Introduction to VB.NET

Difference between ByVal and ByRef

ByVal passes a copy of the value to the procedure, so changes won’t affect the original variable.

ByRef passes a reference to the original variable, allowing it to be modified directly.

Practical example:

If you pass an object ByRef and set it to Nothing inside a Sub, it will actually be destroyed in the calling context.

Strong typing and conversions (CInt(), CStr(), TryParse)

VB.NET is strongly typed, so you must explicitly convert types when needed.

  • CInt(), CStr(), etc. throw exceptions if conversion fails
  • TryParse() avoids errors by returning True/False to indicate validity
Dim numero As Integer
If Integer.TryParse("123", numero) Then
    Console.WriteLine(numero)
Else
    Console.WriteLine("Conversion failed")
End If

Flow control (With, Using, Select Case, Do While)

  • With: simplifies repeated access to object members
  • Using: ensures automatic disposal of resources (e.g. files, DB)
  • Select Case: alternative to multiple If
  • Do While / Do Until: loops with pre/post conditions
Using conn As New SqlConnection(connStr)
    conn.Open()
    '...
End Using

Differences between Module, Class, Structure, Interface

  • Module: contains shared members, cannot be instantiated
  • Class: dynamic reference type, supports inheritance
  • Structure: value type, lighter, good for small data
  • Interface: only defines method signatures, to be implemented by concrete classes

Object-Oriented Programming in VB.NET

OOP concepts: inheritance, polymorphism, encapsulation

  • Inheritance: a class can extend another (Inherits)
  • Polymorphism: different objects respond to common calls
  • Encapsulation: hides internal data using properties and methods

Constructors, properties, Overloading and Overriding

  • The constructor (Sub New) initializes objects
  • Properties (Property) encapsulate field and access
  • Overloads: multiple versions of a method with different parameters
  • Overrides: redefines an inherited method

Difference between Overrides and Overloads

  • Overrides: used to override an inherited method
  • Overloads: defines multiple variants of a method with different parameters

MustInherit vs NotInheritable

  • MustInherit: abstract class, cannot be instantiated, may contain MustOverride methods
  • NotInheritable: cannot be inherited, useful for final classes

Using interfaces (Implements)

Public Interface IPrintable
    Sub Print()
End Interface

Public Class Document
    Implements IPrintable

    Public Sub Print() Implements IPrintable.Print
        Console.WriteLine("Document printed")
    End Sub
End Class

Custom attributes

Attributes allow decorating classes, methods, or properties with metadata.
You can create a custom attribute by inheriting from System.Attribute.

Data Access with ADO.NET

How to connect to SQL Server?

Using conn As New SqlConnection("your_conn_string")
    conn.Open()
    Dim cmd As New SqlCommand("SELECT * FROM Users", conn)
    Using reader = cmd.ExecuteReader()
        While reader.Read()
            Console.WriteLine(reader("Name"))
        End While
    End Using
End Using

Preventing SQL Injection

cmd.CommandText = "SELECT * FROM Users WHERE Name = @name"
cmd.Parameters.AddWithValue("@name", txtName.Text)

This protects against query manipulation.

Difference between DataSet, DataTable, DataReader

  • DataReader: forward-only access, efficient
  • DataTable: represents an in-memory table
  • DataSet: container for multiple DataTable, useful for complex operations

Debugging and Maintenance

How do you handle spaghetti legacy code?

  • Analyze and understand the logic
  • Gradually refactor to separate responsibilities
  • Use debugger to test isolated sections
  • Introduce simple patterns (e.g. Repository, Singleton)
  • Write tests to prevent regressions

Debugging tools

  • Breakpoints
  • Watch / Autos
  • Immediate Window
  • Call Stack

Essential tools in Visual Studio for effective debugging.

Typical Exercises

Function that counts vowels

Function CountVowels(input As String) As Integer
    Dim count As Integer = 0
    For Each c As Char In input.ToLower()
        If "aeiou".Contains(c) Then count += 1
    Next
    Return count
End Function

Persona class with ToString()

Public Class Person
    Public Property Name As String
    Public Property Age As Integer

    Public Sub New(name As String, age As Integer)
        Me.Name = name
        Me.Age = age
    End Sub

    Public Overrides Function ToString() As String
        Return $"{Name} is {Age} years old."
    End Function
End Class

Difference between Shared and Non-Shared

  • Shared: belongs to the class, accessible without creating an instance
  • Non-Shared: each object has its own copy

Useful for utility methods, e.g.: Math.Sqrt()