Value vs Reference Types

C# has two categories of types:

**Value types** store data directly:

```csharp

int x = 10;

bool flag = true;

char letter = 'A';

double price = 9.99;

```

**Reference types** store a reference (pointer) to data:

```csharp

string name = "Alice";

int[] numbers = { 1, 2, 3 };

```

---

Constants

Use `const` for compile-time constants:

```csharp

const double Pi = 3.14159;

const string AppName = "uByte";

```

Use `readonly` for values set once at runtime:

```csharp

readonly DateTime startTime = DateTime.Now;

```

---

Type Conversion

**Implicit** (safe, no data loss):

```csharp

int i = 42;

long l = i; // int → long (widening)

double d = i; // int → double

```

**Explicit cast** (may lose precision):

```csharp

double pi = 3.99;

int truncated = (int)pi; // 3

```

**Parse from string**:

```csharp

int n = int.Parse("42");

bool ok = int.TryParse("abc", out int result); // ok = false, safe

```

---

Nullable Types

Add `?` to allow a value type to hold `null`:

```csharp

int? age = null;

if (age.HasValue) {

Console.WriteLine(age.Value);

}

// Null-coalescing: use default when null

int display = age ?? 0;

```

---

What's Next?

You know the type system. Next: **print formatting** — `Console.Write`, `Console.WriteLine`, string interpolation, and format specifiers.