Constant patterns in C Sharp (C#) - Mini Project: Build & Apply

Using Constant Patterns in C#

📖 Scenario: You are building a simple program that checks the type of fruit and prints a message based on the fruit name.

🎯 Goal: Learn how to use constant patterns with switch expressions in C# to match exact values.

📋 What You'll Learn

Create a string variable called fruit with a specific fruit name.

Create a switch expression using constant patterns to check the fruit.

Print a message for each fruit using Console.WriteLine.

💡 Why This Matters

🌍 Real World

Constant patterns help you check exact values easily, like checking user input or commands in programs.

💼 Career

Understanding constant patterns is useful for writing clear and concise code in many C# applications, including web, desktop, and game development.

1

Create the fruit variable

Create a string variable called fruit and set it to the value "Apple".

C Sharp (C#)

# Create a string variable called fruit and set it to "Apple"
# Your code here

Hint

Use string fruit = "Apple"; to create the variable.

2

Create a switch expression with constant patterns

Create a variable called message and assign it a switch expression that matches fruit using constant patterns for "Apple", "Banana", and "Orange". For each fruit, return a message like "You chose Apple". Use _ as the default pattern returning "Unknown fruit".

C Sharp (C#)

string fruit = "Apple";
# Create a variable message using a switch expression on fruit with constant patterns
# Your code here

Hint

Use the switch expression syntax with constant patterns like "Apple" => "You chose Apple".

3

Add a Console.WriteLine to print the message

Use Console.WriteLine to print the message variable.

C Sharp (C#)

string fruit = "Apple";
string message = fruit switch
{
    "Apple" => "You chose Apple",
    "Banana" => "You chose Banana",
    "Orange" => "You chose Orange",
    _ => "Unknown fruit"
};
# Print the message using Console.WriteLine
# Your code here

Hint

Use Console.WriteLine(message); to display the message.

4

Test with a different fruit value

Change the value of fruit to "Grape" and run the program to see the default message "Unknown fruit" printed.

C Sharp (C#)

# Change the fruit variable to "Grape"
string fruit = "Grape";
string message = fruit switch
{
    "Apple" => "You chose Apple",
    "Banana" => "You chose Banana",
    "Orange" => "You chose Orange",
    _ => "Unknown fruit"
};
Console.WriteLine(message);

Hint

Set fruit to "Grape" and run the program to see the default case message.

Practice

1. What does a constant pattern in C# do when used in a switch statement?

easy

A. It checks if the value is greater than a constant.

B. It checks if the value exactly matches a fixed constant.

C. It checks if the value is less than a constant.

D. It checks if the value is of a certain type.

Solution

1. Step 1: Understand constant pattern purpose

   Constant patterns compare a value to a fixed constant to see if they are equal.

2. Step 2: Apply to switch statement

   In a switch, constant patterns match cases where the value equals the constant exactly.

3. Final Answer:

   It checks if the value exactly matches a fixed constant. -> Option B

4. Quick Check:

   Constant pattern = exact value check [OK]

Hint: Constant pattern means exact value match check [OK]

Common Mistakes:

• Thinking it checks ranges instead of exact values
• Confusing with type patterns
• Assuming it checks inequality

2. Which of the following is the correct syntax for a constant pattern in an if statement in C#?

easy

A. if (is x 10) { }

B. if (x == is 10) { }

C. if (x is 10) { }

D. if (x = 10) { }

Solution

1. Step 1: Recall constant pattern syntax

   The syntax uses is followed by the constant value to check equality.

2. Step 2: Check each option

   if (x is 10) { } uses if (x is 10), which is correct. Others have syntax errors.

3. Final Answer:

   if (x is 10) { } -> Option C

4. Quick Check:

   Correct constant pattern syntax = if (x is 10) [OK]

Hint: Use 'is' keyword followed by constant value [OK]

Common Mistakes:

• Using assignment '=' instead of 'is'
• Placing 'is' incorrectly
• Adding extra operators

3. What will be the output of the following C# code?

int number = 5;
switch (number)
{
    case 3:
        Console.WriteLine("Three");
        break;
    case 5:
        Console.WriteLine("Five");
        break;
    default:
        Console.WriteLine("Other");
        break;
}

medium

A. Five

B. Three

C. Other

D. No output

Solution

1. Step 1: Identify the value of 'number'

   The variable 'number' is assigned the value 5.

2. Step 2: Match the switch cases

   The switch checks cases 3 and 5. Since number is 5, it matches case 5 and prints "Five".

3. Final Answer:

   Five -> Option A

4. Quick Check:

   number = 5 matches case 5 [OK]

Hint: Match value to case constant exactly [OK]

Common Mistakes:

• Choosing default instead of matching case
• Confusing case values
• Ignoring break statements

4. Identify the error in this C# code using constant patterns:

object obj = 10;
if (obj is 10)
{
    Console.WriteLine("Ten");
}

medium

A. No error, code is correct

B. Missing cast to int before pattern matching

C. Cannot use constant pattern with object type

D. Constant pattern requires a variable after 'is'

Solution

1. Step 1: Understand constant pattern with object

   Constant patterns can be used with object type to check if it equals a constant value.

2. Step 2: Check code correctness

   The code uses obj is 10 correctly to check if obj equals 10. This is valid syntax and logic.

3. Final Answer:

   No error, code is correct -> Option A

4. Quick Check:

   Constant pattern works with object type [OK]

Hint: Constant pattern works directly on object values [OK]

Common Mistakes:

• Assuming cast needed before pattern
• Thinking constant pattern needs variable
• Confusing with type patterns

5. Given a list of integers, which C# code correctly uses constant patterns to count how many times the number 7 appears?

var numbers = new List<int> {1, 7, 3, 7, 5};
int count = 0;
foreach (var n in numbers)
{
    // Fill in the blank
}
Console.WriteLine(count);

hard

A. if (n == 7) count = 7;

B. if (n is not 7) count++;

C. if (n is int) count++;

D. if (n is 7) count++;

Solution

1. Step 1: Understand the goal

   We want to count how many times 7 appears in the list.

2. Step 2: Use constant pattern to check for 7

   The correct constant pattern is if (n is 7) which checks if n equals 7, then increments count.

3. Final Answer:

   if (n is 7) count++; -> Option D

4. Quick Check:

   Constant pattern for 7 increments count [OK]

Hint: Use 'is' with constant to check exact value [OK]

Common Mistakes:

• Assigning count instead of incrementing
• Checking type instead of value
• Counting non-7 values