Passing reference types to methods in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to pass the object to the method.

C Sharp (C#)

class Program {
    static void Main() {
        Person p = new Person();
        UpdateName([1]);
    }

    static void UpdateName(Person person) {
        person.Name = "Alice";
    }
}

class Person {
    public string Name { get; set; }
}

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

Anew Person()

BPerson

Cp

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Passing the class name instead of the object variable.

Passing null instead of the object.

You need to pass the existing object p to the method to update its name.

2fill in blank

medium

Complete the method call to change the age of the person.

C Sharp (C#)

class Program {
    static void Main() {
        Person p = new Person();
        SetAge([1], 30);
    }

    static void SetAge(Person person, int age) {
        person.Age = age;
    }
}

class Person {
    public int Age { get; set; }
}

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

Anew Person()

Bp

CPerson

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Passing a new object instead of the existing one.

Passing the class name instead of an object.

You must pass the existing object p to update its age.

3fill in blank

hard

Fix the error in the method to correctly update the person's city.

C Sharp (C#)

class Program {
    static void Main() {
        Person p = new Person();
        ChangeCity(p);
        System.Console.WriteLine(p.City);
    }

    static void ChangeCity(Person person) {
        person.City = [1];
    }
}

class Person {
    public string City { get; set; }
}

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

Anull

BNew York

Cperson.City

D"New York"

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting quotes around the string.

Using an undefined variable.

The city name must be a string literal with quotes.

4fill in blank

hard

Fill both blanks to create a method that updates the person's name and age.

C Sharp (C#)

class Program {
    static void Main() {
        Person p = new Person();
        UpdatePerson(p, [1]);
        System.Console.WriteLine($"Name: {p.Name}, Age: {p.Age}");
    }

    static void UpdatePerson(Person person, int newAge) {
        person.Name = [2];
        person.Age = newAge;
    }
}

class Person {
    public string Name { get; set; }
    public int Age { get; set; }
}

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

A"Bob"

B25

C"Alice"

D30

Attempts:

3 left

💡 Hint

Common Mistakes

Passing the wrong type for age.

Not using quotes for the name string.

The method call passes the new age 25 and sets the name to "Bob".

5fill in blank

hard

Fill all three blanks to create a method that updates name, age, and city of a person.

C Sharp (C#)

class Program {
    static void Main() {
        Person p = new Person();
        UpdateDetails(p, [1], [2], [3]);
        System.Console.WriteLine($"Name: {p.Name}, Age: {p.Age}, City: {p.City}");
    }

    static void UpdateDetails(Person person, string newName, int newAge, string newCity) {
        person.Name = newName;
        person.Age = newAge;
        person.City = newCity;
    }
}

class Person {
    public string Name { get; set; }
    public int Age { get; set; }
    public string City { get; set; }
}

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

A"Charlie"

B40

C"Seattle"

D"Denver"

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up the order of arguments.

Forgetting quotes around string arguments.

The method call passes name "Denver", age 40, and city "Seattle".

Practice

1. What happens when you pass a reference type object to a method in C# without using ref keyword?

easy

A. The method can modify the object's properties, affecting the original object.

B. The method cannot modify the object's properties at all.

C. The method receives a copy of the object and changes do not affect the original.

D. The method can reassign the original reference to a new object automatically.

Solution

1. Step 1: Understand passing reference types

   When a reference type is passed to a method, the method receives a copy of the reference, pointing to the same object.

2. Step 2: Effect on object properties

   Modifying the object's properties inside the method changes the original object because both references point to the same memory.

3. Final Answer:

   The method can modify the object's properties, affecting the original object. -> Option A

4. Quick Check:

   Reference type properties change original [OK]

Hint: Reference types allow property changes to affect original object [OK]

Common Mistakes:

• Thinking the object is copied entirely
• Confusing reference and value types
• Assuming reassignment changes original reference

2. Which of the following method signatures correctly allows a method to reassign a reference type parameter so the caller sees the new object?

easy

A. void UpdateObject(MyClass obj) { obj = new MyClass(); }

B. void UpdateObject(ref MyClass obj) { obj = new MyClass(); }

C. void UpdateObject(out MyClass obj) { obj = new MyClass(); }

D. void UpdateObject(in MyClass obj) { obj = new MyClass(); }

Solution

1. Step 1: Understand parameter passing keywords

   The ref keyword allows the method to reassign the caller's reference variable.

