C# Control Flow: if/else, switch, Loops & Jump Statements

using System;

int score = 78;

// Standard if / else if / else chain
if (score >= 90)
{
    Console.WriteLine("Grade: A");
}
else if (score >= 70)
{
    Console.WriteLine("Grade: B");   // ← this runs for score=78
}
else if (score >= 50)
{
    Console.WriteLine("Grade: C");
}
else
{
    Console.WriteLine("Grade: F");
}

// Nested if — find quadrant
int x = 3, y = -2;
if (x > 0)
{
    if (y > 0) Console.WriteLine("Quadrant I");
    else       Console.WriteLine("Quadrant IV");  // ← x>0, y<0
}
else
{
    if (y > 0) Console.WriteLine("Quadrant II");
    else       Console.WriteLine("Quadrant III");
}

using System;

// 1. Type pattern with 'is' — declare and check in one step
object obj = "Hello";
if (obj is string s)
{
    Console.WriteLine($"String of length {s.Length}");  // s is bound here
}

// 2. Null check pattern
string? name = null;
if (name is not null)   // C# 9 'not' pattern — cleaner than != null
    Console.WriteLine(name);
else
    Console.WriteLine("name is null");

// 3. Type pattern in switch — heterogeneous collection
object[] items = { 42, "hello", 3.14, true, null };
foreach (var item in items)
{
    string desc = item switch
    {
        int i   when i > 0 => $"Positive int: {i}",
        int i               => $"Non-positive int: {i}",
        string str          => $"String: {str}",
        double d            => $"Double: {d}",
        bool b              => $"Bool: {b}",
        null                => "null value",
        _                   => "unknown"
    };
    Console.WriteLine(desc);
}

using System;
using System.Collections.Generic;

// 1. for — count-controlled
Console.WriteLine("for:");
for (int i = 1; i <= 3; i++)
    Console.Write($"{i} ");
Console.WriteLine();

// 2. while — condition-first
Console.WriteLine("while:");
int j = 1;
while (j <= 3)
{
    Console.Write($"{j} ");
    j++;
}
Console.WriteLine();

// 3. do-while — body first, condition after
Console.WriteLine("do-while:");
int k = 1;
do
{
    Console.Write($"{k} ");
    k++;
} while (k <= 3);
Console.WriteLine();

// 4. foreach — collection iteration (preferred)
Console.WriteLine("foreach:");
var colors = new[] { "Red", "Green", "Blue" };
foreach (string color in colors)
    Console.Write($"{color} ");
Console.WriteLine();

// foreach over Dictionary (key-value pairs)
Console.WriteLine("foreach dict:");
var scores = new Dictionary<string, int> { ["Alice"]=95, ["Bob"]=82 };
foreach (var (name, score) in scores)
    Console.WriteLine($"  {name}: {score}");

using System;

// break — exit innermost loop immediately
Console.WriteLine("break:");
for (int i = 1; i <= 6; i++)
{
    if (i == 4) break;
    Console.Write($"{i} ");
}
Console.WriteLine();   // 1 2 3

// continue — skip to next iteration
Console.WriteLine("continue (odds only):");
for (int i = 1; i <= 6; i++)
{
    if (i % 2 == 0) continue;
    Console.Write($"{i} ");
}
Console.WriteLine();   // 1 3 5

// return inside foreach — cleanest way to exit nested loops
static bool ContainsNegative(int[] arr)
{
    foreach (int x in arr)
        if (x < 0) return true;   // exits method + loop
    return false;
}
Console.WriteLine(ContainsNegative(new[] { 1, -3, 5 }));  // True

// goto case — only accepted goto use in switch
int code = 2;
switch (code)
{
    case 1: Console.WriteLine("case 1"); break;
    case 2:
        Console.WriteLine("case 2 → also running case 3");
        goto case 3;   // explicit fall-through
    case 3: Console.WriteLine("case 3"); break;
}

using System;

// Ternary operator — concise two-branch assignment
int score = 75;
string result = score >= 60 ? "Pass" : "Fail";
Console.WriteLine(result);  // Pass

// ?? (null coalescing) — return left if non-null, else right
string? input = null;
string name = input ?? "Anonymous";
Console.WriteLine(name);  // Anonymous

// ??= (null coalescing assignment, C# 8) — assign only if null
input ??= "Default";
Console.WriteLine(input);  // Default

// ?. (null-conditional) — short-circuit on null
string? maybeNull = null;
int? len = maybeNull?.Length;   // null, not NullReferenceException
Console.WriteLine(len ?? -1);    // -1

// Guard clauses — early return to flatten nesting
static string ProcessInput(string? s)
{
    if (s is null)          return "Error: null input";    // guard
    if (s.Length == 0)      return "Error: empty string";  // guard
    if (s.Length > 100)    return "Error: too long";      // guard

    // Happy path — unnested, clearly visible
    return s.Trim().ToUpper();
}

Console.WriteLine(ProcessInput(null));      // Error: null input
Console.WriteLine(ProcessInput("hello"));  // HELLO