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 = iVariable + 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 becauseiVarable
is assumed to be an undeclared variable whose value is 0. If theOption
Explicit
statement had been used, the code would not have compiled, andiVarable
would have been highlighted as the cause.For an ASP.NET page, you use the
@
PAGE
directive rather thanOption
Explicit
to require variable declaration. Its syntax is:<%@ Page Language="VB" Explicit=true|false %>
By default,
Explicit
istrue
in ASP.NET pages.You can also use the <system.web> section of the
WEB.Config
file to require ...
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