Arrays

Option Strict On

Module Program
    Sub Main()
        ' The number in parentheses is the UPPER BOUND, not the length
        Dim shelves(4) As Integer
        Console.WriteLine("Length = " & shelves.Length)
        Console.WriteLine("LBound = " & LBound(shelves) & ", UBound = " & UBound(shelves))
        Console.WriteLine("GetLowerBound(0) = " & shelves.GetLowerBound(0) & ", GetUpperBound(0) = " & shelves.GetUpperBound(0))

        ' Elements start with the default value of the element type
        Dim titles(2) As String
        Dim flags(1) As Boolean
        Console.WriteLine("titles(0) Is Nothing: " & (titles(0) Is Nothing))
        Console.WriteLine("flags(1) = " & flags(1))

        ' Several equivalent ways to say "these three numbers"
        Dim a As Integer() = {10, 20, 30}
        Dim b() As Integer = {10, 20, 30}
        Dim c As Integer() = New Integer() {10, 20, 30}
        Dim d As Integer() = New Integer(2) {10, 20, 30}
        Dim e = {10, 20, 30}
        Console.WriteLine(a.Length & " " & b.Length & " " & c.Length & " " & d.Length & " " & e.Length)
        Console.WriteLine("e is " & e.GetType().Name)

        ' Array literal inference picks the dominant type
        Dim mixed = {1, 2.5, 3}
        Dim wide = {1, 2L, 3}
        Console.WriteLine(mixed.GetType().Name & " " & wide.GetType().Name)

        ' An empty array is not Nothing
        Dim none(-1) As String
        Dim alsoNone As String() = {}
        Dim missing As String() = Nothing
        Console.WriteLine(none.Length & " " & alsoNone.Length & " " & (missing Is Nothing))
        Console.WriteLine("Array.Empty is shared: " & Object.ReferenceEquals(Array.Empty(Of String)(), Array.Empty(Of String)()))

        ' Arrays are reference types: assignment copies the reference, Clone copies the elements
        Dim copyRef = a
        Dim copyElements = CType(a.Clone(), Integer())
        a(0) = 99
        Console.WriteLine(copyRef(0) & " " & copyElements(0))
    End Sub
End Module

Length = 5
LBound = 0, UBound = 4
GetLowerBound(0) = 0, GetUpperBound(0) = 4
titles(0) Is Nothing: True
flags(1) = False
3 3 3 3 3
e is Int32[]
Double[] Int64[]
0 0 True
Array.Empty is shared: True
99 10

Option Strict On

Module Program
    Sub Main()
        Dim tooMany(1) As Integer = {1, 2, 3}
        Dim tooFew(3) As Integer = {1, 2}
        Dim bad As Integer() = New Integer(2) {1, 2}
        Dim noBraces As Integer() = New Integer(3)
        Dim m As Integer(,) = {{1, 2}, {3}}
        Dim wrongRank As Integer() = New Integer(,) {{1}}
    End Sub
End Module

Program.vb(5,13): error BC30672: Explicit initialization is not permitted for arrays declared with explicit bounds.
Program.vb(6,13): error BC30672: Explicit initialization is not permitted for arrays declared with explicit bounds.
Program.vb(7,47): error BC30567: Array initializer is missing 1 elements.
Program.vb(8,49): error BC30057: Too many arguments to 'Public Sub New()'.
Program.vb(9,40): error BC30567: Array initializer is missing 1 elements.
Program.vb(10,38): error BC30414: Value of type 'Integer(*,*)' cannot be converted to 'Integer()' because the array types have different numbers of dimensions.

Option Strict On

Module Program
    Sub Main()
        Dim a As Integer(4)
        Dim b() As Integer(4)
    End Sub
End Module

Program.vb(5,26): error BC30638: Array bounds cannot appear in type specifiers.
Program.vb(6,28): error BC30638: Array bounds cannot appear in type specifiers.

Option Strict On

Module Program
    Sub Main()
        Dim ok(0 To 4) As Integer
        Console.WriteLine(ok.Length)
        Dim vb6(1 To 5) As Integer
    End Sub
End Module

Program.vb(7,17): error BC32059: Array lower bounds can be only '0'.

Option Strict On

Module Program
    Sub Main()
        Dim a = {1, "two", 3.0}
        Dim b = {Nothing, Nothing}
        Dim c As Integer() = {1, 2.5}
    End Sub
End Module

Program.vb(5,17): error BC36915: Cannot infer an element type because more than one type is possible. Specifying the type of the array might correct this error.
Program.vb(6,17): error BC36914: Cannot infer an element type, and Option Strict On does not allow 'Object' to be assumed. Specifying the type of the array might correct this error.
Program.vb(7,34): error BC30512: Option Strict On disallows implicit conversions from 'Double' to 'Integer'.

