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.