Types, Variables and Value vs Reference

Two kinds of types

Every C# type is a value type or a reference type. Value types (int, long, double, decimal, bool, char, DateTime, enums and any struct) hold their data directly; assigning one copies the value. Reference types (string, arrays, class instances, record classes, delegates) hold a reference to an object on the managed heap; assigning one copies the reference, so two variables can point to the same object. Use decimal for money (base-10, no binary rounding surprises) and double for scientific values. var asks the compiler to infer the type; it is still statically typed. string is a reference type but immutable: methods like ToUpper() return new strings. Integer arithmetic wraps on overflow by default; a checked block or the CheckForOverflowUnderflow project setting throws OverflowException instead.

Copying values versus references

The struct copy is independent; the class copy shares one object.

var p1 = new PointStruct { X = 1 };
var p2 = p1;          // copies the value
p2.X = 99;
Console.WriteLine(p1.X);   // still 1

var c1 = new PointClass { X = 1 };
var c2 = c1;          // copies the reference
c2.X = 99;
Console.WriteLine(c1.X);   // 99: same object

decimal price = 0.1m + 0.2m;   // exactly 0.3
double approx = 0.1 + 0.2;     // 0.30000000000000004

struct PointStruct { public int X; }
class PointClass { public int X; }

Large mutable structs are a trap

Structs are copied on every assignment and method call. Keep structs small and preferably immutable (readonly struct), or you will modify a copy and wonder why the original did not change.

त्वरित जाँच: Which type should you use for currency amounts in C#?

• float
• double
• decimal
• int with implicit rounding