07. C# (Scope & Code Blocks)

0) Goal of This Lesson

Understanding where a variable is visible,
and why it suddenly becomes inaccessible.

If you miss this concept:

• if / else will feel inconsistent
• variables will “randomly” stop working
• method parameters won’t make sense later

Scope is not syntax trivia.
It defines what code is allowed to see what data.

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1) The Simplest Rule of Scope

A variable is only visible inside the code block where it is declared,
and any nested blocks inside it.

A code block is anything wrapped in { }.

{
    // this is one scope
}

When execution leaves this block,
variables declared inside it no longer exist.

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2) Variable Declared in the Outer Scope (Works Everywhere Inside)

Run this code:

using System;

class Program
{
    static void Main()
    {
        string userChoice = Console.ReadLine();

        if (userChoice == "A")
        {
            Console.WriteLine("IF block");
        }
        else
        {
            Console.WriteLine("ELSE block");
        }

        Console.WriteLine(userChoice);
    }
}

Observation

• userChoice is accessible:

  • inside if
  • inside else
  • after the if / else

Rule

A variable declared in an outer block
is visible in all inner blocks.

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3) Variable Declared Inside an if Block

Now run this:

using System;

class Program
{
    static void Main()
    {
        string userChoice = "A";

        if (userChoice == "A")
        {
            int number = 10;
            Console.WriteLine(number);
        }
    }
}

Observation

• Code compiles
• number prints correctly

So far, everything looks fine.

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4) Why the Same Variable Fails Outside the Block

Now add one line:

Console.WriteLine(number);

Full code:

if (userChoice == "A")
{
    int number = 10;
}

Console.WriteLine(number); // compile-time error

Observation

• Code does not compile
• Error: The name 'number' does not exist in the current context

Rule

A variable declared inside a block
stops existing when the block ends.

This is checked before execution, by the compiler.

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5) if and else Are Separate Scopes

Run this example:

using System;

class Program
{
    static void Main()
    {
        string userChoice = "A";

        if (userChoice == "A")
        {
            int number = 10;
            Console.WriteLine(number);
        }
        else
        {
            Console.WriteLine(number); // compile-time error
        }
    }
}

Observation

• number exists only in the if block
• else cannot see it

Rule

if and else blocks are sibling scopes, not shared scopes.

Declaring a variable in one does not make it visible in the other.

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6) Nested Blocks Can See Outer Variables

Run this:

using System;

class Program
{
    static void Main()
    {
        string userChoice = "A";

        if (userChoice == "A")
        {
            int number = 10;

            if (number > 5)
            {
                Console.WriteLine(number);
                Console.WriteLine(userChoice);
            }
        }
    }
}

Observation

• Inner if can access:

  • number
  • userChoice

Rule

Inner blocks can see variables from outer blocks.
Outer blocks cannot see variables from inner blocks.

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7) Same Variable Name Rules

7.1 Same scope — NOT allowed

if (true)
{
    int x = 10;
    int x = 20; // compile-time error
}

Rule:

A variable name must be unique within the same scope.

---

7.2 Different scopes — allowed

if (true)
{
    int x = 10;
}
else
{
    int x = 20;
}

Rule:

Different blocks create different scopes,
even if they are part of the same if / else structure.

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8) How to Verify Scope Using Debugging

1. Place a breakpoint inside the block where the variable is declared
2. Start debugging
3. Observe the variable in the debugger
4. Step out of the block

Observation

• The variable disappears from the debugger
• This confirms the scope boundary

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9) The Mental Check That Solves 90% of Scope Confusion

Whenever something “suddenly doesn’t work”, ask:

Was this variable declared inside the braces
where I am currently standing?

If the answer is no,
the compiler is doing exactly what it should.

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10) One-Line Summary

Scope is defined by { }.
Variables exist only inside the block where they are declared,
and disappear when that block ends.