Dictionary Overview in C#

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.

if (!AuthorList.ContainsValue("Value4"))
{
// write code here
}

8. Keys

This attribute returns a collection of keys present in the dictionary object. It returns an object of KeyCollection type.

Dictionary<string, string>.KeyCollection keys = DicObj.Keys;
foreach (string key in keys)
{
// code to process keys
}

9. Values

This attribute returns a collection of values present in the dictionary object. It returns an object of ValueCollection type.

Dictionary<string, string>.ValueCollection values = DicObj.Values;
foreach (string val in values)
{
// code to process values
}

10. Item

The Item property is used to get and set the value associated with the specified key.

// Set Item value
DicObj ["key4"] = value5;
// Get Item value
string value = DicObj ["key4];

11. Count

This property is used to count the number of elements present in the dictionary at any time.

Int noOfElements = DicObj.Count