VB.NET Interview Questions and Answers

​1. How do you implement a thread-safe singleton class in VB.NET?

Answer:
Use double-checked locking with SyncLock and Volatile:

Public Class Singleton Private Shared _instance As Singleton Private Shared ReadOnly lockObj As New Object() Private Sub New() End Sub Public Shared ReadOnly Property Instance As Singleton Get If _instance Is Nothing Then SyncLock lockObj If _instance Is Nothing Then _instance = New Singleton() End If End SyncLock End If Return _instance End Get End Property End Class

2. How do you implement a deadlock in VB.NET?

Answer:

Module DeadlockExample Private lock1 As New Object() Private lock2 As New Object() Sub Main() Dim t1 As New Thread(Sub() SyncLock lock1 SyncLock lock2 Console.WriteLine("Thread 1") End SyncLock End SyncLock End Sub) Dim t2 As New Thread(Sub() SyncLock lock2 SyncLock lock1 Console.WriteLine("Thread 2") End SyncLock End SyncLock End Sub) t1.Start() t2.Start() End Sub End Module

3. How do you avoid deadlocks in VB.NET?

Answer:

• Always acquire locks in a consistent order.

• Use timeouts when acquiring locks.

• Minimize the duration of holding locks.

4. How do you implement the producer-consumer problem in VB.NET?

Answer:

Imports System.Collections.Concurrent Imports System.Threading Module ProducerConsumer Sub Main() Dim queue As New BlockingCollection(Of Integer)(5) Dim producer As New Thread(Sub() queue.Add(1) End Sub) Dim consumer As New Thread(Sub() queue.Take() End Sub) producer.Start() consumer.Start() End Sub End Module

5. What is the difference between Task and Thread in VB.NET?

Answer:

• Thread is low-level and requires manual management.

• Task is higher-level, uses thread pooling, and supports asynchronous programming.

Example:

Dim t As Task = Task.Run(Sub() Console.WriteLine("Running in Task")) t.Wait()

6. How do you implement a custom exception in VB.NET?

Answer:

Class CustomException Inherits Exception Public Sub New(message As String) MyBase.New(message) End Sub End Class

7. How do you use LINQ to find the second-highest number in a list?

Answer:

Dim numbers As List(Of Integer) = New List(Of Integer) From {5, 1, 9, 7} Dim secondHighest As Integer = numbers.OrderByDescending(Function(x) x).Skip(1).FirstOrDefault()

8. How do you detect a cycle in a linked list in VB.NET?

Answer:
Use Floyd’s Cycle Detection Algorithm:

Function HasCycle(head As ListNode) As Boolean Dim slow As ListNode = head, fast As ListNode = head While fast IsNot Nothing AndAlso fast.Next IsNot Nothing slow = slow.Next fast = fast.Next.Next If slow Is fast Then Return True End While Return False End Function

9. How do you implement an LRU Cache in VB.NET?

Answer:

Class LRUCache(Of K, V) Inherits Dictionary(Of K, V) Private capacity As Integer Public Sub New(capacity As Integer) MyBase.New() Me.capacity = capacity End Sub Protected Overrides Function RemoveEldestEntry(eldest As KeyValuePair(Of K, V)) As Boolean Return Me.Count > capacity End Function End Class

10. How do you convert a binary tree to a doubly linked list?

Answer:

Class TreeNode Public val As Integer Public left, right As TreeNode End Class Dim head, prev As TreeNode Sub Convert(root As TreeNode) If root Is Nothing Then Return Convert(root.left) If prev Is Nothing Then head = root Else prev.right = root : root.left = prev prev = root Convert(root.right) End Sub

11. How do you reverse a linked list in VB.NET?

Answer:

Function Reverse(head As ListNode) As ListNode Dim prev As ListNode = Nothing, curr As ListNode = head While curr IsNot Nothing Dim next As ListNode = curr.Next curr.Next = prev prev = curr curr = next End While Return prev End Function

12. How do you implement quicksort in VB.NET?

Answer:

Sub QuickSort(ByRef arr() As Integer, low As Integer, high As Integer) If low < high Then Dim pi As Integer = Partition(arr, low, high) QuickSort(arr, low, pi - 1) QuickSort(arr, pi + 1, high) End If End Sub

13. How do you merge two sorted linked lists?

Answer:

Function Merge(l1 As ListNode, l2 As ListNode) As ListNode If l1 Is Nothing Then Return l2 If l2 Is Nothing Then Return l1 If l1.val < l2.val Then l1.Next = Merge(l1.Next, l2) : Return l1 Else l2.Next = Merge(l1, l2.Next) : Return l2 End Function

