While...End While Statement (Visual Basic)

Artikkeli
09/15/2021

Runs a series of statements as long as a given condition is True.

Syntax

While condition
    [ statements ]
    [ Continue While ]
    [ statements ]
    [ Exit While ]
    [ statements ]
End While

Parts

Term	Definition
`condition`	Required. `Boolean` expression. If `condition` is `Nothing`, Visual Basic treats it as `False`.
`statements`	Optional. One or more statements following `While`, which run every time `condition` is `True`.
`Continue While`	Optional. Transfers control to the next iteration of the `While` block.
`Exit While`	Optional. Transfers control out of the `While` block.
`End While`	Required. Terminates the definition of the `While` block.

Remarks

Use a While...End While structure when you want to repeat a set of statements an indefinite number of times, as long as a condition remains True. If you want more flexibility with where you test the condition or what result you test it for, you might prefer the Do...Loop Statement. If you want to repeat the statements a set number of times, the For...Next Statement is usually a better choice.

Note

The While keyword is also used in the Do...Loop Statement, the Skip While Clause and the Take While Clause.

If condition is True, all of the statements run until the End While statement is encountered. Control then returns to the While statement, and condition is again checked. If condition is still True, the process is repeated. If it’s False, control passes to the statement that follows the End While statement.

The While statement always checks the condition before it starts the loop. Looping continues while the condition remains True. If condition is False when you first enter the loop, it doesn’t run even once.

The condition usually results from a comparison of two values, but it can be any expression that evaluates to a Boolean Data Type value (True or False). This expression can include a value of another data type, such as a numeric type, that has been converted to Boolean.

You can nest While 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.

Exit While

The Exit While statement can provide another way to exit a While loop. Exit While immediately transfers control to the statement that follows the End While statement.

You typically use Exit While after some condition is evaluated (for example, in an If...Then...Else structure). 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 use Exit While when you 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. You can then use Exit While to escape the loop.

You can place any number of Exit While statements anywhere in the While loop.

When used within nested While loops, Exit While transfers control out of the innermost loop and into the next higher level of nesting.

The Continue While statement immediately transfers control to the next iteration of the loop. For more information, see Continue Statement.

Example 1

In the following example, the statements in the loop continue to run until the index variable is greater than 10.

Dim index As Integer = 0
While index <= 10
    Debug.Write(index.ToString & " ")
    index += 1
End While

Debug.WriteLine("")
' Output: 0 1 2 3 4 5 6 7 8 9 10

Example 2

The following example illustrates the use of the Continue While and Exit While statements.

Dim index As Integer = 0
While index < 100000
    index += 1

    ' If index is between 5 and 7, continue
    ' with the next iteration.
    If index >= 5 And index <= 8 Then
        Continue While
    End If

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

    ' If index is 10, exit the loop.
    If index = 10 Then
        Exit While
    End If
End While

Debug.WriteLine("")
' Output: 1 2 3 4 9 10

Example 3

The following example reads all lines in a text file. The OpenText method opens the file and returns a StreamReader that reads the characters. In the While condition, the Peek method of the StreamReader determines whether the file contains additional characters.

Private Sub ShowText(ByVal textFilePath As String)
    If System.IO.File.Exists(textFilePath) = False Then
        Debug.WriteLine("File Not Found: " & textFilePath)
    Else
        Dim sr As System.IO.StreamReader = System.IO.File.OpenText(textFilePath)

        While sr.Peek() >= 0
            Debug.WriteLine(sr.ReadLine())
        End While

        sr.Close()
    End If
End Sub

Jaa