Option Strict On

Module Program
    Const Primes As Integer() = {2, 3, 5}
    Const Names() As String = {"a"}
    Sub Main()
    End Sub
End Module

Program.vb(4,11): error BC30424: Constants must be of an intrinsic or enumerated type, not a class, structure, type parameter, or array type.
Program.vb(5,11): error BC30424: Constants must be of an intrinsic or enumerated type, not a class, structure, type parameter, or array type.

Option Strict On

Module Program
    Sub Main()
        Dim isbns As String() = {"9780132350884", "9780134494166", "9781491950357"}

        For i = 0 To isbns.Length - 1
            Console.WriteLine(i & ": " & isbns(i))
        Next

        For i = LBound(isbns) To UBound(isbns)
            Console.Write(isbns(i).Substring(0, 3) & " ")
        Next
        Console.WriteLine()

        For Each isbn In isbns
            Console.Write(isbn.Length & " ")
        Next
        Console.WriteLine()

        ' Backwards
        For i = isbns.Length - 1 To 0 Step -1
            Console.Write(isbns(i).Substring(10) & " ")
        Next
        Console.WriteLine()

        ' Off by one: the loop bound is the upper bound, not the length
        Try
            For i = 0 To isbns.Length
                Console.Write(i & " ")
            Next
            Console.WriteLine()
            Dim last = isbns(isbns.Length)
            Console.WriteLine(last)
        Catch ex As IndexOutOfRangeException
            Console.WriteLine(ex.GetType().Name & ": " & ex.Message)
        End Try

        ' For Each over a literal
        Dim total = 0
        For Each n In {1, 2, 3, 4}
            total += n
        Next
        Console.WriteLine("total = " & total)

        ' There is no negative index and no range syntax: use arithmetic, Array.Copy or LINQ
        Console.WriteLine("last = " & isbns(isbns.Length - 1))
        Console.WriteLine("last = " & isbns.Last())
        Console.WriteLine("middle = " & String.Join(",", isbns.Skip(1).Take(1)))
    End Sub
End Module

0: 9780132350884
1: 9780134494166
2: 9781491950357
978 978 978
13 13 13
357 166 884
0 1 2 3
IndexOutOfRangeException: Index was outside the bounds of the array.
total = 10
last = 9781491950357
last = 9781491950357
middle = 9780134494166

Option Strict On

Module Program
    Sub Main()
        Dim a = {1, 2, 3}
        Dim s = a(1..2)
    End Sub
End Module

Program.vb(6,21): error BC30203: Identifier expected.

Option Strict On

Module Program
    Sub Main()
        Dim a = {1, 2, 3}
        Dim t = a(^1)
    End Sub
End Module

Program.vb(6,19): error BC30201: Expression expected.

Option Strict On

Module Program
    Sub Show(label As String, values As Integer())
        If values Is Nothing Then
            Console.WriteLine(label & ": Nothing")
        Else
            Console.WriteLine(label & ": [" & String.Join(", ", values) & "] (UBound " & UBound(values) & ")")
        End If
    End Sub

    Sub Main()
        Dim counts(2) As Integer
        counts(0) = 1 : counts(1) = 2 : counts(2) = 3
        Dim before = counts

        ReDim counts(4)                 ' new empty array: the old values are lost
        Show("after ReDim", counts)

        counts(0) = 7 : counts(1) = 8
        ReDim Preserve counts(5)        ' copies the old elements into a new, larger array
        Show("after ReDim Preserve (grow)", counts)

        ReDim Preserve counts(1)        ' shrinking drops the elements above the new bound
        Show("after ReDim Preserve (shrink)", counts)

        Show("the array another variable still refers to", before)
        Console.WriteLine("same object: " & Object.ReferenceEquals(before, counts))

        ' Array.Resize does the same through a ByRef parameter and also allocates a new array
        Dim viaResize = counts
        Array.Resize(counts, 4)
        Show("after Array.Resize", counts)
        Show("viaResize (old array)", viaResize)

        ' Erase sets the variable to Nothing
        Erase counts
        Show("after Erase", counts)

        ' A multidimensional array can change only its last dimension with Preserve
        Dim grid(1, 1) As Integer
        grid(1, 1) = 5
        ReDim Preserve grid(1, 3)
        Console.WriteLine("grid(1, 1) = " & grid(1, 1) & ", GetLength(1) = " & grid.GetLength(1))
        Try
            ReDim Preserve grid(3, 3)
        Catch ex As ArrayTypeMismatchException
            Console.WriteLine(ex.GetType().Name & ": " & ex.Message)
        End Try

