Sub Statement (Visual Basic)
Updated: August 2012
Declares the name, parameters, and code that define a Sub procedure.
[ <attributelist> ] [ Partial ] [ accessmodifier ] [ proceduremodifiers ] [ Shared ] [ Shadows ] Sub name [ (Of typeparamlist) ] [ (parameterlist) ] [ Implements implementslist | Handles eventlist ] [ statements ] [ Exit Sub ] [ statements ] End Sub
Optional. See Attribute List.
Optional. Indicates definition of a partial method. See Partial Methods (Visual Basic).
Optional. Can be one of the following:
Optional. Can be one of the following:
Optional. See Shared.
Optional. See Shadows.
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).
Optional. List of type parameters for a generic procedure. See Type List.
Optional. List of local variable names representing the parameters of this procedure. See Parameter List (Visual Basic).
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.
Required if Implements is supplied. List of Sub procedures being implemented.
implementedprocedure [ , implementedprocedure ... ]
Each implementedprocedure has the following syntax and parts:
interfaceRequired. Name of an interface implemented by this procedure's containing class or structure.
definednameRequired. Name by which the procedure is defined in interface.
Optional. Indicates that this procedure can handle one or more specific events. See Handles Clause (Visual Basic).
Required if Handles is supplied. List of events this procedure handles.
eventspecifier [ , eventspecifier ... ]
Each eventspecifier has the following syntax and parts:
eventvariableRequired. Object variable declared with the data type of the class or structure that raises the event.
eventRequired. Name of the event this procedure handles.
Optional. Block of statements to run within this procedure.
Terminates the definition of this procedure.
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.
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.
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