14. How do you count word occurrences in a file using LINQ?

Answer:

Dim wordCount = File.ReadLines("file.txt") _ .SelectMany(Function(line) line.Split(" "c)) _ .GroupBy(Function(word) word) _ .ToDictionary(Function(g) g.Key, Function(g) g.Count())

15. How do you remove duplicate characters from a string?

Answer:

Function RemoveDuplicates(str As String) As String Return New String(str.Distinct().ToArray()) End Function

16. How do you find the longest substring without repeating characters?

Answer:

Function LongestUniqueSubstring(s As String) As Integer Dim map As New Dictionary(Of Char, Integer) Dim maxLen = 0, left = 0 For right = 0 To s.Length - 1 If map.ContainsKey(s(right)) Then left = Math.Max(left, map(s(right)) + 1) map(s(right)) = right maxLen = Math.Max(maxLen, right - left + 1) Next Return maxLen End Function

17. How do you execute a method at fixed intervals in VB.NET?

Answer:

Dim scheduler = New System.Threading.Timer(Sub() Console.WriteLine("Task"), Nothing, 0, 1000)

18. How do you implement a binary search in VB.NET?

Answer:

Function BinarySearch(arr() As Integer, target As Integer) As Integer Dim left = 0, right = arr.Length - 1 While left <= right Dim mid = left + (right - left) \ 2 If arr(mid) = target Then Return mid If arr(mid) < target Then left = mid + 1 Else right = mid - 1 End While Return -1 End Function

​

19. Q: What is the role of Reflection in VB.NET and how is it used?
A: Reflection is used to inspect metadata, types, and assemblies at runtime. It allows dynamic creation and invocation of objects, which is useful in scenarios like plugins, serialization, or tools like object mappers.
Example:

Dim t As Type = GetType(String) Console.WriteLine("Methods in String class:") For Each m As MethodInfo In t.GetMethods() Console.WriteLine(m.Name) Next

20. Q: Explain the use of Activator.CreateInstance in VB.NET.
A: Activator.CreateInstance is used to dynamically create an instance of a type at runtime. It's often used with reflection when the type is not known at compile time.
Example:

Dim objType As Type = Type.GetType("System.Text.StringBuilder") Dim objInstance As Object = Activator.CreateInstance(objType)

21. Q: What is the difference between Dispose() and Finalize() methods in VB.NET?
A:

• Dispose() is called explicitly to release unmanaged resources using the IDisposable interface.

• Finalize() is called by the garbage collector before the object is destroyed. It's defined using a destructor (Protected Overrides Sub Finalize).
  Use Dispose() for better control and performance with the Using block.

22. Q: How do Delegates work in VB.NET and how are they different from Events?
A: A Delegate is a type-safe function pointer, allowing methods to be passed as parameters. Events are built on delegates but provide additional syntax and encapsulation. Delegates can call multiple methods using multicast.
Example:

Public Delegate Sub MyDelegate(ByVal msg As String) Sub ShowMessage(msg As String) Console.WriteLine(msg) End Sub Dim d As MyDelegate = AddressOf ShowMessage d.Invoke("Hello from delegate!")

23. Q: What are Extension Methods in VB.NET?
A: Extension Methods allow you to add new methods to existing types without modifying them. They must be declared in a module and marked with <Extension()> attribute.
Example:

Imports System.Runtime.CompilerServices Module Extensions <Extension()> Public Function WordCount(str As String) As Integer Return str.Split(" "c).Length End Function End Module ' Usage: Dim txt As String = "VB.NET is powerful" Console.WriteLine(txt.WordCount()) ' Output: 3

24. Q: How does multithreading work in VB.NET?
A: VB.NET supports multithreading through the System.Threading namespace. Threads can be created using the Thread class or using the Task Parallel Library (System.Threading.Tasks).
Example (Thread):

Dim t As New Thread(AddressOf DoWork) t.Start() Sub DoWork() Console.WriteLine("Running on a separate thread") End Sub

25. Q: Explain the role of Async and Await in VB.NET.
A: Async and Await simplify asynchronous programming by allowing non-blocking code execution. An Async method typically returns Task or Task(Of T), and Await is used to pause execution until the awaited task completes.
Example:

Public Async Function GetDataAsync() As Task(Of String) Await Task.Delay(1000) Return "Data received" End Function

​

​