C# Generics

In C#, generics introduce the concept of type parameters which make it possible to design classes and methods that defer the specification of one or more types until the class or method is declared and instantiated.

A generic type is declared by specifying a type parameter T in angle brackets which can be used as a type of field, properties, method parameters, return types, and delegates, etc. The following example, define a generic class.

class MyClass<T>
    public T Val { get; set; }

In the above code, MyClass is a generic class and T is a type parameter. The Val is a generic property because we have used a type parameter T as its type instead of the specific data type.

You can create an instance of generic classes by specifying the data type in angle brackets. Let's consider the following code.

MyClass<int> myClassInt = new MyClass<int>();
MyClass<string> myClassString = new MyClass<string>();
MyClass<DateTime> myClassDateTime = new MyClass<DateTime>();

myClassInt.Val = 34;                       
myClassString.Val = "This is a C# Tutorial";
myClassDateTime.Val = DateTime.Today;

Console.WriteLine("Type: {0}, \t Value: {1}", myClassInt, myClassInt.Val);
Console.WriteLine("Type: {0}, \t Value: {1}", myClassString, myClassString.Val);
Console.WriteLine("Type: {0}, \t Value: {1}", myClassDateTime, myClassDateTime.Val);

In the above code, you can see that we have created three instances of the MyClass for different data types i.e. integer, string, and date-time. When the instances are created T will be replaced with a specified type, wherever T is used in the entire class at compile-time. The three instances of MyClass have been created by replacing T with int, string, and DateTime data types, and these objects are used to store int, string, and DateTime values respectively.

Let's run the above code and you will see the following code.

Type: CSharpTutorialDemo.Generics+MyClass`1[System.Int32],       Value: 34
Type: CSharpTutorialDemo.Generics+MyClass`1[System.String],      Value: This is a C# Tutorial
Type: CSharpTutorialDemo.Generics+MyClass`1[System.DateTime],    Value: 10/22/2020 12:00:00 AM

Generic Collections

In C#, the generic collections represent the System.Collections.Generic namespace which contains interfaces and classes that define generic collections, which allow users to create strongly typed collections that provide better type safety and performance than non-generic strongly typed collections.

  • It will enforce a type of safety so you can store only the elements which are having the same data type.
  • It provides a generic implementation of standard data structures like linked lists, stacks, queues, and dictionaries.
  • These are type-safe because you can store only those items that are type-compatible with the type of collection in a generic collection.

The following table lists some of the frequently used classes in the System.Collections.Generic namespace.

Class Description
Dictionary Represents a collection of keys and values.
List Represents a strongly typed list of objects that can be accessed by index. Provides methods to search, sort, and manipulate lists.
Queue Represents a first-in, first-out (FIFO) collection of objects.
SortedList Represents a collection of key/value pairs that are sorted by the keys and are accessible by key and by index.
Stack Represents a last-in, first-out (LIFO) collection of objects.
HashSet It is an unordered collection of unique elements. It prevents duplicates from being inserted into the collection.
LinkedList It allows fast inserting and removing of elements. It implements a classic linked list.

The following example shows how to create and initialize a List and how to display its values.

List<Customer> customers = new List<Customer>();
customers.Add(new Customer { Id = 1, Name = "John" });
customers.Add(new Customer { Id = 2, Name = "Mark"});
customers.Add(new Customer { Id = 3, Name = "Stella"});

foreach (var customer in customers)
    Console.WriteLine("{0}, {1}", customer.Id, customer.Name);

Let's run the above code and you will see the following code.

1, John
2, Mark
3, Stella

In C#, many of the generic collection types are direct analogs of non-generic types.

  • Dictionary<TKey,TValue> is a generic version of Hashtable, it uses the generic structure KeyValuePair<TKey,TValue> for enumeration instead of DictionaryEntry.
  • List<T> is a generic version of ArrayList.
  • There are generic Queue<T> and Stack<T> classes that correspond to the nongeneric versions.
  • There are generic and nongeneric versions of SortedList<TKey, TValue>, both versions are hybrids of a dictionary and a list.
  • The SortedDictionary<TKey, TValue> generic class is a pure dictionary and has no nongeneric counterpart.
  • The LinkedList<T> generic class is a true linked list and has no nongeneric counterpart.

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All the examples related to the generics and generic collections are available in the Generics.cs file of the source code. Download the source code and try out all the examples for better understanding.