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A Structure is a value type. Its data, its meaning, is found directly in its bytes. Integers, Booleans and DateTimes are built-in Structures.
Copies. When we pass a Structure to a method, its bytes are copied each time. Structures are stored on the evaluation stack (not the heap) when used in a method body.
And: This gives Structures performance advantages—and sometimes hurts performance.
To start, this example has a Structure called Simple. This Structure has three fields: an Integer, a Boolean and a Double. These fields are stored directly as part of the Structure.
And: In Main 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.
Based on: .NET 4.5 VB.NET program that uses 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 Output 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.DateTime
Then: The local d2 copies the values from "d," but the two locals are separate. When "d" is changed, d2 is not affected.
VB.NET program that copies structure 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 Module Output D: 1/1/2020 D2: 1/1/2020 D2: 1/1/2020
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.
Here: A Structure called Box is allocated many times in a loop. The managed heap is not accessed.
And: All the Box instances are stored in local variable memory. Next a Class called Ball is allocated in a similar loop.For Loops
But: On each iteration the managed heap is accessed. This triggers garbage collection at intervals. This reduces performance.
VB.NET program that times Structure 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 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 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 Module Output 2.26 ns Structure 8.20 ns Class
Results. Each allocation of the Structure took around 2 nanoseconds. But each allocation of the Class, which has equivalent fields, took 8 nanoseconds. The Structure allocates faster.
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.Class
Usually, structures will decrease program performance. It is often better to use Classes for custom types. For typical programs, I advise avoiding custom structures.
A 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.