        ' ReDim cannot change the rank
        Dim resizable() As Integer
        ReDim resizable(3)
        Console.WriteLine("resizable: " & resizable.Length)
    End Sub
End Module

after ReDim: [0, 0, 0, 0, 0] (UBound 4)
after ReDim Preserve (grow): [7, 8, 0, 0, 0, 0] (UBound 5)
after ReDim Preserve (shrink): [7, 8] (UBound 1)
the array another variable still refers to: [1, 2, 3] (UBound 2)
same object: False
after Array.Resize: [7, 8, 0, 0] (UBound 3)
viaResize (old array): [7, 8] (UBound 1)
after Erase: Nothing
grid(1, 1) = 5, GetLength(1) = 4
ArrayTypeMismatchException: Attempted to access an element as a type incompatible with the array.
resizable: 4

Option Strict On

Module Program
    Sub Main()
        Dim a As Integer() = {1, 2, 3}
        ReDim a(5)
        ReDim Preserve a(1, 2)
        Dim n As Integer = 3
        ReDim n(2)
        Dim s = "text"
        Erase s
    End Sub
End Module

Program.vb(7,24): error BC30415: 'ReDim' cannot change the number of dimensions of an array.
Program.vb(9,15): error BC30049: 'Redim' statement requires an array.
Program.vb(11,15): error BC30049: 'Erase' statement requires an array.

Option Strict On

Module Program
    Sub Main()
        Dim data() As Byte
        Dim n As Integer = 4
        ReDim data(n) As Byte
        Console.WriteLine(data.Length)
    End Sub
End Module

Program.vb(7,23): error BC30811: 'ReDim' statements can no longer be used to declare array variables.

Option Strict On

Module Program
    Sub Main()
        ' Rectangular: (2, 3) means 3 rows by 4 columns
        Dim matrix(2, 3) As Integer
        For r = 0 To matrix.GetUpperBound(0)
            For c = 0 To matrix.GetUpperBound(1)
                matrix(r, c) = r * 10 + c
            Next
        Next
        Console.WriteLine("Rank = " & matrix.Rank & ", Length = " & matrix.Length)
        Console.WriteLine("GetLength(0) = " & matrix.GetLength(0) & ", GetLength(1) = " & matrix.GetLength(1))
        Console.WriteLine("UBound(matrix, 1) = " & UBound(matrix, 1) & ", UBound(matrix, 2) = " & UBound(matrix, 2))

        ' For Each visits a rectangular array in row-major order
        For Each v In matrix
            Console.Write(v & " ")
        Next
        Console.WriteLine()

        ' Initializers nest braces, one level per dimension
        Dim table As Integer(,) = {{1, 2, 3}, {4, 5, 6}}
        Console.WriteLine("table(1, 2) = " & table(1, 2))

        ' Jagged: an array whose elements are arrays, each with its own length
        Dim jagged(2)() As Integer
        jagged(0) = New Integer() {1}
        jagged(1) = New Integer() {1, 2, 3}
        jagged(2) = Array.Empty(Of Integer)()
        For i = 0 To jagged.Length - 1
            Console.WriteLine("row " & i & " has " & jagged(i).Length & " elements: " & String.Join(",", jagged(i)))
        Next

        Dim triangle = {New Integer() {1}, New Integer() {1, 1}, New Integer() {1, 2, 1}}
        Console.WriteLine("triangle type: " & triangle.GetType().Name & ", triangle(2)(1) = " & triangle(2)(1))

        Dim literal As Integer()() = {({1, 2}), ({3})}
        Console.WriteLine("literal(0)(1) = " & literal(0)(1) & ", literal(1).Length = " & literal(1).Length)

        ' Clone is shallow: the rows are shared
        Dim shallow = CType(jagged.Clone(), Integer()())
        shallow(1)(0) = 100
        Console.WriteLine("jagged(1)(0) after changing the clone: " & jagged(1)(0))

        ' Array.Copy and Clone work on the whole multidimensional array
        Dim copy = CType(table.Clone(), Integer(,))
        copy(0, 0) = -1
        Console.WriteLine("table(0, 0) = " & table(0, 0) & ", copy(0, 0) = " & copy(0, 0))
    End Sub
End Module

Rank = 2, Length = 12
GetLength(0) = 3, GetLength(1) = 4
UBound(matrix, 1) = 2, UBound(matrix, 2) = 3
0 1 2 3 10 11 12 13 20 21 22 23
table(1, 2) = 6
row 0 has 1 elements: 1
row 1 has 3 elements: 1,2,3
row 2 has 0 elements:
triangle type: Int32[][], triangle(2)(1) = 2
literal(0)(1) = 2, literal(1).Length = 1
jagged(1)(0) after changing the clone: 100
table(0, 0) = 1, copy(0, 0) = -1

