Passing reference types to methods in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this code when passing a reference type to a method?

Consider the following C# code:

class Box { public int Value; }

void ChangeBox(Box b) {
    b.Value = 10;
}

Box myBox = new Box { Value = 5 };
ChangeBox(myBox);
Console.WriteLine(myBox.Value);

What will be printed?

C Sharp (C#)

class Box { public int Value; }

void ChangeBox(Box b) {
    b.Value = 10;
}

Box myBox = new Box { Value = 5 };
ChangeBox(myBox);
Console.WriteLine(myBox.Value);

A 5

B 10

C 0

D Compilation error

💡 Hint

Remember that classes are reference types in C#. Changing a property inside the method affects the original object.

The method ChangeBox receives a reference to the same Box object. Changing b.Value changes the original object's Value. So, myBox.Value becomes 10.

❓ Predict Output

intermediate

What happens when you assign a new object to a reference parameter inside a method?

Look at this code:

class Container { public int Number; }

void ResetContainer(Container c) {
    c = new Container { Number = 0 };
}

Container cont = new Container { Number = 5 };
ResetContainer(cont);
Console.WriteLine(cont.Number);

What will be printed?

C Sharp (C#)

class Container { public int Number; }

void ResetContainer(Container c) {
    c = new Container { Number = 0 };
}

Container cont = new Container { Number = 5 };
ResetContainer(cont);
Console.WriteLine(cont.Number);

A 5

B 0

C Compilation error

D NullReferenceException

💡 Hint

Assigning a new object to the parameter inside the method does not change the original reference outside.

The parameter c is a copy of the reference. Assigning a new object to c only changes the local copy. The original cont still points to the old object with Number = 5.

🔧 Debug

advanced

Why does this code throw a NullReferenceException?

Examine this code snippet:

class Data {
    public string Text;
}

void ClearText(Data d) {
    d.Text = null;
}

Data data = null;
ClearText(data);
Console.WriteLine(data.Text);

Why does this code throw a NullReferenceException?

C Sharp (C#)

class Data {
    public string Text;
}

void ClearText(Data d) {
    d.Text = null;
}

Data data = null;
ClearText(data);
Console.WriteLine(data.Text);

A Because Text is null and cannot be assigned

B Because ClearText method is static but called on instance

C Because data is null and accessing d.Text causes an exception

D Because Console.WriteLine cannot print null values

💡 Hint

Think about what happens when you try to access a member of a null object.

The variable data is null. Passing it to ClearText means d is null. Trying to set d.Text causes a NullReferenceException because you cannot access members of a null reference.

📝 Syntax

advanced

Which option correctly passes a reference type by reference to modify the original reference?

Given this code:

class Item { public int Id; }

void ReplaceItem(??? item) {
    item = new Item { Id = 99 };
}

Item myItem = new Item { Id = 1 };
ReplaceItem(myItem);
Console.WriteLine(myItem.Id);

Which method signature for ReplaceItem will make myItem.Id print 99?

C Sharp (C#)

class Item { public int Id; }

void ReplaceItem(??? item) {
    item = new Item { Id = 99 };
}

Item myItem = new Item { Id = 1 };
ReplaceItem(myItem);
Console.WriteLine(myItem.Id);

A void ReplaceItem(ref Item item)

B void ReplaceItem(Item item)

C void ReplaceItem(out Item item)

D void ReplaceItem(in Item item)

💡 Hint

To change the original reference, you must pass the parameter by reference.

Using ref passes the reference itself by reference, so assigning a new object inside the method changes the original variable. out requires initialization inside the method and in is readonly. Passing normally copies the reference.

🚀 Application

expert

How many items are in the list after this method call modifies it?

Analyze this code:

void AddItems(List list) {
    list.Add("A");
    list = new List();
    list.Add("B");
}

var myList = new List();
AddItems(myList);
Console.WriteLine(myList.Count);

What number will be printed?

C Sharp (C#)

void AddItems(List<string> list) {
    list.Add("A");
    list = new List<string>();
    list.Add("B");
}

var myList = new List<string>();
AddItems(myList);
Console.WriteLine(myList.Count);

A Compilation error

B 2

C 0

D 1

💡 Hint

Remember that assigning a new list inside the method does not affect the original list reference.

The method first adds "A" to the original list, so myList has 1 item. Then it assigns a new list to the parameter list, but this does not change myList. Adding "B" is to the new list only. So myList.Count is 1.

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