Control Flow in C#

if / else

if/else is the most fundamental control flow tool — it lets your program take different paths based on a condition. The conditions are boolean expressions, and C# evaluates them top to bottom, taking the first branch whose condition is true.

int temperature = 22;

if (temperature > 30)
{
    Console.WriteLine("Hot");
}
else if (temperature > 20)
{
    Console.WriteLine("Warm");   // This branch runs — first true condition wins
}
else if (temperature > 10)
{
    Console.WriteLine("Cool");
}
else
{
    Console.WriteLine("Cold");
}

$ dotnet run
Warm

Single-line bodies can omit braces, but adding them is generally safer and prevents bugs when adding more lines later:

// Acceptable for very simple cases like guard clauses
if (string.IsNullOrEmpty(name))
    throw new ArgumentNullException(nameof(name));

// Ternary — compact conditional assignment on one line
string label = score >= 50 ? "Pass" : "Fail";

switch Statement

The classic switch tests a single value against a fixed set of constants. It is cleaner than a long chain of if/else if when you have many specific values to handle. Multiple case labels can share a body by falling through to the same break.

string day = "Monday";

switch (day)
{
    case "Monday":
    case "Tuesday":
    case "Wednesday":
    case "Thursday":
    case "Friday":
        Console.WriteLine("Weekday");  // all five cases share this body
        break;

    case "Saturday":
    case "Sunday":
        Console.WriteLine("Weekend");
        break;

    default:
        Console.WriteLine("Unknown day");
        break;
}

$ dotnet run
Weekday

switch Expression (C# 8+)

Switch expressions are more concise than switch statements and return a value directly. The compiler performs exhaustiveness checking — if there is a possible input that no arm handles, you get a compile-time warning. This makes switch expressions safer than chains of ternaries.

string DayType(string day) => day switch
{
    "Saturday" or "Sunday" => "Weekend",
    "Monday" or "Tuesday" or "Wednesday"
         or "Thursday" or "Friday"     => "Weekday",
    _ => throw new ArgumentOutOfRangeException(nameof(day), day, null)
};

Console.WriteLine(DayType("Saturday"));  // Weekend

$ dotnet run
Weekend

Switch expressions really shine when combined with pattern matching, letting you branch on ranges, types, and property values all in one place:

// Range patterns
string GetGrade(int score) => score switch
{
    >= 90 => "A",
    >= 80 => "B",
    >= 70 => "C",
    >= 60 => "D",
    _     => "F"
};

Console.WriteLine(GetGrade(85));  // B

// Type patterns
static string Describe(object obj) => obj switch
{
    int n when n < 0 => $"Negative int: {n}",
    int n            => $"Positive int: {n}",
    string s         => $"String of length {s.Length}",
    null             => "null",
    _                => $"Unknown: {obj.GetType().Name}"
};

Console.WriteLine(Describe("hello"));  // String of length 5
Console.WriteLine(Describe(-42));      // Negative int: -42

// Property patterns
string ShippingLabel(Order order) => order switch
{
    { IsPriority: true, Total: >= 100 } => "Free Priority",
    { IsPriority: true }                => "Priority",
    { Total: >= 50 }                    => "Free Standard",
    _                                   => "Standard ($5)"
};

Console.WriteLine(ShippingLabel(new Order(150, true)));  // Free Priority

// Type declarations must come after top-level statements
record Order(decimal Total, bool IsPriority);

$ dotnet run
B
String of length 5
Negative int: -42
Free Priority

for Loop

Use for when you need an explicit index or a precise number of iterations. The three parts of the for header — initializer, condition, iterator — give you full control over how the loop counter behaves.