Option Strict On

Module Program
    Sub Main()
        Dim a = {1, 2, 3}
        Dim b = {1, 2, 3}
        If a = b Then Console.WriteLine("equal")
        Dim m(2, 2) As Integer
        m(1) = 5
        Dim r As Integer() = m
        ReDim m(2)
        Dim x As Integer() = a.AsSpan()
    End Sub
End Module

Program.vb(7,12): error BC31080: Operator '=' is not defined for types 'Integer()' and 'Integer()'. Use 'Is' operator to compare two reference types.
Program.vb(9,10): error BC30105: Number of indices is less than the number of dimensions of the indexed array.
Program.vb(10,30): error BC30414: Value of type 'Integer(*,*)' cannot be converted to 'Integer()' because the array types have different numbers of dimensions.
Program.vb(11,15): error BC30415: 'ReDim' cannot change the number of dimensions of an array.
Program.vb(12,30): error BC30311: Value of type 'Span(Of Integer)' cannot be converted to 'Integer()'.

Option Strict On

Module Program
    Sub DoubleAll(values As Integer())      ' ByVal: the reference is copied, the elements are shared
        For i = 0 To values.Length - 1
            values(i) *= 2
        Next
    End Sub

    Sub Replace(ByRef values As Integer())  ' ByRef: the caller's variable can be re-pointed
        values = New Integer() {0, 0}
    End Sub

    Sub Rebind(values As Integer())         ' ByVal: re-pointing the parameter is invisible to the caller
        values = New Integer() {0, 0}
    End Sub

    Function Sum(ParamArray numbers As Integer()) As Integer
        Dim total = 0
        For Each n In numbers
            total += n
        Next
        Return total
    End Function

    Function Describe(label As String, ParamArray parts As String()) As String
        If parts Is Nothing Then Return label & ": Nothing"
        Return label & ": " & parts.Length & " part(s)"
    End Function

    Function Squares(count As Integer) As Integer()
        Dim result(count - 1) As Integer
        For i = 0 To count - 1
            result(i) = i * i
        Next
        Return result
    End Function

    Sub Main()
        Dim data = {1, 2, 3}
        DoubleAll(data)
        Console.WriteLine(String.Join(",", data))
        Rebind(data)
        Console.WriteLine(String.Join(",", data))
        Replace(data)
        Console.WriteLine(String.Join(",", data))

        Console.WriteLine(Sum() & " " & Sum(5) & " " & Sum(1, 2, 3) & " " & Sum(New Integer() {4, 5}))
        Console.WriteLine(Describe("none"))
        Console.WriteLine(Describe("two", "a", "b"))
        Console.WriteLine(Describe("array", New String() {"x", "y", "z"}))
        Console.WriteLine(Describe("nothing", Nothing))
        Console.WriteLine(String.Join(",", Squares(5)))
    End Sub
End Module

2,4,6
2,4,6
0,0
0 5 6 9
none: 0 part(s)
two: 2 part(s)
array: 3 part(s)
nothing: Nothing
0,1,4,9,16

Option Strict On

Module Program
    Sub Add(ParamArray numbers As Integer())
    End Sub

    Sub Main()
        Add(1, 2, "three")
        Add({1, 2}, 3)
    End Sub
End Module

Program.vb(8,19): error BC30512: Option Strict On disallows implicit conversions from 'String' to 'Integer'.
Program.vb(9,13): error BC30311: Value of type 'Integer()' cannot be converted to 'Integer'.

Option Strict On

Module Program
    Function Count(ParamArray values As Object()) As Integer
        Return values.Length
    End Function

    Sub Main()
        Dim ints As Integer() = {1, 2, 3}
        Dim strs As String() = {"a", "b"}
        Console.WriteLine("Integer() -> " & Count(ints))
        Console.WriteLine("String()  -> " & Count(strs))
        Console.WriteLine("Object()  -> " & Count(CType(strs, Object())))
        Console.WriteLine("nothing   -> " & Count())
    End Sub
End Module

Integer() -> 1
String()  -> 2
Object()  -> 2
nothing   -> 0

Option Strict On

Module Program
    Sub Main()
        Dim titles As String() = {"Refactoring", "Clean Code", "Domain-Driven Design", "Working Effectively with Legacy Code"}
        Dim copy As String() = CType(titles.Clone(), String())

        Array.Sort(copy)
        Console.WriteLine("Sort: " & String.Join(" | ", copy))

        Array.Sort(copy, Function(left, right) left.Length.CompareTo(right.Length))
        Console.WriteLine("Sort by length: " & String.Join(" | ", copy))

        Array.Reverse(copy)
        Console.WriteLine("Reverse: " & String.Join(" | ", copy))

        Console.WriteLine("IndexOf Clean Code = " & Array.IndexOf(titles, "Clean Code"))
        Console.WriteLine("IndexOf missing    = " & Array.IndexOf(titles, "Missing"))

