w);
}
}
public void ToDictionary() {
var scoreRecords = new [] { new {Name = "Alice", Score = 50},
new {Name = "Bob" , Score = 40},
new {Name = "Cathy", Score = 45}
};
var scoreRecordsDict = scoreRecords.ToDictionary(sr => sr.Name);
Console.WriteLine("Bob's score: {0}", scoreRecordsDict["Bob"]);
}
public void OfType() {
object[] numbers = { null, 1.0, "two", 3, 4.0f, 5, "six", 7.0 };
var doubles = numbers.OfType<double>();
Console.WriteLine("Numbers stored as doubles:");
foreach (var d in doubles) {
Console.WriteLine(d);
}
}
Every other double from highest to lowest:
4.1
2.3
1.7
ToList:Result
The sorted word list:
apple
blueberry
cherry
ToDictionary:Result
Bob's score: {Name=Bob, Score=40}
OfType:Result
Numbers stored as doubles:
1
7