For Each...Next Statement (Visual Basic)

Repeats a group of statements for each element in a collection.

For Each element [ As datatype ] In group
    [ statements ]
    [ Continue For ]
    [ statements ]
    [ Exit For ]
    [ statements ]
Next [ element ]

Parts

Remarks

Use a For Each...Next loop when you want to repeat a set of statements for each element of a collection or array.

Visual Basic evaluates the collection only one time, before the loop starts. If your statement block changes element or group, these changes do not affect the iteration of the loop.

When all the elements in the collection have been successively assigned to element, the For Each loop stops and control passes to the statement following the Next statement.

If element has not been declared outside this loop, you must declare it in the For Each statement. You can declare the type of element explicitly by using an As statement, or you can rely on type inference to assign the type. In either case, the scope of element is the body of the loop. However, you cannot declare element both outside and inside the loop.

You can optionally specify element in the Next statement. This improves the readability of your program, especially if you have nested For Each loops. You must specify the same variable as the one that appears in the corresponding For Each statement.

You might want to avoid changing the value of element inside a loop. Doing this can make it more difficult to read and debug your code. Changing the value of group does not affect the collection or its elements, which were determined when the loop was first entered.

If your code depends on traversing a collection in a particular order, a For Each...Next loop is not the best choice, unless you know the characteristics of the enumerator object the collection exposes. The order of traversal is not determined by Visual Basic, but by the MoveNext method of the enumerator object. Therefore, you might not be able to predict which element of the collection is the first to be returned in element, or which is the next to be returned after a given element. You might achieve more reliable results using a different loop structure, such as For...Next or Do...Loop.

Nesting Loops

You can nest For Each loops by putting one loop within another. However, each loop must have a unique element variable.

You can also nest different kinds of control structures within each other. For more information, see Nested Control Structures (Visual Basic).

If a Next statement of an outer nesting level is encountered before the Next of an inner level, the compiler signals an error. However, the compiler can detect this overlapping error only if you specify element in every Next statement.

Exit For

The Exit For statement causes execution to exit the For…Next loop and transfers control to the statement following the Next statement.

You can put any number of Exit For statements in a For Each loop. When used within nested For Each loops, Exit For causes execution to exit the innermost loop and transfers control to the next higher level of nesting.

Exit For is often used after an evaluation of some condition, for example, in an If...Then...Else structure. You might want to use Exit For for the following conditions:

• Continuing to iterate is unnecessary or impossible. This might be caused by an erroneous value or a termination request.

• An exception is caught in a Try...Catch...Finally. You might use Exit For at the end of the Finally block.

• There an endless loop, which is a loop that could run a large or even infinite number of times. If you detect such a condition, you can use Exit For to escape the loop. For more information, see Do...Loop Statement (Visual Basic).

The Continue For statement transfers control immediately to the next iteration of the loop. For more information, see Continue Statement (Visual Basic).

Data Types

The data type of element must be such that the data type of the elements of group can be converted to it.

The data type of group must be a reference type that refers to a collection or an array that is enumerable. Most commonly this means that group refers to an object that implements the IEnumerable interface of the System.Collections namespace or the IEnumerable<T> interface of the System.Collections.Generic namespace. System.Collections.IEnumerable defines the GetEnumerator method, which returns an enumerator object for the collection. The enumerator object implements the System.Collections.IEnumerator interface of the System.Collections namespace and exposes the Current property and the Reset and MoveNext methods. Visual Basic uses these to traverse the collection.

The elements of group are usually of type Object but can have any run-time data type.

Narrowing Conversions

When Option Strict is set to On, narrowing conversions ordinarily cause compiler errors. In a For Each statement, however, conversions from the elements in group to element are evaluated and performed at run time, and compiler errors caused by narrowing conversions are suppressed.

In the following example, the assignment of m as the initial value for n does not compile when Option Strict is on because the conversion of a Long to an Integer is a narrowing conversion. In the For Each statement, however, no compiler error is reported, even though the assignment to number requires the same conversion from Long to Integer. In the For Each statement that contains a large number, a run-time error occurs when ToInteger is applied to the large number.