        Dim sorted As Integer() = {3, 8, 15, 21, 40}
        Console.WriteLine("BinarySearch 15 = " & Array.BinarySearch(sorted, 15))
        Console.WriteLine("BinarySearch 16 = " & Array.BinarySearch(sorted, 16) & " (complement of insertion point " & (Not Array.BinarySearch(sorted, 16)) & ")")

        Console.WriteLine("Exists > 30: " & Array.Exists(sorted, Function(n) n > 30))
        Console.WriteLine("Find > 10: " & Array.Find(sorted, Function(n) n > 10))
        Console.WriteLine("FindIndex > 10: " & Array.FindIndex(sorted, Function(n) n > 10))
        Console.WriteLine("FindAll even: " & String.Join(",", Array.FindAll(sorted, Function(n) n Mod 2 = 0)))
        Console.WriteLine("TrueForAll > 0: " & Array.TrueForAll(sorted, Function(n) n > 0))

        Dim lengths = Array.ConvertAll(titles, Function(t) t.Length)
        Console.WriteLine("ConvertAll: " & String.Join(",", lengths))

        Dim buffer(5) As Integer
        Array.Fill(buffer, 7)
        Array.Clear(buffer, 1, 2)
        Console.WriteLine("Fill + Clear: " & String.Join(",", buffer))

        Dim target(7) As Integer
        Array.Copy(sorted, 1, target, 3, 3)
        Console.WriteLine("Copy: " & String.Join(",", target))
        sorted.CopyTo(target, 0)
        Console.WriteLine("CopyTo: " & String.Join(",", target))

        ' Two parallel arrays sorted together
        Dim keys As Integer() = {3, 1, 2}
        Dim items As String() = {"three", "one", "two"}
        Array.Sort(keys, items)
        Console.WriteLine("Sort(keys, items): " & String.Join(",", items))

        Array.ForEach(sorted, Sub(n) Console.Write(n & ";"))
        Console.WriteLine()

        ' Comparing arrays: = is not defined, Is compares references
        Dim x = {1, 2, 3}
        Dim y = {1, 2, 3}
        Console.WriteLine("Is: " & (x Is y) & ", Equals: " & x.Equals(y) & ", SequenceEqual: " & x.SequenceEqual(y))
        Console.WriteLine("Array.MaxLength = " & Array.MaxLength)
    End Sub
End Module

Sort: Clean Code | Domain-Driven Design | Refactoring | Working Effectively with Legacy Code
Sort by length: Clean Code | Refactoring | Domain-Driven Design | Working Effectively with Legacy Code
Reverse: Working Effectively with Legacy Code | Domain-Driven Design | Refactoring | Clean Code
IndexOf Clean Code = 1
IndexOf missing    = -1
BinarySearch 15 = 2
BinarySearch 16 = -4 (complement of insertion point 3)
Exists > 30: True
Find > 10: 15
FindIndex > 10: 2
FindAll even: 8,40
TrueForAll > 0: True
ConvertAll: 11,10,20,36
Fill + Clear: 7,0,0,7,7,7
Copy: 0,0,0,8,15,21,0,0
CopyTo: 3,8,15,21,40,21,0,0
Sort(keys, items): one,two,three
3;8;15;21;40;
Is: False, Equals: False, SequenceEqual: True
Array.MaxLength = 2147483591

Option Strict On

Module Program
    Sub Main()
        Dim words As String() = {"alpha", "Beta", "gamma", "Delta"}
        Array.Sort(words)
        Console.WriteLine("default order:     " & String.Join(",", words))
        Array.Sort(words, StringComparer.Ordinal)
        Console.WriteLine("ordinal:           " & String.Join(",", words))
        Array.Sort(words, StringComparer.OrdinalIgnoreCase)
        Console.WriteLine("ignoring case:     " & String.Join(",", words))

        Dim withNothing As String() = {"b", Nothing, "a"}
        Array.Sort(withNothing)
        Console.WriteLine("Nothing sorts first: " & (withNothing(0) Is Nothing))

        Dim mixed As Object() = {1, "two"}
        Try
            Array.Sort(mixed)
        Catch ex As InvalidOperationException
            Console.WriteLine(ex.GetType().Name & ": " & ex.InnerException.GetType().Name)
        End Try

        ' Split and Join round-trip an array and a string
        Dim fields = "9780132350884;Refactoring;Fowler".Split(";"c)
        Console.WriteLine(fields.Length & " fields, last = " & fields(UBound(fields)))
    End Sub
End Module

default order:     alpha,Beta,Delta,gamma
ordinal:           Beta,Delta,alpha,gamma
ignoring case:     alpha,Beta,Delta,gamma
Nothing sorts first: True
InvalidOperationException: ArgumentException
3 fields, last = Fowler

Option Strict On

Module Program
    Sub Main()
        Dim names As String() = {"b", "a"}
        Array.Sort(names, Function(x, y) String.CompareOrdinal(x, y))
        Dim x = 1
    End Sub
End Module

Program.vb(6,36): error BC36641: Lambda parameter 'x' hides a variable in an enclosing block, a previously defined range variable, or an implicitly declared variable in a query expression.

