What are generics and how to use them in C#

Generics in C# are a feature that allows us to define classes, interfaces, and methods dependent on a data type (which, logically, can vary).

This data type is specified at compile time, providing flexibility and type safety without sacrificing performance.

Creation and use of generic classes

Definition of a generic class

To define a generic class, the symbol <T> is used after the class name. Here, T is a type parameter that can be replaced by any specific type when an instance of the class is created.

public class MyClass<T>
{
    private T content;

    public MyClass(T content)
    {
        this.content = content;
    }

    public T GetContent()
    {
        return content;
    }

    public void ShowContent()
    {
        Console.WriteLine($"The content of the box is: {content}");
    }
}

public class MyClass<type1>
{
   // ... content of MyClass
}

Instantiation of a generic class

Now, to instantiate a generic class, we must specify the data type to be used in place of the type parameter T (or whatever you named the type).

// For integer
MyClass<int> intBox = new MyClass<int>(123);
intBox.ShowContent(); // The content of the box is: 123

// For string
MyClass<string> stringBox = new MyClass<string>("Hello");
stringBox.ShowContent(); // The content of the box is: Hello

public interface IRepository<T>
{
    void Add(T item);
    T Get(int id);
}

public delegate T Function<T>(T arg);

Generics in collections

Generic collections in .NET (in the System.Collections.Generic namespace) are widely used and offer a safer and more efficient alternative to non-generic collections.

For example, let’s see how to create a List for integers and for text strings.

// integer list
List<int> numbers = new List<int> { 1, 2, 3, 4, 5 };

// string list
List<string> letters = new List<string> { "A", "B", "C", "D", "E" };

Generic type constraints

Constraints allow limiting the types that can be used as arguments for type parameters. This is achieved using the where keyword.

public class Storage<T> where T : class
{
    private List<T> items = new List<T>();

    public void Add(T item)
    {
        items.Add(item);
    }

    public T Get(int index)
    {
        return items[index];
    }
}

Some common constraints include:

Generic methods with multiple types

A generic method with multiple type parameters allows specifying more than one type in the method definition.

public class Converter
{
    public TResult Convert<TInput, TResult>(TInput input)
    {
        TResult result = // whatever with input        
        return result;
    }
}