VB.NET Language in a Nutshell, Second Edition

Name

Option Explicit Statement

Syntax

Option Explicit [On | Off]

Description

Use Option Explicit to generate a compile-time error whenever a variable that has not been declared is encountered.

Rules at a Glance

• The Option Explicit statement must appear in the declarations section of a module before any procedures.

• In modules where the Option Explicit statement is not used, any undeclared variables are automatically cast as Objects.

• The default is Option Explicit On. In other words, the statement:

  Option Explicit

  is equivalent to:

  Option Explicit On

Programming Tips and Gotchas

• It is considered good programming practice to always use the Option Explicit statement. The following example shows why:

  1:    Dim iVariable As Integer
  
  2:    iVariable = 100
  3:    iVariable = iVarable + 50
  4:    MsgBox iVariable

  In this code snippet, an integer variable, iVariable, has been declared. However, because the name of the variable has been mistyped in line 3, the message box shows its value as only 50 instead of 150. This is because iVarable is assumed to be an undeclared variable whose value is 0. If the Option Explicit statement had been used, the code would not have compiled, and iVarable would have been highlighted as the cause.

• For an ASP.NET page, you use the @ PAGE directive rather than Option Explicit to require variable declaration. Its syntax is:

  <%@ Page Language="VB" Explicit=true|false %>

  By default, Explicit is true in ASP.NET pages.

  You can also use the <system.web> section of the WEB.Config file to require ...