How can I delete an item from an array in VB.NET?

Question

How can I delete an item from an array in VB.NET?

Answer 1

As Heinzi said, an array has a fixed size. In order to 'remove an item' or 'resize' it, you'll have to create a new array with the desired size and copy the items you need as appropriate.

Here's code to remove an item from an array:

<System.Runtime.CompilerServices.Extension()> _ Function RemoveAt(Of T)(ByVal arr As T(), ByVal index As Integer) As T()     Dim uBound = arr.GetUpperBound(0)     Dim lBound = arr.GetLowerBound(0)     Dim arrLen = uBound - lBound      If index < lBound OrElse index > uBound Then         Throw New ArgumentOutOfRangeException( _         String.Format("Index must be from {0} to {1}.", lBound, uBound))      Else         'create an array 1 element less than the input array         Dim outArr(arrLen - 1) As T         'copy the first part of the input array         Array.Copy(arr, 0, outArr, 0, index)         'then copy the second part of the input array         Array.Copy(arr, index + 1, outArr, index, uBound - index)          Return outArr     End If End Function 

You can use it as such:

Module Module1      Sub Main()         Dim arr = New String() {"abc", "mno", "xyz"}         arr.RemoveAt(1)     End Sub End Module 

The code above removes the second element ("mno") [which has an index of 1] from the array.

You need to be developing in .NET 3.5 or higher in order to use the extension method. If you're using .NET 2.0 or 3.0, you can call the method as such

arr = RemoveAt(arr, 1) 

I hope this is what you need.

Update

After running tests based on ToolMakerSteve's comment it appears the initial code does not modify the array you want to update because of the ByVal used in the function's declaration. However, writing code like arr = arr.RemoveAt(1) or arr = RemoveAt(arr, 1) does modify the array because it reassigns the modified array to the original.

Find below the updated method (subroutine) for removing an element from an array.

<System.Runtime.CompilerServices.Extension()> _ Public Sub RemoveAt(Of T)(ByRef arr As T(), ByVal index As Integer)     Dim uBound = arr.GetUpperBound(0)     Dim lBound = arr.GetLowerBound(0)     Dim arrLen = uBound - lBound      If index < lBound OrElse index > uBound Then         Throw New ArgumentOutOfRangeException( _         String.Format("Index must be from {0} to {1}.", lBound, uBound))      Else         'create an array 1 element less than the input array         Dim outArr(arrLen - 1) As T         'copy the first part of the input array         Array.Copy(arr, 0, outArr, 0, index)         'then copy the second part of the input array         Array.Copy(arr, index + 1, outArr, index, uBound - index)          arr = outArr     End If End Sub 

Usage of the method is similar to the original, except there is no return value this time so trying to assign an array from the return value will not work because nothing is returned.

Dim arr = New String() {"abc", "mno", "xyz"} arr.RemoveAt(1)  ' Output: {"abc", "mno"} (works on .NET 3.5 and higher) RemoveAt(arr, 1) ' Output: {"abc", "mno"} (works on all versions of .NET fx) arr = arr.RemoveAt(1)  'will not work; no return value arr = RemoveAt(arr, 1) 'will not work; no return value 

Note:

1. I use a temporary array for the process because it makes my intentions clear and that is exactly what VB.NET does behind the scenes when you do Redim Preserve. If you would like to modify the array in-place using Redim Preserve, see ToolmakerSteve's answer.

2. The RemoveAt methods written here are extension methods. In order for them to work, you will have to paste them in a Module. Extension methods will not work in VB.NET if they are placed in a Class.

3. Important If you will be modifying your array with lots of 'removes', it is highly recommended to use a different data structure such as List(Of T) as suggested by other answerers to this question.