Option Strict On

Module Module1
    Sub Main()
        ' The assignment of m to n causes a compiler error when 
        ' Option Strict is on.
        Dim m As Long = 987
        'Dim n As Integer = m

        ' The For Each loop requires the same conversion but
        ' causes no errors, even when Option Strict is on.
        For Each number As Integer In New Long() {45, 3, 987}
            Console.Write(number & " ")
        Next
        Console.WriteLine()
        ' Output: 45 3 987

        ' Here a run-time error is raised because 9876543210
        ' is too large for type Integer.
        'For Each number As Integer In New Long() {45, 3, 9876543210}
        '    Console.Write(number & " ")
        'Next

        Console.ReadKey()
    End Sub
End Module

IEnumerator Calls

When execution of a For Each...Next loop starts, Visual Basic verifies that group refers to a valid collection object. If not, it throws an exception. Otherwise, it calls the MoveNext method and the Current property of the enumerator object to return the first element. If MoveNext indicates that there is no next element, that is, if the collection is empty, the For Each loop stops and control passes to the statement following the Next statement. Otherwise, Visual Basic sets element to the first element and runs the statement block.

Each time Visual Basic encounters the Next statement, it returns to the For Each statement. Again it calls MoveNext and Current to return the next element, and again it either runs the block or stops the loop depending on the result. This process continues until MoveNext indicates that there is no next element or an Exit For statement is encountered.

Modifications

Modifying the Collection. The enumerator object returned by GetEnumerator normally does not let you change the collection by adding, deleting, replacing, or reordering any elements. If you change the collection after you have initiated a For Each...Next loop, the enumerator object becomes invalid, and the next attempt to access an element causes an InvalidOperationException exception.

However, this blocking of modification is not determined by Visual Basic, but rather by the implementation of the IEnumerable interface. It is possible to implement IEnumerable in a way that allows for modification during iteration. If you are considering doing such dynamic modification, make sure that you understand the characteristics of the IEnumerable implementation on the collection you are using.

Modifying Collection Elements. The Current property of the enumerator object is ReadOnly (Visual Basic), and it returns a local copy of each collection element. This means that you cannot modify the elements themselves in a For Each...Next loop. Any modification you make affects only the local copy from Current and is not reflected back into the underlying collection. However, if an element is a reference type, you can modify the members of the instance to which it points. The following example illustrates this.

Sub lightBlueBackground(ByVal thisForm As System.Windows.Forms.Form)
    For Each thisControl As System.Windows.Forms.Control In thisForm.Controls
        thisControl.BackColor = System.Drawing.Color.LightBlue
    Next thisControl
End Sub

The previous example can modify the BackColor member of each thisControl element, although it cannot modify thisControl itself.

Traversing Arrays. Because the Array class implements the IEnumerable interface, all arrays expose the GetEnumerator method. This means that you can iterate through an array with a For Each...Next loop. However, you can only read the array elements. You cannot change them. For an illustration, see How to: Run Several Statements for Each Element in a Collection or Array (Visual Basic).

Example

The following example shows how to use the For Each…Next statement.

' Create a list of strings by using a
' collection initializer.
Dim lst As New List(Of String) _
    From {"abc", "def", "ghi"}

' Iterate through the list.
For Each item As String In lst
    Debug.Write(item & " ")
Next
Debug.WriteLine("")
'Output: abc def ghi

The following example demonstrates nested For Each…Next structures.

' Create lists of numbers and letters
' by using array initializers.
Dim numbers() As Integer = {1, 4, 7}
Dim letters() As String = {"a", "b", "c"}

' Iterate through the list by using nested loops.
For Each number As Integer In numbers
    For Each letter As String In letters
        Debug.Write(number.ToString & letter & " ")
    Next
Next
Debug.WriteLine("")
'Output: 1a 1b 1c 4a 4b 4c 7a 7b 7c 

The following example shows how to use the Continue For and Exit For statements.

Dim numberSeq() As Integer =
    {1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12}

For Each number As Integer In numberSeq
    ' If number is between 5 and 7, continue
    ' with the next iteration.
    If number >= 5 And number <= 8 Then
        Continue For
    End If

    ' Display the number.
    Debug.Write(number.ToString & " ")

    ' If number is 10, exit the loop.
    If number = 10 Then
        Exit For
    End If
Next
Debug.WriteLine("")
' Output: 1 2 3 4 9 10

The following example lists all the folders in the C:\ directory by using the DirectoryInfo class.

Dim dInfo As New System.IO.DirectoryInfo("c:\")
For Each dir As System.IO.DirectoryInfo In dInfo.GetDirectories()
    Debug.WriteLine(dir.Name)
Next

The following example shows how to sort a collection by using the IComparable interface.

