Sub Statement (Visual Basic)

Declares the name, parameters, and code that define a Sub procedure.

[ <attributelist> ] [ Partial ] [ accessmodifier ] [ proceduremodifiers ] [ Shared ] [ Shadows ] [ Async ]
Sub name [ (Of typeparamlist) ] [ (parameterlist) ] [ Implements implementslist | Handles eventlist ]
    [ statements ]
    [ Exit Sub ]
    [ statements ]
End Sub

Parts

Term	Definition
attributelist	Optional. See Attribute List.
Partial	Optional. Indicates definition of a partial method. See Partial Methods (Visual Basic).
accessmodifier	Optional. Can be one of the following: Public Protected Friend Private Protected Friend See Access Levels in Visual Basic.
proceduremodifiers	Optional. Can be one of the following: Overloads Overrides Overridable NotOverridable MustOverride MustOverride Overrides NotOverridable Overrides
Shared	Optional. See Shared.
Shadows	Optional. See Shadows.
Async	Optional. See Async.
name	Required. Name of the procedure. See Declared Element Names (Visual Basic). To create a constructor procedure for a class, set the name of a Sub procedure to the New keyword. For more information, see Object Lifetime: How Objects Are Created and Destroyed (Visual Basic).
typeparamlist	Optional. List of type parameters for a generic procedure. See Type List.
parameterlist	Optional. List of local variable names representing the parameters of this procedure. See Parameter List (Visual Basic).
Implements	Optional. Indicates that this procedure implements one or more Sub procedures, each one defined in an interface implemented by this procedure's containing class or structure. See Implements Statement.
implementslist	Required if Implements is supplied. List of Sub procedures being implemented. implementedprocedure [ , implementedprocedure ... ] Each implementedprocedure has the following syntax and parts: interface.definedname Part Description interface Required. Name of an interface implemented by this procedure's containing class or structure. definedname Required. Name by which the procedure is defined in interface.
Handles	Optional. Indicates that this procedure can handle one or more specific events. See Handles Clause (Visual Basic).
eventlist	Required if Handles is supplied. List of events this procedure handles. eventspecifier [ , eventspecifier ... ] Each eventspecifier has the following syntax and parts: eventvariable.event Part Description eventvariable Required. Object variable declared with the data type of the class or structure that raises the event. event Required. Name of the event this procedure handles.
statements	Optional. Block of statements to run within this procedure.
EndSub	Terminates the definition of this procedure.

Remarks

All executable code must be inside a procedure. Use a Sub procedure when you don't want to return a value to the calling code. Use a Function procedure when you want to return a value.

Defining a Sub Procedure

You can define a Sub procedure only at the module level. The declaration context for a sub procedure must, therefore, be a class, a structure, a module, or an interface and can't be a source file, a namespace, a procedure, or a block. For more information, see Declaration Contexts and Default Access Levels (Visual Basic).

Sub procedures default to public access. You can adjust their access levels by using the access modifiers.

If the procedure uses the Implements keyword, the containing class or structure must have an Implements statement that immediately follows its Class or Structure statement. The Implements statement must include each interface that's specified in implementslist. However, the name by which an interface defines the Sub (in definedname) doesn't have to match the name of this procedure (in name).

Returning from a Sub Procedure

When a Sub procedure returns to the calling code, execution continues with the statement after the statement that called it.

The following example shows a return from a Sub procedure.

Sub mySub(ByVal q As String)
    Return
End Sub 

The Exit Sub and Return statements cause an immediate exit from a Sub procedure. Any number of Exit Sub and Return statements can appear anywhere in the procedure, and you can mix Exit Sub and Return statements.

Calling a Sub Procedure

You call a Sub procedure by using the procedure name in a statement and then following that name with its argument list in parentheses. You can omit the parentheses only if you don't supply any arguments. However, your code is more readable if you always include the parentheses.

A Sub procedure and a Function procedure can have parameters and perform a series of statements. However, a Function procedure returns a value, and a Sub procedure doesn't. Therefore, you can't use a Sub procedure in an expression.

