.VB File Format – Statements and control flow

Conditions and selection

If n > 10 Then
    Console.WriteLine("big")
ElseIf n > 5 Then
    Console.WriteLine("medium")
Else
    Console.WriteLine("small")
End If

If n = 0 Then Console.WriteLine("zero") Else Console.WriteLine("not zero")

Select Case n
    Case 1, 2, 3
        Console.WriteLine("low")
    Case 4 To 9
        Console.WriteLine("middle")
    Case Is >= 10
        Console.WriteLine("high")
    Case Else
        Console.WriteLine("other")
End Select

• If has a block form (If … Then, any number of ElseIf … Then, one Else, End If) and a single-line form (If c Then a Else b, where the parts may hold several statements separated by colons). The condition must be Boolean (with Option Strict Off a number is accepted).
• Select Case compares one expression with the cases in order and runs the first that matches. A case is a list of values, ranges (4 To 9), comparisons (Is >= 10) or a mix; Case Else is the fallback. There is no fall-through, and the cases may be strings, enumerations and any type with the operators needed.

Loops

For i As Integer = 0 To 10 Step 2
    If i = 4 Then Continue For
    If i > 8 Then Exit For
    Console.WriteLine(i)
Next

For Each ch As Char In text
    Console.Write(ch)
Next

While k < 3
    k += 1
End While

Do While k < 6
    k += 1
Loop

Do
    k -= 1
Loop Until k < 3

Exceptions

Try
    Using reader As New StreamReader(path)
        Console.WriteLine(reader.ReadLine())
    End Using
Catch ex As FileNotFoundException When path.Length > 0
    Console.WriteLine("missing " & ex.FileName)
Catch ex As IOException
    Throw
Catch ex As Exception
    Throw New InvalidOperationException("failed", ex)
Finally
    Console.WriteLine("done")
End Try

• Try … Catch … Finally … End Try: the first Catch whose type matches (and whose When filter is true) handles the exception; Finally always runs. Catch alone catches everything. Exit Try leaves the block.
• Throw throws an exception object; Throw with nothing rethrows the current one and keeps its stack.
• On Error is the unstructured style from VB6, still allowed in a procedure that has no Try: On Error GoTo label, On Error GoTo 0 (off), On Error GoTo -1 (clear), On Error Resume Next; Resume, Resume Next and Resume label continue after a handler; the Err object holds the error (Err.Number, Err.Description, Err.Raise); Error raises one by number.

Function Old(x As Integer) As Integer
    On Error GoTo Handler
    Dim r = 10 \ x
    Return r
Handler:
    If Err.Number = 11 Then Return -1
    Resume Next
End Function

Resources, blocks and branches

Dim o As New Object
SyncLock o
    Console.WriteLine("locked")
End SyncLock

Dim sb As New System.Text.StringBuilder
With sb
    .Append("a")
    .Append("b")
End With

Dim i = 0
Again:
i += 1
If i < 3 Then GoTo Again
Dim a(3) As Integer
Erase a
ReDim a(7)
Dim s = "hello"
Mid(s, 1, 1) = "J"
Call Console.WriteLine("called")
Stop

• Using declares or evaluates a resource and calls its Dispose at End Using, whatever happens (version 8).
• SyncLock takes the monitor of an object so only one thread runs the block.
• With evaluates an object once; inside, a name that starts with a dot is a member of it.
• GoTo jumps to a label in the same procedure; a label is a name (or number) followed by a colon at the start of a line. It cannot jump into a block. GoSub and Return without a value are VB6 forms.
• Return leaves a procedure (with the value in a function); Exit Sub, Exit Function and Exit Property do the same, and a function can also assign to its own name.
• ReDim / Erase resize and clear arrays; Stop breaks into a debugger; End ends the program; Call is the optional keyword for a call statement.
• RaiseEvent, AddHandler, RemoveHandler belong to events; Await and Yield to async and iterator methods.

Not described

Known, not yet covered

• Reference: the exact rules of For loop control variables (type of the counter, overflow, loop-variable scope and the closure rules), For Each over arrays of arrays and the enumeration of dictionary entries are summarised, not specified.
• Reference: the semantics of Resume forms, the ordering of Finally with On Error, and the definite-assignment warnings are in the statements chapter of the specification and are not listed here.

See also

• Operators and expressions – the expressions inside statements
• VB6 and VBA – the file and I/O statements of the older languages