Control Flow

The flow of the program flows from top to bottom, branching with conditions and repeating with repeat. The switch expression, added after C# 8, provides clean pattern matching in the functional style.

Learning Objectives

• Conditional branch to if / else if / else
• Know the difference between the switch statement and the switch expression (=>)
• Distinguish the use of for / while / do-while / foreach
• Control repetition with break and continue

Key concepts

1) if / else if / else

int score = 85;
if (score >= 90)        Console.WriteLine("A");
else if (score >= 80)   Console.WriteLine("B");
else if (score >= 70)   Console.WriteLine("C");
else                    Console.WriteLine("F");

Conditions are evaluated in order from the top, so that only the branch that is true first is executed.

2) switch statement vs switch expression

statement type:

switch (day)
{
    case 1: Console.WriteLine("month"); break;
    case 2: Console.WriteLine("angry"); break;
    default: Console.WriteLine("?"); break;
}

expression type — C# 8+:

string name = day switch
{
    1 => "month",
    2 => "angry",
    _ => "?"        //default role
};

Since the expression type returns a value, it can be directly assigned to a variable, and there is no need for break. _ is a wildcard that “matches any value”.

3) for / while / do-while

• for: Repeat with a set number of times
• while: Repeat while the condition is true (possible 0 times)
• do-while: Check conditions after executing once (at least once)

for (int i = 0; i < 3; i++)  Console.WriteLine(i);  // 0 1 2

int n = 3;
while (n > 0) { Console.WriteLine(n); n--; }        // 3 2 1

int m = 0;
do { Console.WriteLine("once"); } while (m > 0);    //just once

4) foreach

Iterates over IEnumerable, such as an array or list. Suitable for simple traversal that does not require an index.

int[] nums = [10, 20, 30];
foreach (int x in nums)
    Console.WriteLine(x);

5) break / continue

• break: Terminates the current loop immediately
• continue: Skip current iteration to next iteration

for (int i = 1; i <= 10; i++)
{
    if (i == 5) break;       //Escape from i==5
    if (i % 2 == 0) continue;//Skip even numbers
    Console.WriteLine(i);    //1, 3 output
}

See as example

Example 1 — IfElse/Program.cs: Determining grade level

Console.Write("Score:");
int score = int.Parse(Console.ReadLine()!);

string grade;
if (score >= 90)      grade = "A";
else if (score >= 80) grade = "B";
else if (score >= 70) grade = "C";
else if (score >= 60) grade = "D";
else                  grade = "F";

Console.WriteLine($"Grade: {grade}");

Execution result

Score: 85
Grade: B

Note: Since it is evaluated from top to bottom, you need to sort it from narrow condition or big value first to work as intended.

Example 2 — SwitchExpression/Program.cs: Day of the week

Console.Write("Day number (1~7):");
int day = int.Parse(Console.ReadLine()!);

string name = day switch
{
    1 => "Monday",
    2 => "Tuesday",
    3 => "Wednesday",
    4 => "Thursday",
    5 => "Friday",
    6 => "Saturday",
    7 => "Sunday",
    _ => "Invalid input"
};

Console.WriteLine(name);

Execution result

Day number (1~7): 3
Wednesday

Note: The switch expression will issue a warning if it does not cover all cases. Be sure to leave the default value as _.

Example 3 — ForWhile/Program.cs : for sum, while countdown

//sum from 1 to 10
int sum = 0;
for (int i = 1; i <= 10; i++)
    sum += i;
Console.WriteLine($"1~10 sum = {sum}");

//Countdown from 5 to 1
int n = 5;
while (n > 0)
{
    Console.Write($"{n} ");
    n--;
}
Console.WriteLine("Fire!");

Execution result

1~10 sum = 55
5 4 3 2 1 Fire!

Note: Accumulated variables must be initialized outside the loop for the results to persist.

Example 4 — Foreach/Program.cs: Array sum

int[] scores = [78, 92, 64, 85, 100];

int total = 0;
foreach (int s in scores)
    total += s;

double avg = (double)total / scores.Length;
Console.WriteLine($"Total score: {total}, average: {avg:F2}");

Execution result

Total score: 419, average: 83.80

Memo: Detailed usage of arrays is covered in Part 11. “Iterating over a bundle” is sufficient here.

Common Mistakes

1. Confusing <= and < like for (int i = 0; i <= n; i++) — off-by-one bug.
2. break is missing from the switch statement — it is not in the expression type, but the statement type is required.
3. Infinite loop — break condition is missing in while (true).
4. Modifying collection during foreach — exception occurred. Use for to fix.
5. Incorrectly written as if (a == 0 || 1) — if (a == 0 || a == 1) is correct.

Cleanup

• Branches: if/else and switch (recommended expression type)
• Repeat: Count — for, Condition — while, At least 1 time — do-while, Collection — foreach
• Escape to break, skip to continue
• The switch expression + _ pattern is the recommended style in modern C#.

Try it yourself

cd src/IfElse && dotnet run
cd ../SwitchExpression && dotnet run
cd ../ForWhile && dotnet run
cd ../Foreach && dotnet run