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Learn more about: Function Statement (Visual Basic)
Function Statement
05/12/2018
vb.Function
procedures [Visual Basic], creating
Function procedures [Visual Basic], Function statement syntax
functions [Visual Basic], function procedures
ParamArray keyword [Visual Basic], Function statements
Private keyword [Visual Basic], Function statements
declarations [Visual Basic], procedures
procedures [Visual Basic], declaration
Public keyword [Visual Basic], in Function statement
ByVal keyword [Visual Basic], Function statements
procedures [Visual Basic], recursive
Implements keyword [Visual Basic], Function statements
procedures [Visual Basic], returning values
Exit statement [Visual Basic], in Function procedures
recursive procedures
As keyword [Visual Basic], in Function statement
Optional keyword [Visual Basic], Function statements
Function statement [Visual Basic]
Visual Basic code, Function procedures
procedures [Visual Basic], function
ByRef keyword [Visual Basic], Function statements
Friend keyword [Visual Basic], Function statements
End keyword [Visual Basic], Function statements
Handles keyword [Visual Basic], Function statements
a4497077-0f46-4ede-a27f-9e8670df52b9

Function Statement (Visual Basic)

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

Syntax

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

Parts

  • attributelist

    Optional. See Attribute List.

  • accessmodifier

    Optional. Can be one of the following:

    See Access levels in Visual Basic.

  • proceduremodifiers

    Optional. Can be one of the following:

  • Shared

    Optional. See Shared.

  • Shadows

    Optional. See Shadows.

  • Async

    Optional. See Async.

  • Iterator

    Optional. See Iterator.

  • name

    Required. Name of the procedure. See Declared Element Names.

  • 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.

  • returntype

    Required if Option Strict is On. Data type of the value returned by this procedure.

  • Implements

    Optional. Indicates that this procedure implements one or more Function 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 Function 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.

  • 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 be executed within this procedure.

  • End Function

    Terminates the definition of this procedure.

Remarks

All executable code must be inside a procedure. Each procedure, in turn, is declared within a class, a structure, or a module that is referred to as the containing class, structure, or module.

To return a value to the calling code, use a Function procedure; otherwise, use a Sub procedure.

Defining a Function

You can define a Function procedure only at the module level. Therefore, the declaration context for a function must 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.

Function procedures default to public access. You can adjust their access levels with the access modifiers.

A Function procedure can declare the data type of the value that the procedure returns. You can specify any data type or the name of an enumeration, a structure, a class, or an interface. If you don't specify the returntype parameter, the procedure returns Object.

If this procedure uses the Implements keyword, the containing class or structure must also 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 Function (in definedname) doesn't need to match the name of this procedure (in name).

Note

You can use lambda expressions to define function expressions inline. For more information, see Function Expression and Lambda Expressions.

Returning from a Function

When the Function procedure returns to the calling code, execution continues with the statement that follows the statement that called the procedure.

To return a value from a function, you can either assign the value to the function name or include it in a Return statement.

The Return statement simultaneously assigns the return value and exits the function, as the following example shows.

[!code-vbVbVbalrStatements#24]

The following example assigns the return value to the function name myFunction and then uses the Exit Function statement to return.

[!code-vbVbVbalrStatements#23]

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

If you use Exit Function without assigning a value to name, the procedure returns the default value for the data type that's specified in returntype. If returntype isn't specified, the procedure returns Nothing, which is the default value for Object.

Calling a Function

You call a Function procedure by using the procedure name, followed by the argument list in parentheses, in an expression. You can omit the parentheses only if you aren't supplying any arguments. However, your code is more readable if you always include the parentheses.

You call a Function procedure the same way that you call any library function such as Sqrt, Cos, or ChrW.

You can also call a function by using the Call keyword. In that case, the return value is ignored. Use of the Call keyword isn't recommended in most cases. For more information, see Call Statement.

Visual Basic sometimes rearranges arithmetic expressions to increase internal efficiency. For that reason, you shouldn't use a Function procedure in an arithmetic expression when the function changes the value of variables in the same expression.

Async Functions

The Async feature allows you to invoke asynchronous functions without using explicit callbacks or manually splitting your code across multiple functions or lambda expressions.

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

Note

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

An Async function can have a return type of xref:System.Threading.Tasks.Task%601 or xref:System.Threading.Tasks.Task. An example of an Async function that has a return type of xref:System.Threading.Tasks.Task%601 is provided below.

An Async function cannot declare any ByRef parameters.

A Sub Statement can also be marked with the Async modifier. This is primarily used for event handlers, where a value cannot be returned. An Async Sub procedure can't be awaited, and the caller of an Async Sub procedure can't catch exceptions that are thrown by the Sub procedure.

For more information about Async functions, see Asynchronous Programming with Async and Await, Control Flow in Async Programs, and Async Return Types.

Iterator Functions

An iterator function performs a custom iteration over a collection, such as a list or array. An iterator function uses the Yield statement to return each element one at a time. When a Yield statement is reached, the current location in code is remembered. Execution is restarted from that location the next time the iterator function is called.

You call an iterator from client code by using a For Each…Next statement.

The return type of an iterator function can be xref:System.Collections.IEnumerable, xref:System.Collections.Generic.IEnumerable%601, xref:System.Collections.IEnumerator, or xref:System.Collections.Generic.IEnumerator%601.

For more information, see Iterators.

Example 1

The following example uses the Function statement to declare the name, parameters, and code that form the body of a Function procedure. The ParamArray modifier enables the function to accept a variable number of arguments.

[!code-vbVbVbalrStatements#25]

Example 2

The following example invokes the function declared in the preceding example.

[!code-vbVbVbalrStatements#26]

Example 3

In the following example, DelayAsync is an Async Function that has a return type of xref:System.Threading.Tasks.Task%601. DelayAsync has a Return statement that returns an integer. Therefore the function declaration of DelayAsync needs to 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. This is demonstrated in this statement: 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 demonstrates: Await DoSomethingAsync(). The startButton_Click Sub procedure must be defined with the Async modifier because it has an Await expression.

[!code-vbcsAsyncMethod#1]

See also