Reasons to use array.resize instead of redim

Question

I know that "redim" is older than "Array.Resize" but do not understand the reasons that make better to use the latter to the former.

Answer 1

As explained in MSDN, ReDim and Array.Resize seems to work equal by allocationg a new Array with fixed size to copy the contents of the old awwat, however there are some internal differences (that I ignore) that makes ReDim faster, and ReDim Preserve a performance killer when compared.

By the result of the next test, which performs a simple iteration to redimensionate an array 100.000 times, you can proof that ReDim is much faster than Array.Resize, however, both are slower than a custom generic solution using Array.Copy method.

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Results

ReDim Elapsed Time: 00:00:07.3918219

Array.Resize Elapsed Time: 00:00:28.9773317

Custom Resize Elapsed Time: 00:00:03.2659880

As you see, there is a difference of +20 seconds between ReDim and Array.Resize

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Test source-code

Dim myArray As String()
Dim sw As New Stopwatch

myArray = New String() {"Q", "W", "E", "R", "T", "Y"}
sw.Start()
For x As Integer = 1 To 100000 : ReDim myArray(x) : Next
sw.Stop()
Console.WriteLine("ReDim Elapsed Time: " & sw.Elapsed.ToString)

myArray = New String() {"Q", "W", "E", "R", "T", "Y"}
sw.Restart()
For x As Integer = 1 To 100000 : Array.Resize(myArray, x) : Next
sw.Stop()
Console.WriteLine("Array.Resize Elapsed Time: " & sw.Elapsed.ToString)

myArray = New String() {"Q", "W", "E", "R", "T", "Y"}
sw.Restart()
For x As Integer = 1 To 100000 : ResizeArray(myArray, x) : Next
sw.Stop()
Console.WriteLine("Custom Resize Elapsed Time: " & sw.Elapsed.ToString)

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Custom Array-Resize solution:

Very useful to add it as a extension method.

' <Extension>
<DebuggerStepThrough>
Public Function ResizeArray(Of T)(ByVal sender As T(),
                                  ByVal newSize As Integer) As T()

    If (newSize <= 0) Then
        Throw New ArgumentOutOfRangeException(paramName:="newSize",
                                              message:="Value bigger than 0 is required.")
    End If

    Dim preserveLength As Integer = Math.Min(sender.Length, newSize)

    If (preserveLength > 0) Then
        Dim newArray As Array = Array.CreateInstance(sender.GetType.GetElementType, newSize)
        Array.Copy(sender, newArray, preserveLength)
        Return DirectCast(newArray, T())

    Else
        Return sender

    End If

End Function

Disclaimer: I'm not the author of the function, it is a custom Vb.Net adapttaion from a C# code that I found time ago in a random StackOverflow answer about Array.Resize performance.