03. C# (Operators)

0) Goal of This Lesson

Operators define how values are calculated or combined,
and when multiple operators appear together, the order of evaluation matters.

This is not a memorization problem.
It’s something you understand by running the code and observing the result.

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1) The Most Basic Operators

In C#, the most fundamental arithmetic operators are:

• + addition
• - subtraction
• * multiplication
• / division

▶ Run this example

using System;

class Program
{
    static void Main()
    {
        int a = 10;
        int b = 5;

        Console.WriteLine(a + b); // 15
        Console.WriteLine(a - b); // 5
        Console.WriteLine(a * b); // 50
        Console.WriteLine(a / b); // 2
    }
}

What to notice

• int / int results in an integer
• The results match basic arithmetic expectations

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2) Same Operator, Different Behavior Depending on Type

One important idea:

An operator can behave differently depending on the data type.

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2-1) The + operator with strings

With numbers, + means addition.
With strings, + means concatenation.

using System;

class Program
{
    static void Main()
    {
        string firstName = "Sabin";
        string lastName = "Sim";

        Console.WriteLine(firstName + " " + lastName);
    }
}

Output

Sabin Sim

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2-2) String + Number

This is also valid in C#.

using System;

class Program
{
    static void Main()
    {
        int age = 36;
        Console.WriteLine("Age: " + age);
    }
}

Output

Age: 36

Why this works:

• The number is converted to a string
• Then the strings are concatenated

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3) Why This Code Fails

Console.WriteLine("Result: " - 5);

👉 Compile-time error

Meaning:

C# does not know how to subtract a number from a string.

Important rule

• string + int ✅
• string - int ❌
• string * int ❌
• string / int ❌

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4) Operator Precedence

Now for a key concept:

When multiple operators appear in one expression,
C# follows a predefined order of evaluation.

This is called operator precedence.

For now, remember only this:

1. * and / first
2. + and - later
3. Same precedence → evaluated left to right

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5) Understanding Precedence by Running Code

5-1) Why does this work?

using System;

class Program
{
    static void Main()
    {
        Console.WriteLine("Result: " + 10 * 2);
    }
}

Output

Result: 20

Evaluation order

1. 10 * 2 → 20
2. "Result: " + 20 → string concatenation

---

5-2) Why does this fail?

Console.WriteLine("Result: " + 10 - 2);

👉 Compile-time error

What actually happens

1. "Result: " + 10 → "Result: 10"
2. "Result: 10" - 2 → ❌ invalid

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6) Using Parentheses to Control Order

Parentheses tell the compiler exactly what to evaluate first.

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6-1) Unexpected result

Console.WriteLine("Sum: " + 10 + 5);

Output

Sum: 105

Why?

• "Sum: " + 10 → "Sum: 10"
• "Sum: 10" + 5 → "Sum: 105"

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6-2) Correcting the intent

Console.WriteLine("Sum: " + (10 + 5));

Output

Sum: 15

📌 When strings and numbers are mixed, always think about parentheses.

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7) Unary Operators (Single-Value Operators)

You only need to know two for now:

• ++ increment
• -- decrement

Example

using System;

class Program
{
    static void Main()
    {
        int a = 10;
        int b = 10;

        a++; // a = a + 1
        b--; // b = b - 1

        Console.WriteLine(a); // 11
        Console.WriteLine(b); // 9
    }
}

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8) What Not to Memorize

❌ Full operator precedence tables
❌ All possible operators

✅ Remember this instead:

When strings and numbers are mixed,
always question which part becomes a string first.

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9) 30-Second Self Check

Explain the results without running the code:

Console.WriteLine("A" + 1 + 2);
Console.WriteLine("A" + (1 + 2));

If you can explain both results clearly,
👉 this lesson is complete.