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Learn more about: Nested Control Structures (Visual Basic) |
Nested Control Structures |
07/20/2015 |
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cf60b061-65d9-44a8-81f2-b0bdccd23a05 |
You can place control statements inside other control statements, for example an If...Then...Else
block within a For...Next
loop. A control statement placed inside another control statement is said to be nested.
Control structures in Visual Basic can be nested to as many levels as you want. It is common practice to make nested structures more readable by indenting the body of each one. The integrated development environment (IDE) editor automatically does this.
In the following example, the procedure sumRows
adds together the positive elements of each row of the matrix.
Public Sub sumRows(ByVal a(,) As Double, ByRef r() As Double)
Dim i, j As Integer
For i = 0 To UBound(a, 1)
r(i) = 0
For j = 0 To UBound(a, 2)
If a(i, j) > 0 Then
r(i) = r(i) + a(i, j)
End If
Next j
Next i
End Sub
In the preceding example, the first Next
statement closes the inner For
loop and the last Next
statement closes the outer For
loop.
Likewise, in nested If
statements, the End If
statements automatically apply to the nearest prior If
statement. Nested Do
loops work in a similar fashion, with the innermost Loop
statement matching the innermost Do
statement.
Note
For many control structures, when you click a keyword, all of the keywords in the structure are highlighted. For instance, when you click If
in an If...Then...Else
construction, all instances of If
, Then
, ElseIf
, Else
, and End If
in the construction are highlighted. To move to the next or previous highlighted keyword, press CTRL+SHIFT+DOWN ARROW or CTRL+SHIFT+UP ARROW.
You can nest one kind of control structure within another kind. The following example uses a With
block inside a For Each
loop and nested If
blocks inside the With
block.
For Each ctl As System.Windows.Forms.Control In Me.Controls
With ctl
.BackColor = System.Drawing.Color.Yellow
.ForeColor = System.Drawing.Color.Black
If .CanFocus Then
.Text = "Colors changed"
If Not .Focus() Then
' Insert code to process failed focus.
End If
End If
End With
Next ctl
You cannot overlap control structures. This means that any nested structure must be completely contained within the next innermost structure. For example, the following arrangement is invalid because the For
loop terminates before the inner With
block terminates.
The Visual Basic compiler detects such overlapping control structures and signals a compile-time error.