Object type as universal base in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a variable that can hold any type of value.

C Sharp (C#)

object [1] = 42;

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

Aobj

Bany

Cdata

Dvalue

Attempts:

3 left

💡 Hint

Common Mistakes

Using a reserved keyword as variable name

Using a type name instead of a variable name

In C#, object is the universal base type. You can name the variable anything, but obj is a common choice.

2fill in blank

medium

Complete the code to assign a string value to an object variable.

C Sharp (C#)

object obj = [1];

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

Anull

B100

C"hello"

Dtrue

Attempts:

3 left

💡 Hint

Common Mistakes

Using single quotes instead of double quotes for strings

Assigning a number instead of a string

Strings in C# are enclosed in double quotes. Assigning a string literal to an object variable is allowed.

3fill in blank

hard

Fix the error in the code by completing the cast from object to string.

C Sharp (C#)

object obj = "test";
string s = ([1])obj;

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

Astring

Bint

Cbool

Ddouble

Attempts:

3 left

💡 Hint

Common Mistakes

Casting to a wrong type causes runtime errors

Forgetting to cast causes compile errors

To convert an object holding a string back to a string, you must cast it to string.

4fill in blank

hard

Fill both blanks to create an object array and assign different types of values.

C Sharp (C#)

object[] arr = new object[] { [1], [2] };

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

A"hello"

B42

Ctrue

D3.14

Attempts:

3 left

💡 Hint

Common Mistakes

Using quotes around numbers

Mixing up boolean and numeric values

An object array can hold any types. Here, a string and an integer are assigned.

5fill in blank

hard

Fill all three blanks to create a method that returns an object and assigns different types inside.

C Sharp (C#)

public object GetValue(int choice) {
    return choice switch {
        1 => [1],
        2 => [2],
        _ => [3]
    };
}

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

A"apple"

B100

Cfalse

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Using quotes around booleans or numbers

Returning incompatible types without casting

The method returns different types as objects based on the input choice: a string, an integer, or a boolean.

Practice

1. What is the role of the object type in C#?

easy

A. object is used only for reference types, not value types.

B. object is used only for strings.

C. object is a special type for numeric calculations.

D. object is the universal base type for all data types.

Solution

1. Step 1: Understand the base type concept

   In C#, object is the root type from which all other types derive, including value and reference types.

2. Step 2: Analyze the options

   Only object is the universal base type for all data types. correctly states that object is the universal base type for all data types.

3. Final Answer:

   object is the universal base type for all data types. -> Option D

4. Quick Check:

   object = universal base type [OK]

Hint: Remember: all types inherit from object in C# [OK]

Common Mistakes:

• Thinking object is only for reference types
• Confusing object with string or numeric types
• Assuming object is a special numeric type

2. Which of the following is the correct way to store an integer value in an object variable?

easy

A. int obj = new object(42);

B. object obj = 42;

C. object obj = new int(42);

D. object obj = "42";

Solution

1. Step 1: Understand boxing of value types

   In C#, value types like int can be stored in an object variable by implicit boxing, e.g., object obj = 42;.

2. Step 2: Check each option

   object obj = 42; correctly assigns an integer to an object variable. Options B and C use invalid syntax. object obj = "42"; stores a string, not an integer.

3. Final Answer:

   object obj = 42; -> Option B

4. Quick Check:

   Boxing int to object = object obj = 42; [OK]

Hint: Assign value directly to object for boxing [OK]

Common Mistakes:

• Trying to use new with int type
• Confusing string and int assignments
• Using invalid constructors for value types

3. What will be the output of this code?

object obj = 10;
int x = (int)obj + 5;
Console.WriteLine(x);

medium

A. Runtime error

B. 10

C. 15

D. Compilation error

Solution

1. Step 1: Understand boxing and unboxing

   The integer 10 is boxed into obj. Casting back to int unboxes it correctly.

2. Step 2: Calculate the expression

   Unboxed value 10 plus 5 equals 15, which is printed.

3. Final Answer:

   15 -> Option C

4. Quick Check:

   Unbox 10 + 5 = 15 [OK]

Hint: Cast object back to int before arithmetic [OK]

Common Mistakes:

• Forgetting to cast before adding
• Expecting string output
• Confusing compile and runtime errors

4. Identify the error in this code snippet:

object obj = "hello";
int num = (int)obj;
Console.WriteLine(num);

medium

A. Invalid cast exception at runtime

B. Compilation error due to type mismatch

C. No error, prints 'hello'

D. Syntax error in variable declaration

Solution

1. Step 1: Analyze the cast operation

   The object contains a string "hello" but is cast to int, which is invalid.

2. Step 2: Identify the error type

   This causes an InvalidCastException at runtime, not a compile-time error.

3. Final Answer:

   Invalid cast exception at runtime -> Option A

4. Quick Check:

   Wrong cast string to int = runtime error [OK]

Hint: Cast only to the original stored type [OK]

Common Mistakes:

• Assuming compile-time error instead of runtime
• Casting string to int directly
• Ignoring exception possibility

5. Given this code, what is the best way to safely retrieve the integer value stored in obj?

object obj = 25;
// Retrieve integer value safely here

hard

A. int value = obj is int i ? i : 0;

B. int value = obj as int;

C. int value = Convert.ToInt32(obj);

D. int value = (int)obj;

Solution

1. Step 1: Understand safe casting options

   Direct cast (A) can throw exceptions. as operator (B) cannot be used with value types like int.

2. Step 2: Evaluate options for safety

   int value = obj is int i ? i : 0; uses pattern matching to check type before assignment, avoiding exceptions. int value = Convert.ToInt32(obj); converts but may throw if obj is not convertible.

3. Final Answer:

   int value = obj is int i ? i : 0; -> Option A

4. Quick Check:

   Use pattern matching for safe unboxing [OK]

Hint: Use pattern matching to safely cast object [OK]

Common Mistakes:

• Using 'as' with value types
• Not checking type before casting
• Assuming Convert.ToInt32 is always safe