Option Strict On

Public Class CatalogItem
    Public Property Title As String = ""
End Class

Public Class Book
    Inherits CatalogItem
End Class

Public Class Magazine
    Inherits CatalogItem
End Class

Module Program
    Sub Main()
        Dim books As Book() = {New Book() With {.Title = "Refactoring"}}

        ' Arrays of a reference type are covariant: Book() converts to CatalogItem()
        Dim items As CatalogItem() = books
        Console.WriteLine("same array: " & Object.ReferenceEquals(books, items))
        Console.WriteLine("runtime type: " & items.GetType().Name)

        Try
            items(0) = New Magazine() With {.Title = "Byte"}   ' compiles, fails at run time
        Catch ex As ArrayTypeMismatchException
            Console.WriteLine(ex.GetType().Name & ": " & ex.Message)
        End Try

        ' Arrays of value types are not covariant, but an array of String is an array of Object
        Dim objects As Object() = New String() {"a", "b"}
        Console.WriteLine("Object() from String(): " & objects.Length)
        Dim numbers As Integer() = {1, 2}
        Console.WriteLine("Integer() is Object(): " & (TypeOf CObj(numbers) Is Object()))

        ' An array also implements the generic collection interfaces
        Dim list As IList(Of Integer) = numbers
        Dim readOnlyView As IReadOnlyList(Of Integer) = numbers
        Console.WriteLine("IList.Count = " & list.Count & ", IsReadOnly = " & list.IsReadOnly)
        Console.WriteLine("IReadOnlyList(0) = " & readOnlyView(0))
        Try
            list.Add(3)
        Catch ex As NotSupportedException
            Console.WriteLine(ex.GetType().Name & ": " & ex.Message)
        End Try

        ' The safest read-only wrapper
        Dim wrapper = Array.AsReadOnly(numbers)
        numbers(0) = 100
        Console.WriteLine("wrapper(0) sees the change: " & wrapper(0))
    End Sub
End Module

same array: True
runtime type: Book[]
ArrayTypeMismatchException: Attempted to access an element as a type incompatible with the array.
Object() from String(): 2
Integer() is Object(): False
IList.Count = 2, IsReadOnly = True
IReadOnlyList(0) = 1
NotSupportedException: Collection was of a fixed size.
wrapper(0) sees the change: 100

Option Strict On

Module Program
    Sub Main()
        Dim numbers As Integer() = {1, 2, 3}
        Dim view As Span(Of Integer) = numbers.AsSpan()
        Console.WriteLine(view.Length)
    End Sub
End Module

Program.vb(6,21): error BC30668: 'Span(Of Integer)' is obsolete: 'Types with embedded references are not supported in this version of your compiler.'.

Option Strict On

Imports System.Buffers
Imports System.Collections.Immutable

Module Program
    Sub Main()
        Dim numbers As Integer() = {10, 20, 30, 40, 50}

        ' A copy of part of an array
        Dim part(2) As Integer
        Array.Copy(numbers, 1, part, 0, 3)
        part(0) = -1
        Console.WriteLine("copy:    " & String.Join(",", part) & " original: " & String.Join(",", numbers))

        ' A view over part of an array: ArraySegment shares the storage
        Dim segment As New ArraySegment(Of Integer)(numbers, 1, 3)
        segment(0) = 21                      ' writes through to numbers(1)
        Console.WriteLine("segment: " & String.Join(",", segment) & " original: " & String.Join(",", numbers))

        ' Memory(Of T) is a view that VB can store in a variable
        Dim memory As Memory(Of Integer) = numbers.AsMemory(2, 2)
        numbers(2) = 31
        Console.WriteLine("memory:  " & String.Join(",", memory.ToArray()) & " sees the change to numbers(2)")

        ' A span can be used inside one expression, never stored in a variable
        Console.WriteLine("span slice: " & String.Join(",", numbers.AsSpan().Slice(1, 2).ToArray()) & ", parsed from a span: " & Integer.Parse("123".AsSpan()))

        ' LINQ makes copies
        Dim page = numbers.Skip(1).Take(2).ToArray()
        Console.WriteLine("LINQ:    " & String.Join(",", page))
        For Each chunk In numbers.Chunk(2)
            Console.Write("[" & String.Join(",", chunk) & "] ")
        Next
        Console.WriteLine()