You can use the Call keyword when you call a Sub procedure, but that keyword isn't recommended for most uses. For more information, see Call Statement (Visual Basic).

Visual Basic sometimes rearranges arithmetic expressions to increase internal efficiency. For that reason, if your argument list includes expressions that call other procedures, you shouldn't assume that those expressions will be called in a particular order.

Async Sub Procedures

By using the Async feature, you can invoke asynchronous functions without using explicit callbacks or manually splitting your code across multiple functions or lambda expressions.

If you mark a procedure with the Async modifier, you can use the Await operator in the procedure. When control reaches an Await expression in the Async procedure, control returns to the caller, and progress in the procedure is suspended until the awaited task completes. When the task is complete, execution can resume in the procedure.

Note

An Async procedure returns to the caller when either the first awaited object that’s not yet complete is encountered or the end of the Async procedure is reached, whichever occurs first.

You can also mark a Function Statement (Visual Basic) with the Async modifier. An Async function can have a return type of Task<TResult> or Task. An example later in this topic shows an Async function that has a return type of Task<TResult>.

AsyncSub procedures are primarily used for event handlers, where a value can't be returned. An AsyncSub procedure can't be awaited, and the caller of an AsyncSub procedure can't catch exceptions that the Sub procedure throws.

An Async procedure can't declare any ByRef parameters.

For more information about Async procedures, see Asynchronous Programming with Async and Await (C# and Visual Basic), Control Flow in Async Programs (C# and Visual Basic), and Async Return Types (C# and Visual Basic).

Example

The following example uses the Sub statement to define the name, parameters, and code that form the body of a Sub procedure.

Sub computeArea(ByVal length As Double, ByVal width As Double)
    ' Declare local variable. 
    Dim area As Double 
    If length = 0 Or width = 0 Then 
        ' If either argument = 0 then exit Sub immediately. 
        Exit Sub 
    End If 
    ' Calculate area of rectangle.
    area = length * width
    ' Print area to Immediate window.
    Debug.WriteLine(area)
End Sub

In the following example, DelayAsync is an an AsyncFunction that has a return type of Task<TResult>. DelayAsync has a Return statement that returns an integer. Therefore, the function declaration of DelayAsync must have a return type of Task(Of Integer). Because the return type is Task(Of Integer), the evaluation of the Await expression in DoSomethingAsync produces an integer, as the following statement shows: Dim result As Integer = Await delayTask.

The startButton_Click procedure is an example of an Async Sub procedure. Because DoSomethingAsync is an Async function, the task for the call to DoSomethingAsync must be awaited, as the following statement shows: Await DoSomethingAsync(). The startButton_ClickSub procedure must be defined with the Async modifier because it has an Await expression.

' Imports System.Diagnostics 
' Imports System.Threading.Tasks 

' This Click event is marked with the Async modifier. 
Private Async Sub startButton_Click(sender As Object, e As RoutedEventArgs) Handles startButton.Click
    Await DoSomethingAsync()
End Sub 

Private Async Function DoSomethingAsync() As Task
    Dim delayTask As Task(Of Integer) = DelayAsync()
    Dim result As Integer = Await delayTask

    ' The previous two statements may be combined into 
    ' the following statement. 
    ' Dim result As Integer = Await DelayAsync()

    Debug.WriteLine("Result: " & result)
End Function 

Private Async Function DelayAsync() As Task(Of Integer)
    Await Task.Delay(100)
    Return 5
End Function 

'  Output: 
'   Result: 5

See Also

Tasks

How to: Use a Generic Class (Visual Basic)

Troubleshooting Procedures (Visual Basic)

Reference

Implements Statement

Function Statement (Visual Basic)

Parameter List (Visual Basic)

Dim Statement (Visual Basic)

Call Statement (Visual Basic)

Of Clause (Visual Basic)

Concepts

Parameter Arrays (Visual Basic)

Partial Methods (Visual Basic)