读书笔记 C# Lookup<TKey,TElement>和ToLookup方法的浅析

Lookup<TKey,TElement>类型对象和分组是一样的，就好比使用Linq的group关键字后所查询出来的结果，使用foreach的时候，都可以用IGrouping<TKey,TElement>来迭代它们。Lookup<TKey,TElement>也是一种字典，不过它是一对多，不像Dictionary<TKey,TElement>一样是一对一的。Lookup<int,int>和Dictionary<int,List<int>>是一样的。

Lookup<TKey,TElement>的对象可以存储ToLookup方法的结果。代码如下：

class Package

{

    public string Company;

    public double Weight;

    public long TrackingNumber;

}

public static void LookupExample()

{

    // Create a list of Packages to put into a Lookup data structure.

    List<Package> packages = new List<Package> { new Package { Company = "Coho Vineyard", Weight = 25.2, TrackingNumber = 89453312L },

                                                 new Package { Company = "Lucerne Publishing", Weight = 18.7, TrackingNumber = 89112755L },

                                                 new Package { Company = "Wingtip Toys", Weight = 6.0, TrackingNumber = 299456122L },

                                                 new Package { Company = "Contoso Pharmaceuticals", Weight = 9.3, TrackingNumber = 670053128L },

                                                 new Package { Company = "Wide World Importers", Weight = 33.8, TrackingNumber = 4665518773L } };

    // Create a Lookup to organize the packages. Use the first character of Company as the key value.

    // Select Company appended to TrackingNumber for each element value in the Lookup.

    Lookup<char, string> lookup = (Lookup<char, string>)packages.ToLookup(p => Convert.ToChar(p.Company.Substring(, )),

                                                    p => p.Company + " " + p.TrackingNumber);

    // Iterate through each IGrouping in the Lookup and output the contents.

    foreach (IGrouping<char, string> packageGroup in lookup)

    {

        // Print the key value of the IGrouping.

        Console.WriteLine(packageGroup.Key);

        // Iterate through each value in the IGrouping and print its value.

        foreach (string str in packageGroup)

            Console.WriteLine("    {0}", str);

    }

    // This code produces the following output:

    //

    // C

    //     Coho Vineyard 89453312

    //     Contoso Pharmaceuticals 670053128

    // L

    //     Lucerne Publishing 89112755

    // W

    //     Wingtip Toys 299456122

    //     Wide World Importers 4665518773

    // Get the number of key-collection pairs in the Lookup.

    int count = lookup.Count;

    // Select a collection of Packages by indexing directly into the Lookup.

    IEnumerable<string> cgroup = lookup['C'];

    // Output the results.

    Console.WriteLine("\nPackages that have a key of 'C':");

    foreach (string str in cgroup)

        Console.WriteLine(str);

    // This code produces the following output:

    //

    // Packages that have a key of 'C'

    // Coho Vineyard 89453312

    // Contoso Pharmaceuticals 670053128

    // Determine if there is a key with the value 'G' in the Lookup.

    bool hasG = lookup.Contains('G');