C＃|哈希表类

Hashtable类表示基于键的哈希码组织的键/值对的集合。此类位于System.Collections命名空间下。 Hashtable类提供了各种类型的方法，这些方法用于对哈希表执行不同类型的操作。在Hashtable中，键用于访问集合中存在的元素。对于非常大的Hashtable对象，可以在64位系统上将最大容量增加到20亿个元素。

哈希表类的特征：

• 在Hashtable中，键不能为null，而值可以为null。
• 在哈希表中，密钥对象必须是不可变的，只要它们在哈希表中用作键即可。
• 哈希表的容量是哈希表可以容纳的元素数。
• 哈希表中的每个键对象都提供了哈希函数。

建设者

CONSTRUCTOR	DESCRIPTION
Hashtable()	Initializes a new, empty instance of the Hashtable class using the default initial capacity, load factor, hash code provider, and comparer.
Hashtable(IDictionary)	Initializes a new instance of the Hashtable class by copying the elements from the specified dictionary to the new Hashtable object. The new Hashtable object has an initial capacity equal to the number of elements copied, and uses the default load factor, hash code provider, and comparer.
Hashtable(IDictionary, IEqualityComparer)	Initializes a new instance of the Hashtable class by copying the elements from the specified dictionary to a new Hashtable object. The new Hashtable object has an initial capacity equal to the number of elements copied, and uses the default load factor and the specified IEqualityComparer object.
Hashtable(IDictionary, IHashCodeProvider, IComparer)	Initializes a new instance of the Hashtable class by copying the elements from the specified dictionary to the new Hashtable object. The new Hashtable object has an initial capacity equal to the number of elements copied, and uses the default load factor, and the specified hash code provider and comparer. This API is obsolete. For an alternative, see Hashtable(IDictionary, IEqualityComparer).
Hashtable(IDictionary, Single)	Initializes a new instance of the Hashtable class by copying the elements from the specified dictionary to the new Hashtable object. The new Hashtable object has an initial capacity equal to the number of elements copied, and uses the specified load factor, and the default hash code provider and comparer.
Hashtable(IDictionary, Single, IEqualityComparer)	Initializes a new instance of the Hashtable class by copying the elements from the specified dictionary to the new Hashtable object. The new Hashtable object has an initial capacity equal to the number of elements copied, and uses the specified load factor and IEqualityComparer object.
Hashtable(IDictionary, Single, IHashCodeProvider, IComparer)	Initializes a new instance of the Hashtable class by copying the elements from the specified dictionary to the new Hashtable object. The new Hashtable object has an initial capacity equal to the number of elements copied, and uses the specified load factor, hash code provider, and comparer.
Hashtable(IEqualityComparer)	Initializes a new, empty instance of the Hashtable class using the default initial capacity and load factor, and the specified IEqualityComparer object.
Hashtable(IHashCodeProvider, IComparer)	Initializes a new, empty instance of the Hashtable class using the default initial capacity and load factor, and the specified hash code provider and comparer.
Hashtable(Int32)	Initializes a new, empty instance of the Hashtable class using the specified initial capacity, and the default load factor, hash code provider, and comparer.
Hashtable(Int32, IEqualityComparer)	Initializes a new, empty instance of the Hashtable class using the specified initial capacity and IEqualityComparer, and the default load factor.
Hashtable(Int32, IHashCodeProvider, IComparer)	Initializes a new, empty instance of the Hashtable class using the specified initial capacity, hash code provider, comparer, and the default load factor.
Hashtable(Int32, Single)	Initializes a new, empty instance of the Hashtable class using the specified initial capacity and load factor, and the default hash code provider and comparer.
Hashtable(Int32, Single, IEqualityComparer)	Initializes a new, empty instance of the Hashtable class using the specified initial capacity, load factor, and IEqualityComparer object.
Hashtable(Int32, Single, IHashCodeProvider, IComparer)	Initializes a new, empty instance of the Hashtable class using the specified initial capacity, load factor, hash code provider, and comparer.
Hashtable(SerializationInfo, StreamingContext)	Initializes a new, empty instance of the Hashtable class that is serializable using the specified SerializationInfo and StreamingContext objects.

例子：

// C# program to create Hashtable
using System;
using System.Collections;
  
class GFG {
  
    // Main method
    static void Main(string[] args)
    {
  
        // create and initalize Hash table
        // using Add() method
        Hashtable g = new Hashtable();
        g.Add(1, "welcome");
        g.Add(2, "to");
        g.Add(3, "tutorials");
        g.Add(4, "of");
        g.Add(5, "C#");
  
        ICollection key = g.Keys;
  
        // Print Hash table
        Console.WriteLine("Hashtable:");
        Console.WriteLine();
        foreach(var val in key)
        {
            Console.WriteLine(val + "-" + g[val]);
        }
    }
}

输出：

Hashtable:

5-C#
4-of
3-tutorials
2-to
1-welcome

特性

PROPERTY	DESCRIPTION
comparer	Gets or sets the IComparer to use for the Hashtable.
Count	Gets the number of key/value pairs contained in the Hashtable.
EqualityComparer	Gets the IEqualityComparer to use for the Hashtable.
hcp	Gets or sets the object that can dispense hash codes.
IsFixedSize	Gets a value indicating whether the Hashtable has a fixed size.
IsReadOnly	Gets a value indicating whether the Hashtable is read-only.
IsSynchronized	Gets a value indicating whether access to the Hashtable is synchronized (thread safe).
Item[Object]	Gets or sets the value associated with the specified key.
Keys	Gets an ICollection containing the keys in the Hashtable.
SyncRoot	Gets an object that can be used to synchronize access to the Hashtable.
Values	Gets an ICollection containing the values in the Hashtable.

例子：

// C# Program to illustrate the
// Properties of Hashtable
using System;
using System.Collections;
  
class GFG {
  
    // Main method
    static void Main(string[] args)
    {
  
        // create and initalize Hash table
        // using Add() method
        Hashtable has1 = new Hashtable();
        has1.Add("1", "Welcome");
        has1.Add("2", "to");
        has1.Add("3", "geeks");
        has1.Add("4", "for");
        has1.Add("5", "geeks");
  
        // --------- Using IsSynchronized Property 
  
        // Creating a synchronized packing
        // around the Hashtable
        Hashtable has2 = Hashtable.Synchronized(has1);
  
        // print the status of both Hashtables
        Console.WriteLine("has1 Hashtable is {0}.", 
             has1.IsSynchronized ? "synchronized" : 
                               "not synchronized");
  
        Console.WriteLine("has2 Hashtable is {0}.", 
             has2.IsSynchronized ? "synchronized" : 
                               "not synchronized");
  
        // --------- Using Count Property
  
        Console.WriteLine("Total Number of Elements in has1: "
                                                + has1.Count);
    }
}

输出：

has1 Hashtable is not synchronized.
has2 Hashtable is synchronized.
Total Number of Elements in has1: 5

方法

METHOD	DESCRIPTION
Add(Object, Object)	Adds an element with the specified key and value into the Hashtable.
Clear()	Removes all elements from the Hashtable.
Clone()	Creates a shallow copy of the Hashtable.
Contains(Object)	Determines whether the Hashtable contains a specific key.
ContainsKey(Object)	Determines whether the Hashtable contains a specific key.
ContainsValue(Object)	Determines whether the Hashtable contains a specific value.
CopyTo(Array, Int32)	Copies the Hashtable elements to a one-dimensional Array instance at the specified index.
Equals(Object)	Determines whether the specified object is equal to the current object.
GetEnumerator()	Returns an IDictionaryEnumerator that iterates through the Hashtable.
GetHash(Object)	Returns the hash code for the specified key.
GetHashCode()	Serves as the default hash function.
GetObjectData(SerializationInfo, StreamingContext)	Implements the ISerializable interface and returns the data needed to serialize the Hashtable.
GetType()	Gets the Type of the current instance.
KeyEquals(Object, Object)	Compares a specific Object with a specific key in the Hashtable.
MemberwiseClone()	Creates a shallow copy of the current Object.
OnDeserialization(Object)	Implements the ISerializable interface and raises the deserialization event when the deserialization is complete.
Remove(Object)	Removes the element with the specified key from the Hashtable.
Synchronized(Hashtable)	Returns a synchronized (thread-safe) wrapper for the Hashtable.
ToString()	Returns a string that represents the current object.

例子：

// C# program to illustrate Add()
// and Remove() Methods
using System;
using System.Collections;
  
class Program {
  
    // Main method
    static void Main(string[] args)
    {
  
        // -------- Add() Method --------
  
        // create and initalize Hash table
        // using Add() method
        Hashtable g = new Hashtable();
        g.Add("1", "Welcome");
        g.Add("2", "to");
        g.Add("3", "Geeks");
        g.Add("4", "for");
        g.Add("5", "Geeks");
  
        ICollection key = g.Keys;
  
        // Print Hash table
        Console.WriteLine("Hashtable Contains:");
  
        foreach(var val in key)
        {
            Console.WriteLine(val + "-" + g[val]);
        }
  
        Console.WriteLine();
  
        // --------- Remove() Method ----------'
  
        // Remove element 4
        // using Remove() method
        g.Remove("4");
  
        // printing updated Hash table
        Console.WriteLine("Hashtable after removing element 4:");
  
        foreach(var val in key)
        {
            Console.WriteLine(val + "-" + g[val]);
        }
    }
}

输出：

Hashtable Contains:
2-to
3-Geeks
5-Geeks
1-Welcome
4-for

Hashtable after removing element 4:
2-to
3-Geeks
5-Geeks
1-Welcome

参考：

• https://docs.microsoft.com/zh-cn/dotnet/api/system.collections.hashtable?view=netframework-4.7.2#definition