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

Článek
07/22/2011

The For Each statement construction is similar to the For...Next loop, but it runs the statement block for each element in a collection, instead of a specified number of times. A For Each...Next loop uses an element variable that represents a different element of the collection during each repetition of the loop.

Repeating a Statement Block for a Collection

To run a group of statements for each element in a collection

Identify the collection on which to run the statement block, and use the For Each...Next Statement (Visual Basic) to specify both the element variable and the collection.
```
For Each thisControl As System.Windows.Forms.Control In thisForm.Controls 
```
If the element variable is not declared outside the loop, you can use the As clause to declare it as part of the For Each statement.

Visual Basic automatically substitutes a different element of the collection for each iteration.
Complete the For Each...Next construction with a Next statement following the last statement to be repeated. You can specify the element variable in the Next statement.
```
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
```
You can read the elements of the collection, but you cannot make any changes to the collection itself, such as adding, deleting, or replacing any element. However, if an element is a reference type, you can access and set its members. Because each Control element is a reference type, the code in the preceding example can change its BackColor property.

Repeating a Statement Block for an Array

You can also iterate through an array with a For Each...Next loop. However, as with collections, you can only read the array elements, not change them.

To run a group of statements for each element of an array

Use the For Each statement to specify both the element variable and the array. Do not follow the array name with parentheses.

Visual Basic treats the array the same way it treats a collection.
Follow the statement block with a Next statement. You can specify the element variable in the Next statement.

The following procedure is intended to find the sum of the elements of an array and reset each element to zero.
```
Function sumAndReset(ByRef numbers() As Integer) As Integer
    Dim sum As Integer = 0
    For Each elt As Integer In numbers 
        sum += elt
        ' The following statement works only on the local copy
        ' of the array, not on the original array.
        elt = 0
    Next elt
    Return sum
End Function
```
The summation works as intended because it relies only on reading the elements. However, the resetting to zero does not work as intended, because only the local copy of each element is reset to zero, while the original element is left unchanged in the original array.

Sdílet prostřednictvím

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

Repeating a Statement Block for a Collection

To run a group of statements for each element in a collection

Repeating a Statement Block for an Array

To run a group of statements for each element of an array

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