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Do...Loop Statement (Visual Basic) 

Repeats a block of statements while a Boolean condition is True or until the condition becomes True.


Do { While | Until } condition
    [ statements ]
    [ Exit Do ]
    [ statements ]
Loop
-or-
Do
    [ statements ]
    [ Exit Do ]
    [ statements ]
Loop { While | Until } condition

While

Required unless Until is used. Repeat the loop until condition is False.

Until

Required unless While is used. Repeat the loop until condition is True.

condition

Optional. Boolean expression. If condition is Nothing, Visual Basic treats it as False.

statements

Optional. One or more statements that are repeated while, or until, condition is True.

Exit Do

Optional. Transfers control out of the Do loop.

Loop

Required. Terminates the definition of the Do loop.

Use a Do...Loop structure when you want to repeat a set of statements an indefinite number of times, until a condition is satisfied. If you want to repeat the statements a set number of times, the For...Next Statement is usually a better choice.

The Do...Loop structure gives you more flexibility than the While...End While Statement (Visual Basic) because it allows you to choose whether to end the loop when condition stops being True or when it first becomes True. It also allows you to test condition at either the beginning or the end of the loop.

Rules

  • Nature of Condition. The condition usually results from a comparison of two values, but it can be any expression that evaluates to a Boolean Data Type (Visual Basic) value (True or False). This includes values of other data types, such as numeric types, that have been converted to Boolean.

  • Testing the Condition. You can test condition only once, at either the beginning or the end of the loop. You can use either While or Until to specify condition, but not both.

  • Number of Iterations. If you test condition at the beginning of the loop (in the Do statement), the loop might never run even once. If you test at the end of the loop (in the Loop statement), the loop always runs at least once.

  • Nesting Loops. You can nest Do loops by placing one loop within another. You can also nest different kinds of control structures within one another. For more information, see Nested Control Structures.

  • Transferring Out of the Loop. The Exit Statement (Visual Basic) transfers control immediately to the statement following the Loop statement. You might want to exit a loop if you detect a condition that makes it unnecessary or impossible to continue iterating, such as an erroneous value or a termination request. You can place any number of Exit Do statements anywhere in the Do loop. Exit Do is often used after evaluating some condition, for example in an If...Then...Else structure.

Endless Loops

One use of Exit Do is to test for a condition that could cause an endless loop, which is a loop that could run an extremely large or even infinite number of times. If you detect such a condition, you can use Exit Do to escape the loop. Otherwise, the loop continues running.

In the following example, number is assigned a value that could cause the loop to run more than 2 ^ 31 times. The If statement checks for this condition and exits if it exists, preventing endless looping.

Sub exitDoExample()
    Dim counter As Integer = 0
    Dim number As Integer = 8
    Do Until number = 10
        If number <= 0 Then Exit Do
        number -= 1
        counter += 1
    Loop
    MsgBox("The loop ran " & counter & " times.")
End Sub

NoteNote

To stop an endless loop, press ESC or CTRL+BREAK.

The following example illustrates nested Do...Loop structures, as well as the use of While and Until, and testing at the beginning (Do statement) and end (Loop statement) of the loop.

    Dim check As Boolean = True
    Dim counter As Integer = 0
    Do
        Do While counter < 20
            counter += 1
            If counter = 10 Then
                check = False
                Exit Do
            End If
        Loop
    Loop Until check = False
End Sub

In the preceding example, the inner Do...Loop structure loops 10 times, sets the value of the flag to False, and exits prematurely using the Exit Do statement. The outer loop exits immediately upon checking the value of the flag.

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