C# Generics

C# allows you to define generic classes, interfaces, abstract classes, fields, methods, static methods, properties, events, delegates, and operators using the type parameter and without the specific data type. A type parameter is a placeholder for a particular type specified when creating an instance of the generic type.

Generic Class

class DataStore<T>
{
    public T Data { get; set; }
}

You can also define multiple type parameters separated by a comma.

class KeyValuePair<TKey, TValue>
{
    public TKey Key { get; set; }
    public TValue Value { get; set; }
}

Instantiating Generic Class

DataStore<string> store = new DataStore<string>();

DataStore<string> store = new DataStore<string>();
store.Data = "Hello World!";
//store.Data = 123; //compile-time error

You can specify the different data types for different objects, as shown below.

DataStore<string> strStore = new DataStore<string>();
strStore.Data = "Hello World!";
//strStore.Data = 123; // compile-time error

DataStore<int> intStore = new DataStore<int>();
intStore.Data = 100;
//intStore.Data = "Hello World!"; // compile-time error

KeyValuePair<int, string> kvp1 = new KeyValuePair<int, string>();
kvp1.Key = 100;
kvp1.Value = "Hundred";

KeyValuePair<string, string> kvp2 = new KeyValuePair<string, string>();
kvp2.Key = "IT";
kvp2.Value = "Information Technology";

Generic Fields

class DataStore<T>
{
    public T data;
}

class DataStore<T>
{
    public T[] data = new T[10];
}

Generic Methods

class DataStore<T>
{
    private T[] _data = new T[10];
    
    public void AddOrUpdate(int index, T item)
    {
        if(index >= 0 && index < 10)
            _data[index] = item;
    }

    public T GetData(int index)
    {
        if(index >= 0 && index < 10)
            return _data[index];
        else 
            return default(T);
    }
}

DataStore<string> cities = new DataStore<string>();
cities.AddOrUpdate(0, "Mumbai");
cities.AddOrUpdate(1, "Chicago");
cities.AddOrUpdate(2, "London");

DataStore<int> empIds = new DataStore<int>();
empIds.AddOrUpdate(0, 50);
empIds.AddOrUpdate(1, 65);
empIds.AddOrUpdate(2, 89);

class Printer
{
    public void Print<T>(T data)
    {
        Console.WriteLine(data);
    }
}

Printer printer = new Printer();
printer.Print<int>(100);
printer.Print(200); // type infer from the specified value
printer.Print<string>("Hello");
printer.Print("World!"); // type infer from the specified value

References
https://www.tutorialsteacher.com/csharp/csharp-generics
https://learn.microsoft.com/en-us/dotnet/csharp/fundamentals/types/generics