Dictionary Overview in C#

Ravi Satyadarshi
Jun 27, 2017
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Dictionary in C#

Dictionary is a generic class that belongs to the System.Collection namespace in .NET.
The dictionary type in C# allows the user to retrieve a corresponding value very quickly for a specified key value.
A dictionary can store Keys and Values of any data type in .NET.
It provides a mapping from a set of keys to a set of values .
Represented as Dictionary<TKey, TValue> pair where TKey represents Key and TValue represents the value associated with the key.
Each Key in a Dictionary is associated with a corresponding Value.
Retrieving a value using its key is very fast, close to O(1), because the Dictionary<TKey, TValue> class is implemented as a hash table.
Each key in the dictionary must be unique and cannot be NULL but a value corresponding to a key can be NULL and/or a duplicate.
The capacity of a Dictionary<TKey, TValue> varies according to the number of elements present in the Dictionary.
As elements are added to a Dictionary<TKey, TValue>, the capacity is automatically increased as required by reallocating the internal array.
The maximum capacity of a dictionary is up to 2 billion elements on a 64-bit system by setting the enabled attribute of the gcAllowVeryLargeObjects configuration element to true in the run-time environment.
Each item in the Dictionary can be iterated through KeyValuePair<TKey, TValue> structure representing a value and it's key.
In case of multiuser environment a Dictionary<TKey, TValue> can support multiple readers concurrently until the collection is modified even though enumerating through a collection is intrinsically not a thread-safe procedure.
In the rare case where an enumeration contends with write accesses, the collection must be locked during the entire enumeration.
So if you have to iterate through each member of the collection use a List type but if have want to look up a value in a collection then always choose Dictionary.
Has better performance than any other type if the number of elements is large in a collection.
Almost the same performance as that of an array so sometimes an array and dictionary objects are interchangeable (refhttp://www.dotnetperls.com/array-dictionary-test).

Methods and Attributes with Example

1. Declaration

A Dictionary can be declared as follows:

//If capacity of dictionary is not known
Dictionary<string, string> dicObj = new Dictionary<string, string>();
//If capacity of dictionary is known
Dictionary<string, string> dicObj = new Dictionary<string, string>(5);

2. Add()

This method is used to add elements with key value pair in the dictionary object.

DicObj.Add("key1", "val1");
DicObj.Add("key2", " val2");
DicObj.Add("key3", " val3");
DicObj.Add("key4", " val4");

3. Reading or iterating through Dictionary element

Since the Dictionary type represents a collection so we can use the foreach loop to go through all the items and read them using they Key and Value properties.

foreach (KeyValuePair<string, Int16> author in AuthorList)
{
Console.WriteLine("Key: {0}, Value: {1}", author.Key, author.Value);
}

4. Remove()

This method is used to remove an element from a dictionary object.

DicObj.Remove("key4");

5. Clear()

This method removes all the elements from a dictionary object.

DicObj.Clear();

6. ContainsKey()

This method is used to find a key from the Dictionary and it returns a Boolean value indicating whether that key is found in the collection or not.

if (!AuthorList.ContainsKey("key4"))
{
DicObj ["key4"] = 20;
}

7. ContainsValue()

This method is used to check whether a value exists in the dictionary or not and it returns a Boolean value indicating whether that value is found in the collection or not.