var, const, and the null concept

var: inference, not dynamic

var count = 10;          // compiler infers int — count IS an int, forever
var name = "Ada";        // string
var price = 19.99m;      // decimal (the suffix drives inference)

var is not JavaScript's var. The type is inferred at compile time and then fixed; var x = 5; x = "hi"; is a compile error. Rule of thumb: use var when the right-hand side makes the type obvious (new List<int>(), a constructor call); write the explicit type when it aids reading (long total = 0;).

const

const int MaxRetries = 3;
const double Pi = 3.14159;

const is a compile-time constant: its value is baked into every use site, it's implicitly static, and it can never be reassigned. Use it for values that are truly permanent (MaxRetries, DaysInWeek). Naming: PascalCase is the C# convention for constants.

null: the absence of an object

A variable of a reference type (like string) either refers to an object or is null — "refers to nothing". Dereferencing null throws NullReferenceException at runtime:

string nickname = null;
Console.WriteLine(nickname.Length);   // throws at runtime

C#'s compiler flow analysis warns you about likely nulls (nullable reference types), and null checks are a normal part of validating input. For now: know the word, expect the exception name, and treat every incoming value as possibly-absent until validated. Value types (int, decimal, bool) cannot be null — they always hold a value.