Type patterns in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to check if the object is an integer using a type pattern.

C Sharp (C#)

if (obj is [1] number) {
    Console.WriteLine($"Number: {number}");
}

Drag options to blanks, or click blank then click option'

Astring

Bint

Cbool

Ddouble

Attempts:

3 left

💡 Hint

Common Mistakes

Using a wrong type like 'string' instead of 'int'.

Forgetting to declare the variable after the type.

The 'is' keyword with 'int' checks if obj is an integer and assigns it to 'number'.

2fill in blank

medium

Complete the code to check if the object is a string and has length greater than 5.

C Sharp (C#)

if (obj is [1] text && text.Length > 5) {
    Console.WriteLine($"Long text: {text}");
}

Drag options to blanks, or click blank then click option'

Aint

Bdouble

Cstring

Dbool

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'int' instead of 'string' for text.

Not declaring the variable after the type.

The 'is string text' pattern checks if obj is a string and assigns it to 'text'.

3fill in blank

hard

Fix the error in the type pattern to correctly check for a double value.

C Sharp (C#)

if (obj is [1] value) {
    Console.WriteLine($"Double value: {value}");
}

Drag options to blanks, or click blank then click option'

Adouble

Bint

Cstring

Dbool

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'int' instead of 'double'.

Using 'string' or 'bool' for numeric checks.

The correct type to check for a double value is 'double'.

4fill in blank

hard

Fill both blanks to create a type pattern that checks if obj is a bool and true.

C Sharp (C#)

if (obj is [1] flag && flag [2] true) {
    Console.WriteLine("Flag is true");
}

Drag options to blanks, or click blank then click option'

Abool

B==

C!=

Dint

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'int' instead of 'bool' for the type.

Using '!=' instead of '==' for comparison.

The pattern checks if obj is a bool and if the flag equals true.

5fill in blank

hard

Fill both blanks to create a type pattern that checks if obj is a string with length less than 4 and prints it.

C Sharp (C#)

if (obj is [1] text && text.Length [2] 4) {
    Console.WriteLine($"Short text: {text}");
}

Drag options to blanks, or click blank then click option'

Aint

B<

Cstring

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'int' instead of 'string' for the type.

Using '>' instead of '<' for the length comparison.

The code checks if obj is a string and if its length is less than 4.

Practice

1. What does a type pattern in C# do when used in an if statement like if (obj is string s)?

easy

A. Always returns true regardless of obj type

B. Converts obj to a string without checking

C. Checks if obj is a string and assigns it to s if true

D. Throws an exception if obj is not a string

Solution

1. Step 1: Understand the is keyword with type pattern

   The is keyword checks if the object is of the specified type.

2. Step 2: Recognize the pattern variable assignment

   If the check is true, the object is assigned to the variable s of that type.

3. Final Answer:

   Checks if obj is a string and assigns it to s if true -> Option C

4. Quick Check:

   Type pattern = Check and assign [OK]

Hint: Type pattern checks type and assigns in one step [OK]

Common Mistakes:

• Thinking it converts without checking
• Assuming it always returns true
• Expecting an exception on type mismatch

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

easy

A. if (obj as int number) { }

B. if (int obj is number) { }

C. if (obj int is number) { }

D. if (obj is int number) { }

Solution

1. Step 1: Recall correct type pattern syntax

   The correct syntax is if (variable is Type name).

2. Step 2: Check each option

   if (obj is int number) { } matches the correct syntax. Options B, C, and D have incorrect order or keywords.

3. Final Answer:

   if (obj is int number) { } -> Option D

4. Quick Check:

   Correct syntax = if (obj is Type name) [OK]

Hint: Remember: if (obj is Type var) [OK]

Common Mistakes:

• Swapping type and variable order
• Using 'as' instead of 'is' for pattern
• Placing keywords incorrectly

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

object obj = 42;
if (obj is int number)
{
    Console.WriteLine(number + 10);
}
else
{
    Console.WriteLine("Not an int");
}

medium

A. 52

B. Not an int

C. Compilation error

D. Runtime exception

Solution

1. Step 1: Check the type of obj

   obj holds the integer 42, so obj is int is true.

2. Step 2: Calculate the output inside the if block

   The variable number is assigned 42, so number + 10 equals 52, which is printed.

3. Final Answer:

   52 -> Option A

4. Quick Check:

   Type check true, output = 42 + 10 = 52 [OK]

Hint: If type matches, variable holds value for use [OK]

Common Mistakes:

• Expecting 'Not an int' output
• Confusing type check with cast
• Thinking code throws error

4. Identify the error in this code snippet using type patterns:

object obj = "hello";
if (obj is int number)
{
    Console.WriteLine(number);
}

medium

A. No output because obj is not an int

B. Compilation error due to missing else block

C. No error, code runs and prints 'hello'

D. Runtime exception when casting string to int

Solution

1. Step 1: Analyze the type check condition

   obj is a string, so obj is int number is false.

2. Step 2: Understand the effect on output

   Since the if condition is false and no else block exists, nothing is printed.

3. Final Answer:

   No output because obj is not an int -> Option A

4. Quick Check:

   False type check = no output [OK]

Hint: False type check skips block, no output [OK]

Common Mistakes:

• Expecting runtime exception
• Thinking else block is mandatory
• Assuming it prints the string

5. Given this method using type patterns, what will it return?
<pre>string Describe(object input)
{
return input switch
{
int i => $"Integer: {i}",
string s => $"String: {s}",
null => "Null value",
_ => "Unknown type"
};
}

What is the result of Describe(3.14)?

hard

A. "Integer: 3"

B. "Unknown type"

C. "String: 3.14"

D. Compilation error

Solution

1. Step 1: Check the switch cases for input 3.14

   3.14 is a double, which does not match int, string, or null.

2. Step 2: Identify the default case

   The underscore _ matches any other type, so it returns "Unknown type".

3. Final Answer:

   "Unknown type" -> Option B

4. Quick Check:

   Switch default case = "Unknown type" [OK]

Hint: Switch default (_) handles unmatched types [OK]

Common Mistakes:

• Assuming 3.14 is int or string
• Expecting compilation error
• Ignoring default case