// Classic for loop — index goes 0 to 9
for (int i = 0; i < 10; i++)
    Console.Write(i + " ");  // 0 1 2 3 4 5 6 7 8 9

// Reverse — count down from 9 to 0
for (int i = 9; i >= 0; i--)
    Console.Write(i + " ");  // 9 8 7 6 5 4 3 2 1 0

// Multiple variables in one for loop
for (int i = 0, j = 10; i < j; i++, j--)
    Console.Write($"({i},{j}) ");

// Loop over array by index — useful when you need the position
string[] names = { "Alice", "Bob", "Carol" };
for (int i = 0; i < names.Length; i++)
    Console.WriteLine($"{i}: {names[i]}");

$ dotnet run
0 1 2 3 4 5 6 7 8 9
9 8 7 6 5 4 3 2 1 0
(0,10) (1,9) (2,8) (3,7) (4,6)
0: Alice
1: Bob
2: Carol

foreach Loop

foreach is the idiomatic way to iterate a collection when you do not need the index. It is cleaner than a for loop and works with any type that implements IEnumerable<T> — not just arrays and lists.

var fruits = new List<string> { "apple", "banana", "cherry" };

// Simple iteration — no index management needed
foreach (string fruit in fruits)
    Console.WriteLine(fruit);

// With index using LINQ's Select overload — when you need both value and position
foreach (var (fruit, index) in fruits.Select((f, i) => (f, i)))
    Console.WriteLine($"{index}: {fruit}");

// Iterating a dictionary — deconstruct each key-value pair
var scores = new Dictionary<string, int>
{
    ["Alice"] = 95,
    ["Bob"]   = 82
};

foreach (var (name, score) in scores)
    Console.WriteLine($"{name}: {score}");

// Iterate a string — yields each character in order
foreach (char c in "Hello")
    Console.Write(c + " ");  // H e l l o

$ dotnet run
apple
banana
cherry
0: apple
1: banana
2: cherry
Alice: 95
Bob: 82
H e l l o 

while and do-while

while runs as long as a condition is true, checking it before each iteration. do-while always executes the body at least once before checking. Use do-while for input loops and retry patterns where you always need at least one attempt.

// while — checks condition before each iteration
int count = 0;
while (count < 5)
{
    Console.Write(count + " ");
    count++;
}
// Output: 0 1 2 3 4

$ dotnet run
0 1 2 3 4

do-while always executes the body at least once before checking the condition. This makes it ideal for input-validation loops where you need at least one prompt:

// do-while — body runs at least once, condition checked afterwards
// Perfect for "keep asking until valid input" patterns
string input;
do
{
    Console.Write("Enter 'quit' to exit: ");
    input = Console.ReadLine() ?? "";
} while (input != "quit");

break and continue

break and continue give you fine-grained control over loop execution without restructuring the entire loop. break exits immediately; continue skips the rest of the current iteration and moves on to the next check.

// break — exit the loop immediately when condition is met
for (int i = 0; i < 100; i++)
{
    if (i == 5)
        break;
    Console.Write(i + " ");  // 0 1 2 3 4
}

// continue — skip even numbers, process only odd ones
for (int i = 0; i < 10; i++)
{
    if (i % 2 == 0)
        continue;  // jump to next iteration
    Console.Write(i + " ");  // 1 3 5 7 9
}

// break in nested loops — only breaks the innermost loop
for (int i = 0; i < 3; i++)
{
    for (int j = 0; j < 3; j++)
    {
        if (j == 1) break;           // exits inner loop only
        Console.Write($"({i},{j}) ");
    }
}
// (0,0) (1,0) (2,0)

$ dotnet run
0 1 2 3 4
1 3 5 7 9
(0,0) (1,0) (2,0)

To break out of an outer loop, either use a flag or restructure into a method. The method approach is generally cleaner:

// Using a flag — works but adds noise
bool found = false;
for (int i = 0; i < rows && !found; i++)
    for (int j = 0; j < cols && !found; j++)
        if (grid[i][j] == target)
            found = true;

// Better: extract to a method — return exits all loops at once
bool ContainsTarget(int[][] grid, int target)
{
    for (int i = 0; i < grid.Length; i++)
        for (int j = 0; j < grid[i].Length; j++)
            if (grid[i][j] == target) return true;
    return false;
}

Pattern Matching in Control Flow

Pattern matching lets if and switch do more than just compare values. You can simultaneously check the type, bind a variable, and test properties — all in one readable expression.

