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A Structure is a value type. Its data is contained directly inside its bytes. Integers, Booleans and DateTimes are built-in Structures. When we pass a Structure to a method, its bytes are copied each time.
Tip:Structures are stored on the evaluation stack (not the managed heap) when used in a method body.
Tip 2:This gives Structures performance advantages—and sometimes hurts performance.
To start, this program has a Structure called Simple.
This Structure has three fields:
and a Double. These fields are stored directly as part of the Simple Structure itself. In Main we create an instance of Simple.
Note:The Simple Structure is created and used without calling its constructor (Sub New). It is used in the same way as an Integer.
Program that uses Structure: VB.NET 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
The performance difference between a Structure and a Class comes from how the types are allocated. A Class reference points to data stored in a separate location—the managed heap. A Structure variable stores data in the variable itself.
Here, a Structure called Box is allocated many times in a loop. The managed heap is not accessed. 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.
Program that times Structure: VB.NET 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
Each allocation of the Structure took around 2 nanoseconds.
But each allocation of the Class,
which has equivalent fields,
took 8 nanoseconds. Allocating a Structure in this kind of loop has performance advantages.
However, the Structure, when passed as an argument to a Function, will be slower. It is larger.
The Class is only four bytes—
or eight bytes,
depending on the system. When more bytes are copied, Function calls are slower.
Further:This is not true if the JIT-compiler happens to inline the Function call. We have little control over this.
We explored Structures in the VB.NET programming language. Structures are unique in their allocation behavior. They are not references to the managed heap. Instead they are stored directly inside their bytes.
Usually:Structures will decrease program performance. It is often better to use Classes for custom types.