Answer

Property in Visual Basic is used to encapsulate access to an object's data with the ability to perform checks, computations, and manage access control. Properties contain Get blocks (for reading values) and Set blocks (for setting values).

Basics of declaration and usage:

Private _age As Integer

Public Property Age() As Integer
    Get
        Return _age
    End Get
    Set(ByVal value As Integer)
        If value < 0 Or value > 120 Then
            Throw New ArgumentException("Age must be between 0 and 120!")
        End If
        _age = value
    End Set
End Property

With such a property, you can implement value validation, caching, on-the-fly computation, etc., and for the user, the object appears to be a regular field.

Automatic properties (VB.NET):

Public Property ItemId As Integer ' automatically creates a hidden field, no access to logic

Nuance: automatic properties cannot be extended with internal logic without explicit declaration of get/set.

Trick question

Question: Can a property in Visual Basic be implemented with only a public set and a private get? How can different access levels be set for get and set?

Answer: Yes, starting from VB.NET, you can specify different access levels for get and set:

Public Property Salary As Decimal
    Private Get
        Return _salary
    End Get
    Set(ByVal value As Decimal)
        _salary = value
    End Set
End Property

In this example, only the class can get the value of Salary, while external objects can only set it.

Examples of real errors due to ignorance of the nuances of the topic

---

Story

A developer used automatic properties for all fields of a business object. Later, validation for negative values became necessary, but switching from an automatic property to an explicit one required manually adjusting a large part of the code, leading to errors and increased workload.

---

Story

During the migration from VB6 to VB.NET, some properties were implemented as fields with public access modifiers: this opened full unauthorized access to the internal data of the class, leading to unwanted modifications of the state by external objects.

---

Story

In a complex object, a property contained additional computations in the get block, including file operations. When the property was accessed frequently, the application's performance dropped sharply; it turned out that using a private field with cached results would have been better than computing the value on-the-fly during each get.