Public Sub ListCars()

    ' Create some new cars.
    Dim cars As New List(Of Car) From
    {
        New Car With {.Name = "car1", .Color = "blue", .Speed = 20},
        New Car With {.Name = "car2", .Color = "red", .Speed = 50},
        New Car With {.Name = "car3", .Color = "green", .Speed = 10},
        New Car With {.Name = "car4", .Color = "blue", .Speed = 50},
        New Car With {.Name = "car5", .Color = "blue", .Speed = 30},
        New Car With {.Name = "car6", .Color = "red", .Speed = 60},
        New Car With {.Name = "car7", .Color = "green", .Speed = 50}
    }

    ' Sort the cars by color, alphabetically, and then by speed
    ' in descending order.
    cars.Sort()

    ' View all of the cars.
    For Each thisCar As Car In cars
        Debug.Write(thisCar.Color.PadRight(5) & " ")
        Debug.Write(thisCar.Speed.ToString & " ")
        Debug.Write(thisCar.Name)
        Debug.WriteLine("")
    Next

    ' Output:
    '  blue  50 car4
    '  blue  30 car5
    '  blue  20 car1
    '  green 50 car7
    '  green 10 car3
    '  red   60 car6
    '  red   50 car2
End Sub

Public Class Car
    Implements IComparable

    Public Property Name As String
    Public Property Speed As Integer
    Public Property Color As String

    Public Function CompareTo(ByVal obj As Object) As Integer Implements System.IComparable.CompareTo
        ' Determine the relative order of the objects being compared.
        ' This is used to sort by color alphabetically, and then by
        ' speed in descending order.

        ' Obtain the other object for the comparison.
        Dim other As Car = CType(obj, Car)

        ' Compare the colors.
        Dim compare As Integer
        compare = String.Compare(Me.Color, other.Color, True)

        ' If the colors are the same, compare the speeds.
        If compare = 0 Then
            compare = Me.Speed.CompareTo(other.Speed)

            ' Use descending order for speed.
            compare = -compare
        End If

        Return compare
    End Function
End Class

The following example includes a custom collection class that has a custom enumerator.

Public Sub ListColors()
    Dim colors As New AllColors()

    For Each theColor As Color In colors
        Debug.Write(theColor.Name & " ")
    Next
    Debug.WriteLine("")
    ' Output: red blue green
End Sub

' Collection class.
Public Class AllColors
    Implements System.Collections.IEnumerable

    Private _colors() As Color =
    {
        New Color With {.Name = "red"},
        New Color With {.Name = "blue"},
        New Color With {.Name = "green"}
    }

    Public Function GetEnumerator() As System.Collections.IEnumerator _
        Implements System.Collections.IEnumerable.GetEnumerator

        Return New ColorEnumerator(_colors)

        ' Instead of creating a using a custom enumerator, you could
        ' use the GetEnumerator of the array.
        'Return _colors.GetEnumerator
    End Function

    ' Custom enumerator.
    Private Class ColorEnumerator
        Implements System.Collections.IEnumerator

        Private _colors() As Color
        Private _position As Integer = -1

        Public Sub New(ByVal colors() As Color)
            _colors = colors
        End Sub

        Public ReadOnly Property Current() As Object Implements System.Collections.IEnumerator.Current
            Get
                Return _colors(_position)
            End Get
        End Property

        Public Function MoveNext() As Boolean Implements System.Collections.IEnumerator.MoveNext
            _position += 1
            Return (_position < _colors.Length)
        End Function

        Public Sub Reset() Implements System.Collections.IEnumerator.Reset
            _position = -1
        End Sub
    End Class
End Class

' Element class.
Public Class Color
    Public Property Name As String
End Class

See Also

Tasks

How to: Run Several Statements for Each Element in a Collection or Array (Visual Basic)

Reference

For...Next Statement (Visual Basic)

While...End While Statement (Visual Basic)

Do...Loop Statement (Visual Basic)

Concepts

Loop Structures (Visual Basic)

Collections in Visual Basic

Widening and Narrowing Conversions (Visual Basic)

Object Initializers: Named and Anonymous Types (Visual Basic)

Collection Initializers Overview (Visual Basic)

Other Resources

Arrays in Visual Basic

Change History