        ' Rent a scratch buffer instead of allocating one per call
        Dim rented = ArrayPool(Of Integer).Shared.Rent(16)
        Console.WriteLine("rented length >= 16: " & (rented.Length >= 16))
        ArrayPool(Of Integer).Shared.Return(rented)

        ' ImmutableArray is a value-type wrapper whose Add returns a new array
        Dim frozen = ImmutableArray.Create(1, 2, 3)
        Dim bigger = frozen.Add(4)
        Console.WriteLine("immutable: " & frozen.Length & " and " & bigger.Length)
        Dim unset As ImmutableArray(Of Integer) = Nothing
        Console.WriteLine("default ImmutableArray: IsDefault = " & unset.IsDefault)
        Try
            Console.WriteLine(unset.Length)
        Catch ex As NullReferenceException
            Console.WriteLine(ex.GetType().Name)
        End Try
    End Sub
End Module

copy:    -1,30,40 original: 10,20,30,40,50
segment: 21,30,40 original: 10,21,30,40,50
memory:  31,40 sees the change to numbers(2)
span slice: 21,31, parsed from a span: 123
LINQ:    21,31
[10,21] [31,40] [50]
rented length >= 16: True
immutable: 3 and 4
default ImmutableArray: IsDefault = True
NullReferenceException

Option Strict On
Option Base 1

Module Program
    Sub Main()
    End Sub
End Module

Program.vb(2,8): error BC30206: 'Option' must be followed by 'Compare', 'Explicit', 'Infer', or 'Strict'.

Option Strict On

Module Program
    Sub Main()
        ' Dim a(0 To 4) is allowed: the lower bound is always zero
        Dim fine(0 To 4) As Integer
        Console.WriteLine("fine: " & fine.Length)

        ' System.Array can create arrays whose lower bound is not zero
        Dim lengths As Integer() = {3}
        Dim lowers As Integer() = {1}
        Dim oneBased As Array = Array.CreateInstance(GetType(String), lengths, lowers)
        oneBased.SetValue("first", 1)
        oneBased.SetValue("third", 3)
        Console.WriteLine("Type = " & oneBased.GetType().Name & ", Rank = " & oneBased.Rank)
        Console.WriteLine("LowerBound = " & oneBased.GetLowerBound(0) & ", UpperBound = " & oneBased.GetUpperBound(0))
        Console.WriteLine("Value at 3 = " & CStr(oneBased.GetValue(3)))
        Try
            Dim asStrings = CType(oneBased, String())
            Console.WriteLine(asStrings.Length)
        Catch ex As InvalidCastException
            Console.WriteLine(ex.GetType().Name)
        End Try
        Try
            oneBased.SetValue("zero", 0)
        Catch ex As IndexOutOfRangeException
            Console.WriteLine(ex.GetType().Name & ": " & ex.Message)
        End Try
    End Sub
End Module

fine: 5
Type = String[*], Rank = 1
LowerBound = 1, UpperBound = 3
Value at 3 = third
InvalidCastException
IndexOutOfRangeException: Index was outside the bounds of the array.

Option Strict On

Module Program
    Sub Report(label As String, action As Action)
        Try
            action()
            Console.WriteLine(label & ": ok")
        Catch ex As Exception
            Console.WriteLine(label & ": " & ex.GetType().Name & " - " & ex.Message)
        End Try
    End Sub

    Sub Main()
        Dim missing As Integer() = Nothing
        Dim titles(1) As String
        Dim grid(1, 1) As Integer
        Dim size = -2

        Report("Length of Nothing", Sub() Console.WriteLine(missing.Length))
        Report("UBound of Nothing", Sub() Console.WriteLine(UBound(missing)))
        Report("element Is Nothing, then .Length", Sub() Console.WriteLine(titles(0).Length))
        Report("negative size", Sub()
                                    Dim a(size) As Integer
                                    Console.WriteLine(a.Length)
                                End Sub)
        Report("Integer.MaxValue as upper bound", Sub()
                                                      Dim a(Integer.MaxValue) As Byte
                                                      Console.WriteLine(a.Length)
                                                  End Sub)
        Report("Sort a 2-D array", Sub() Array.Sort(grid))
        Report("GetLength(2) on a 2-D array", Sub() Console.WriteLine(grid.GetLength(2)))
        Report("UBound dimension 3", Sub() Console.WriteLine(UBound(grid, 3)))
        Report("copy into a too-small array", Sub() Array.Copy({1, 2, 3}, New Integer(1) {}, 3))

        Dim empty(-1) As Integer
        Console.WriteLine("empty: Length = " & empty.Length & ", UBound = " & UBound(empty) & ", LBound = " & LBound(empty))
        Console.WriteLine("an array is an Array: " & (TypeOf titles Is Array) & ", base type " & titles.GetType().BaseType.Name)

