`Structure`

This VB.NET keyword is used to represent data. A Structure's data is found directly in its bytes: Integers, Booleans and DateTimes are built-in Structures.

When we pass a Structure to a method, its bytes are copied. Structures are stored on the evaluation stack (not the heap) when used in a method body. This can help (or hurt) performance.

To start, this example has a Structure called Simple. This Structure has 3 fields: an Integer, a Boolean and a Double. These fields are stored directly as part of the Structure.

Here We create an instance of Simple—we do not need to use a New Sub (a constructor).

So A Structure, of any type, is used in the same way as an Integer. And an Integer itself is a kind of Structure.

Structure Simple
    Public _position As Integer
    Public _exists As Boolean
    Public _lastValue As Double
End Structure

Module Module1
    Sub Main()
        Dim s As Simple
        s._position = 1
        s._exists = False
        s._lastValue = 5.5

        Console.WriteLine(s._position)
    End Sub
End Module
1

Copy

A structure is self-contained in its memory region. So when we assign one Structure local to another, it is copied. And the two values, when changed, do not affect each other.

Here We create a DateTime structure, as a local, and initialize to a value in the year 2020.

Then The local d2 copies the values from "d," but the two locals are separate. When "d" is changed, d2 is not affected.

Module Module1

    Sub Main()
        ' Create a structure and copy it.
        Dim d As DateTime = New DateTime(2020, 1, 1)
        Dim d2 As DateTime = d

        Console.WriteLine("D:  " + d)
        Console.WriteLine("D2: " + d2)

        ' Reassign "d" and the copy "d2" does not change.
        d = DateTime.MinValue
        Console.WriteLine("D2: " + d2)
    End Sub

End ModuleD:  1/1/2020
D2: 1/1/2020
D2: 1/1/2020

A benchmark

A speed difference between Structure and Class comes from allocation models. A Class reference points to data externally stored. A Structure's data is in the variable itself.

Version 1 A Structure called Box is allocated many times in a loop. The managed heap is not accessed.

Version 2 In this version of the code a Class called Ball is allocated in a similar loop.

Result Each allocation of the Structure took around 2 nanoseconds, and the class took 8. The Structure allocates faster.

Structure Box
    Public _a As Integer
    Public _b As Boolean
    Public _c As DateTime
End Structure

Class Ball
    Public _a As Integer
    Public _b As Boolean
    Public _c As DateTime
End Class

Module Module1
    Sub Main()
        Dim m As Integer = 100000000
        Dim s1 As Stopwatch = Stopwatch.StartNew
        ' Version 1: use Structure.
        For i As Integer = 0 To m - 1
            Dim b As Box
            b._a = 1
            b._b = False
            b._c = DateTime.MaxValue
        Next
        s1.Stop()

        Dim s2 As Stopwatch = Stopwatch.StartNew
        ' Version 2: use Class.
        For i As Integer = 0 To m - 1
            Dim b As Ball = New Ball
            b._a = 1
            b._b = False
            b._c = DateTime.MaxValue
        Next
        s2.Stop()

        Dim u As Integer = 1000000
        Console.WriteLine(((s1.Elapsed.TotalMilliseconds * u) / m).ToString("0.00 ns"))
        Console.WriteLine(((s2.Elapsed.TotalMilliseconds * u) / m).ToString("0.00 ns"))
    End Sub
End Module2.26 ns   Structure
8.20 ns   Class

Arguments

The Structure, when passed as an argument to a Function, will be slower. It is larger. The Class is only 4 (or 8) bytes. When more bytes are copied, Function calls are slower.

Usually, structures will decrease program performance. It is often better to use Classes for custom types. For typical programs, I advise avoiding custom structures.

Summary

Structures are often used for built-in types. Structures are unique in their allocation behavior. Their data, their fields and values, are stored directly inside the variable.