Generic methods and classes

The duplication problem

Swap two values, find the max, reverse a list — for int, then string, then Money... The logic is identical; only the type changes. Copy-pasting per type is how bugs breed.

<T> writes it once

static T Max<T>(T a, T b) where T : IComparable<T>
{
    return a.CompareTo(b) >= 0 ? a : b;
}

Max(3, 7);            // T = int
Max("apple", "pear"); // T = string

T is a type parameter: the caller's argument type fills it in, and the compiler enforces every operation on T against what the constraint allows. Inside the method there is no casting, no boxing, and no way to sneak in the wrong type.

Generic classes work the same way:

class Box<T>
{
    public T Content { get; set; }
}

var ib = new Box<int> { Content = 42 };
var sb = new Box<string> { Content = "hi" };
// ib.Content is int, sb.Content is string — statically

You already use generics everywhere: List<T>, Dictionary<TKey, TValue> — now you can write your own.