        ' Value equality of arrays needs a structural comparer
        Dim a1 = {1, 2, 3}
        Dim a2 = {1, 2, 3}
        Dim comparer = System.Collections.StructuralComparisons.StructuralEqualityComparer
        Console.WriteLine("structural equality: " & comparer.Equals(a1, a2) & ", SequenceEqual: " & a1.SequenceEqual(a2))
    End Sub
End Module

Length of Nothing: NullReferenceException - Object reference not set to an instance of an object.
UBound of Nothing: ArgumentNullException - Value cannot be null. (Parameter 'Array')
element Is Nothing, then .Length: NullReferenceException - Object reference not set to an instance of an object.
negative size: OverflowException - Arithmetic operation resulted in an overflow.
Integer.MaxValue as upper bound: OverflowException - Arithmetic operation resulted in an overflow.
Sort a 2-D array: RankException - Only single dimension arrays are supported here.
GetLength(2) on a 2-D array: IndexOutOfRangeException - Array does not have that many dimensions.
UBound dimension 3: RankException - Argument 'Rank' is not valid for the array.
copy into a too-small array: ArgumentException - Destination array was not long enough. Check the destination index, length, and the array's lower bounds. (Parameter 'destinationArray')
empty: Length = 0, UBound = -1, LBound = 0
an array is an Array: True, base type Array
structural equality: True, SequenceEqual: True

Option Strict On

Public Class Book
    Public ReadOnly Property Title As String
    Public ReadOnly Property Pages As Integer

    Public Sub New(title As String, pages As Integer)
        Me.Title = title
        Me.Pages = pages
    End Sub

    Public Overrides Function ToString() As String
        Return Title & " (" & Pages & " pages)"
    End Function
End Class

Module Program
    Const LevelCount As Integer = 3
    Const SlotsPerLevel As Integer = 4

    ' Returns a new array; the caller owns it
    Function ByLength(books As Book()) As Book()
        Dim sorted = CType(books.Clone(), Book())
        Array.Sort(sorted, Function(left, right) left.Pages.CompareTo(right.Pages))
        Return sorted
    End Function

    Sub Main()
        Dim books As Book() = {
            New Book("Refactoring", 448),
            New Book("Clean Code", 464),
            New Book("The Pragmatic Programmer", 352),
            New Book("Domain-Driven Design", 560),
            New Book("Working Effectively with Legacy Code", 456)
        }

        ' Number of elements is Length; the last valid index is Length - 1
        Console.WriteLine(books.Length & " books, last index " & (books.Length - 1))

        ' A rectangular array: levels by slots (the declaration gives UPPER bounds)
        Dim slots(LevelCount - 1, SlotsPerLevel - 1) As Book
        Dim nextBook = 0
        For level = 0 To slots.GetUpperBound(0)
            For slot = 0 To slots.GetUpperBound(1)
                If nextBook < books.Length Then
                    slots(level, slot) = books(nextBook)
                    nextBook += 1
                End If
            Next
        Next

        For level = 0 To slots.GetLength(0) - 1
            Dim used = 0
            For slot = 0 To slots.GetLength(1) - 1
                If slots(level, slot) IsNot Nothing Then used += 1
            Next
            Console.WriteLine("level " & level & ": " & used & " of " & slots.GetLength(1) & " slots used")
        Next

        ' A jagged array: books grouped by size class, each group with its own length
        Dim groups(2)() As Book
        groups(0) = Array.FindAll(books, Function(b) b.Pages < 400)
        groups(1) = Array.FindAll(books, Function(b) b.Pages >= 400 AndAlso b.Pages < 500)
        groups(2) = Array.FindAll(books, Function(b) b.Pages >= 500)
        For i = 0 To groups.Length - 1
            Console.WriteLine("group " & i & ": " & groups(i).Length & " book(s)")
        Next

        For Each book In ByLength(books)
            Console.WriteLine(book)
        Next
        Console.WriteLine("original first: " & books(0).Title)

        ' The total uses a variable declared before the loop; a Sum call would do the same
        Dim totalPages = 0
        For Each book In books
            totalPages += book.Pages
        Next
        Console.WriteLine("average pages = " & (totalPages / books.Length).ToString("0.0"))
    End Sub
End Module

5 books, last index 4
level 0: 4 of 4 slots used
level 1: 1 of 4 slots used
level 2: 0 of 4 slots used
group 0: 1 book(s)
group 1: 3 book(s)
group 2: 1 book(s)
The Pragmatic Programmer (352 pages)
Refactoring (448 pages)
Working Effectively with Legacy Code (456 pages)
Clean Code (464 pages)
Domain-Driven Design (560 pages)
original first: Refactoring
average pages = 456.0