LINQ之非延迟执行标准查询操作符(上)

操作符：ToArray

原型：

 

public static TSource[] ToArray<TSource>(
this IEnumerable<TSource> source
)

 

 

描述：将一个IEnumerable<T>的输入序列，转换成T类型的数组，这个方法经常用来缓存一个序列，以防止在我们遍历它之前， 序列的数据发生改变。

 

例子：

 

List<Product> products = new List<Product>
{
new Product{ProductId = 201,ProductName = "Chuckit",CategoryId = 2},
new Product{ProductId = 202,ProductName = "SafeMade",CategoryId = 2},
new Product{ProductId = 101,ProductName = "Taste",CategoryId = 1},
new Product{ProductId = 102,ProductName = "Canidae",CategoryId = 1},
new Product{ProductId = 103,ProductName = "Flavor",CategoryId = 1}

};
string [] productArray = products.Select(p => p.ProductName).ToArray();
foreach (string product in productArray)
{
Console.WriteLine(product);
}

操作符：ToList

原型：

public static List<TSource> ToList<TSource>(
this IEnumerable<TSource> source
)

 

描述：将输入序列转换成T类型的List对象，同ToArray一样，这个方法经常用来缓存一个序列，以防止在我们遍历它之前， 序列的数据发生改变。

 

string[] allBrands = new string[] { "Exuviance", "Avene", "Baby Quasar", "Ecoya", "Alterna", "Ecru New York" };
List<int> brandLength = allBrands.Select(b => b.Length).ToList();
foreach (int length in brandLength)
{
Console.WriteLine(length);
}

操作符：ToDictonary

原型：四种

public static Dictionary<TKey, TSource> ToDictionary<TSource, TKey>(
this IEnumerable<TSource> source,
Func<TSource, TKey> keySelector
)

 

public static Dictionary<TKey, TSource> ToDictionary<TSource, TKey>(
this IEnumerable<TSource> source,
Func<TSource, TKey> keySelector,
IEqualityComparer<TKey> comparer
)

 

public static Dictionary<TKey, TElement> ToDictionary<TSource, TKey, TElement>(
this IEnumerable<TSource> source,
Func<TSource, TKey> keySelector,
Func<TSource, TElement> elementSelector
)

 

public static Dictionary<TKey, TElement> ToDictionary<TSource, TKey, TElement>(
this IEnumerable<TSource> source,
Func<TSource, TKey> keySelector,
Func<TSource, TElement> elementSelector,
IEqualityComparer<TKey> comparer
)

 

描述：根据指定的key，返回一个Dictionary序列

原型一和原型二，原型三和原型四，提供了IEqualityComparer用于比较2个对象是否相等，原型一和原型三，主要区别在于与原型三允许返回与输入序列不一样类型的序列，

因此，我们主要看看原型一。

 

List<Product> products = new List<Product>
{
new Product{ProductId = 201,ProductName = "Chuckit",CategoryId = 2},
new Product{ProductId = 202,ProductName = "SafeMade",CategoryId = 2},
new Product{ProductId = 101,ProductName = "Taste",CategoryId = 1},
new Product{ProductId = 102,ProductName = "Canidae",CategoryId = 1},
new Product{ProductId = 103,ProductName = "Flavor",CategoryId = 1}

};
Dictionary<int, Product> productDic = products.ToDictionary(p => p.ProductId);
Product someProduct = productDic[201];
Console.WriteLine("{0},{1}",someProduct.ProductId,someProduct.ProductName);

 

 

操作符：ToLookup

原型：ToLookup和ToDictionary一样提供了四种原型，且四种都是对应的。

 

public static ILookup<TKey, TSource> ToLookup<TSource, TKey>(
this IEnumerable<TSource> source,
Func<TSource, TKey> keySelector
)

 

描述：ToLookup返回的是一个ILookup类型的序列，ILookup允许我们根据key来查找，跟Dictionary不一样的地方是，key可以不唯一，且多个元素可以存在一个key里面

 

List<Product> products = new List<Product>
{
new Product{ProductId = 201,ProductName = "Chuckit",CategoryId = 2},
new Product{ProductId = 202,ProductName = "SafeMade",CategoryId = 2},
new Product{ProductId = 101,ProductName = "Taste",CategoryId = 1},
new Product{ProductId = 102,ProductName = "Canidae",CategoryId = 1},
new Product{ProductId = 103,ProductName = "Flavor",CategoryId = 1}

};
ILookup<int, Product> productLookups = products.ToLookup(p => p.CategoryId);
IEnumerable < Product >  productLookup = productLookups[1];

foreach (Product product in productLookup)
{
Console.WriteLine("{0},{1}",product.ProductId,product.ProductName);
}

 

在这个例子中，通过key在ILookup序列里面查找，返回的是一个IEnumerable<Product>的序列，而不再像Dictionary一样，返回唯一的一个对象。

 

操作符：SequenceEqual

原型：

 

public static bool SequenceEqual<TSource>(
this IEnumerable<TSource> first,
IEnumerable<TSource> second
)

 

public static bool SequenceEqual<TSource>(
this IEnumerable<TSource> first,
IEnumerable<TSource> second,
IEqualityComparer<TSource> comparer
)

 

描述：判断2个序列是否相等，注意，这里的相等，要求两个序列的元素个数要相等，相同索引位置的元素也要相等，原型二提供了相等比较器，允许自定义对象相等的算法。

 

来看个原型二的例子：

首先定义相等比较器：

 

public class StringNumberComparee:IEqualityComparer<string>
{
public bool Equals(string x, string y)
{
return x.TrimStart('0').Equals(y.TrimStart('0'));
}

public int GetHashCode(string obj)
{
return obj.GetHashCode();
}
}

 

string[] productIds = new string[] { "1", "3", "5", "12", "34", "21" };

string[] productIds2 = new string[] { "0001", "0003", "0005", "0012", "0034", "0021" };

bool twoProductIdsAreSame = productIds.SequenceEqual(productIds2, new StringNumberComparee());

Console.WriteLine(twoProductIdsAreSame);

返回结果正如我们预料的为True。