2. Step 2: Analyze each signature

   void UpdateObject(ref MyClass obj) { obj = new MyClass(); } uses ref and assigns a new object, so the caller's reference changes. void UpdateObject(MyClass obj) { obj = new MyClass(); } reassigns local copy only. void UpdateObject(out MyClass obj) { obj = new MyClass(); } requires assignment but is for output parameters. void UpdateObject(in MyClass obj) { obj = new MyClass(); } is readonly and cannot reassign.

3. Final Answer:

   void UpdateObject(ref MyClass obj) { obj = new MyClass(); } -> Option B

4. Quick Check:

   Use ref to reassign reference type parameter [OK]

Hint: Use ref keyword to let method reassign caller's reference [OK]

Common Mistakes:

• Using no keyword and expecting reassignment to affect caller
• Confusing out and ref keywords
• Trying to reassign with in parameter

3. What is the output of the following C# code?

class Person {
  public string Name;
}

void ChangeName(Person p) {
  p.Name = "Alice";
  p = new Person();
  p.Name = "Bob";
}

Person person = new Person();
person.Name = "John";
ChangeName(person);
Console.WriteLine(person.Name);

medium

A. John

B. null

C. Bob

D. Alice

Solution

1. Step 1: Trace property change before reassignment

   The method sets p.Name = "Alice", changing the original object's Name to "Alice".

2. Step 2: Reassignment inside method does not affect caller

   Then p is assigned a new Person object with Name "Bob", but this does not affect the original reference outside the method.

3. Final Answer:

   Alice -> Option D

4. Quick Check:

   Property change affects original; reassignment does not [OK]

Hint: Property changes affect original; reassignment inside method does not [OK]

Common Mistakes:

• Thinking reassignment changes original reference
• Expecting output to be 'Bob'
• Ignoring property change before reassignment

4. Identify the error in the following code that tries to update a reference type parameter so the caller sees the new object:

class Data {
  public int Value;
}

void ResetData(Data d) {
  d = new Data();
  d.Value = 0;
}

Data data = new Data();
data.Value = 5;
ResetData(data);
Console.WriteLine(data.Value);

medium

A. The method should use ref keyword to reassign the caller's reference.

B. The method should not create a new object inside.

C. The method should assign d.Value = 5 instead of 0.

D. The method should return the new object instead of void.

Solution

1. Step 1: Understand reassignment effect without ref

   Assigning d = new Data() inside the method only changes the local copy of the reference, not the caller's variable.

2. Step 2: Fix by using ref keyword

   Using ref in the method parameter and call allows the method to reassign the caller's reference to the new object.

3. Final Answer:

   The method should use ref keyword to reassign the caller's reference. -> Option A

4. Quick Check:

   Use ref to reassign reference type parameter [OK]

Hint: Use ref keyword to let method reassign caller's reference [OK]

Common Mistakes:

• Expecting reassignment without ref to affect caller
• Thinking returning new object is required
• Confusing property assignment with reference reassignment

5. You want a method to update a reference type parameter's properties and also optionally replace the whole object with a new one, so the caller sees all changes. Which approach correctly achieves this in C#?

hard

A. Pass the object with out keyword; assign new object inside method but do not modify properties.

B. Pass the object normally and modify properties; reassigning inside method updates caller automatically.

C. Pass the object with ref keyword; modify properties and reassign new object inside method.

D. Pass the object with in keyword; modify properties but cannot reassign the object.

Solution

1. Step 1: Understand property modification vs reassignment

   Modifying properties affects original object without ref, but reassignment does not affect caller unless ref is used.

2. Step 2: Use ref to allow both property changes and reassignment

   Passing with ref lets method modify properties and replace the object so caller sees all changes.

3. Final Answer:

   Pass the object with ref keyword; modify properties and reassign new object inside method. -> Option C

4. Quick Check:

   Use ref to modify and reassign reference type parameter [OK]

Hint: Use ref to modify properties and reassign object in method [OK]

Common Mistakes:

• Assuming normal pass allows reassignment to affect caller
• Using out when input object is needed
• Using in which forbids reassignment