// is pattern with variable binding — type check and cast in one step
object result = "Operation completed";

if (result is string message)
    Console.WriteLine($"Success: {message}");
else if (result is Exception ex)
    Console.WriteLine($"Error: {ex.Message}");
else if (result is int code and > 0)
    Console.WriteLine($"Code: {code}");

// Tuple patterns — match on multiple values simultaneously
var point = (x: 3, y: -1);

string quadrant = point switch
{
    (> 0, > 0) => "Q1",
    (< 0, > 0) => "Q2",
    (< 0, < 0) => "Q3",
    (> 0, < 0) => "Q4",
    _           => "On axis"
};
Console.WriteLine(quadrant);  // Q4

$ dotnet run
Success: Operation completed
Q4

Exception-Based Control Flow

Exceptions exist for exceptional situations — unexpected failures, not expected outcomes. Using exceptions for ordinary control flow (like checking if a string is a valid number) is expensive and obscures intent. Prefer Try* methods that return bool for expected failure cases.

// Avoid using exceptions for normal control flow
// Bad — exceptions are expensive and not meant for this
bool ParseIntBad(string s, out int result)
{
    try
    {
        result = int.Parse(s);
        return true;
    }
    catch
    {
        result = 0;
        return false;
    }
}

// Good — TryParse is specifically designed for "try this, it might fail"
bool ParseIntGood(string s, out int result)
    => int.TryParse(s, out result);

Practical Example: FizzBuzz with Switch

This example shows how tuple patterns and switch expressions combine into readable, compact logic — no nested ifs required.

for (int i = 1; i <= 20; i++)
{
    // Tuple pattern: test both (i % 3) and (i % 5) simultaneously
    string output = (i % 3, i % 5) switch
    {
        (0, 0) => "FizzBuzz",  // divisible by both
        (0, _) => "Fizz",      // divisible by 3 only
        (_, 0) => "Buzz",      // divisible by 5 only
        _       => i.ToString() // neither
    };
    Console.WriteLine(output);
}

$ dotnet run
1
2
Fizz
4
Buzz
Fizz
7
8
Fizz
Buzz
11
Fizz
13
14
FizzBuzz
16
17
Fizz
19
Buzz

Practice

1. What does break do inside a switch case? Modify the if/else temperature example to use a switch statement instead. What happens if you remove all break statements?

switch (temp)
{
    case <= 0:
        Console.WriteLine("Freezing");
        break;
    case <= 15:
        Console.WriteLine("Cold");
        break;
    case <= 25:
        Console.WriteLine("Warm");
        break;
    default:
        Console.WriteLine("Hot");
        break;
}

2. Extend the FizzBuzz. Modify the FizzBuzz example to also print “Bazz” for every number divisible by 7. What will the output for 21 look like?

for (int i = 1; i <= 21; i++)
{
    string output = (i % 3, i % 5, i % 7) switch
    {
        (0, 0, _) => "FizzBuzz",
        (0, _, 0) => "FizzBazz",
        (_, 0, 0) => "BuzzBazz",
        (0, _, _) => "Fizz",
        (_, 0, _) => "Buzz",
        (_, _, 0) => "Bazz",
        _ => i.ToString()
    };
    Console.WriteLine(output);
}

3. Convert a for loop to foreach. Take the array iteration example (iterating names by index) and rewrite it using foreach. When would you prefer one over the other?

// for loop — need index to modify or access other arrays by position
for (int i = 0; i < names.Length; i++)
    Console.WriteLine(names[i]);

// foreach — simpler, but no index available
foreach (string name in names)
    Console.WriteLine(name);