Statements

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In VB.NET, statements represent executable code. The next section outlines the statements found and VB.NET and their usage.

Blocks

A block statement allows multiple statements to be written in contexts where a single statement is allowed. A block statement consists of a statement list enclosed by braces:

 Sub Main End Sub 

Statement Lists

A statement list is simply one or more statements written in sequence and enclosed in blocks as outlined following:

  Sub Main     Refresh()     Repaint() End Sub 

Labeled

A labeled statement allows a statement to be referenced by a Goto statement. The scope of the label is the block that encloses it, including any nested blocks:

  Sub Main     If (X>0) Then Goto MyLabel     X = X + 1     MyLabel:         return X End Sub 

Declarations

A declaration statement declares a local variable or constant:

  Dim X as Integer = 3 Dim Const pi As Decimal = 3.14 

Expressions

As expected, an expression statement evaluates a given statement:

  Return (x * y) Repaint(Me) 

Selection

Selection statements allow for the execution of one of a list of possible statements based upon the value of a controlling expression. Selection statements include the If...Then...Else and Select...Case statements.

If

The If statement selects a statement for execution based on a Boolean expression:

  If (x > 0) Then    x = x -1 End If 

This statement may be coupled with an else clause in order to specify a statement for execution if the condition fails:

  If (x > 0) Then     x = x - 1 Else     return x End If 

If statements may also be nested:

  If (x <> 3) Then     If (x > 0) Then        x = x - 1     Else         Return x     End If End If 
Select Case

To provide multiple statements for possible execution based upon the value of a condition, we use the Select Case statement:

  Select Case (x)     Case 1         MessageBox.Show("X = 1")     Case 2         MessageBox.Show("X = 2")     Case 3         MessageBox.Show("X = 3")     Case Else         MessageBox.Show("X does not equal 1,2, or 3") End Select 

The Select...Case statement evaluates the expression and looks for a match with a constant value from one of the labeled statements. When a match is found, execution control is transferred to the statement list for the matching label. If no match is found, control is transferred to the Case Else block and its statement list.

Loops

Loop statements allow for the execution of a statement list as long as some expression evaluates True . VB.NET includes four such iteration statements: While...End While , Do...Loop , For...Next , and For Each...Next .

While...End While

The While...End While statement executes a statement list zero or more times, depending on the value of the conditional expression:

  While (x < 100)     MessageBox.Show(x)     X = X + 1 End While 
Do...Loop

Similar to the While statement, the Do...Loop statement executes one or more times:

  Do     MessageBox.Show(x)     X = X + 1 Loop While(X < 100) 

Note that the Do...Loop statement can use the While keyword as used above, which breaks the loop if the expression evaluates to False or the Until keyword, which breaks the loop if the expression evaluates to True.

For...Next

The For...Next statement loops on a set of bounds. The statement specifies a lower bound, an upper bound, and an optional step for the iteration. Upon commencement of the loop, the three values are evaluated. If the step is omitted, the iteration defaults to a step of 1 :

  For x = 0 to 100    MessageBox.Show(x) Next For x = 100 to 0 Step 1    MessageBox.Show(x) Next 
For Each...Next

Very similar to the For...Next statement, the For Each...Next statement enumerates a collection and executes the statement list once for each item in the collection:

  For Each x in Args    MessageBox.Show(x) Next 

Control-Flow Statements

Another type of statement, the control-flow statement allows the unconditional transfer of control execution in a VB.NET program. These statements include Goto , Exit , Stop , Return , and End statements.

Goto

The Goto statement transfers control to the statement matching the specified label:

  Sub Main     If (X>0) Then Goto MyLabel     X = X + 1     MyLabel:         return X End Sub 
Exit

An Exit statement transfers control to the next statement after the end of the enclosing block statement. If the block is a method, execution is transferred back to the caller of the method.

  Do     x = x + 10     If x > 35 Then Exit Do Loop While (x > 0) 
Return

The Return statement returns control to the caller of the procedure in which the Return statement appears. If enclosed within a function, the Return statement may take a return value argument:

  Function GetStudentID(FullName As String) As Integer     Return "Jim Smith" End Function 
Stop

Using the Stop statement permits putting breakpoints in your code:

  Dim x As Integer For x = 1 To 5    Debug.WriteLine (x)     Stop 'Stop during each iteration. Next I 
End

Similar to the Stop statement, the End statement terminates program execution and can only appear in executables (.exes) and not libraries (. dlls ).

Try

The try statement provides exception handling during execution of a statement block. The try block may be followed by one or more catch blocks and may be followed by one finally block:

  Try     Repaint() Catch (e As Exception)     'handle gracefully     If(x < 0) Then ' out of range         Throw New Exception("Index out of Range")     End If Finally     ' any clean-up code here End Try 
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Asp. Net. By Example
ASP.NET by Example
ISBN: 0789725622
EAN: 2147483647
Year: 2001
Pages: 154

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