Most programming languages, such as C and
Fortran, require that you explicitly declare all variables before
you use them. Visual Basic 6 does not—you can use a variable in
code without declaring it. Some VB programmers tout this as an
advantage of VB, but, in fact, it’s a serious shortcoming. Here’s

When a VB program does not explicitly declare
variables (with a Dim, Private, or Public statement), VB creates a
new variable in memory every time it encounters a new variable name
in code. If you misspell a name, VB cannot know that it’s an error;
it just creates a new variable and initializes it, a process called
implicit variable declaration. This can cause serious problems and
make it difficult to find bugs in a program when, for example, a
calculation uses a misspelled variable that was initialized to zero
rather than the correct variable that you intended.

Another shortcoming is that such implicitly
declared variables are always of type Variant, which denies you the
program efficiencies that result from using the proper data type
for your variables.

Fortunately, there is a way around this. If you
include the Option Explicit statement in a module, then VB will
require explicit declarations for all variables in that module. Any
misspelled variables are flagged as syntax errors when you try to
run the program, and then you can make the necessary corrections.
Place the Option Explicit statement at the start of each module
(before any procedures). You can make this automatic by selecting
Tools | Options and, on the Editor tab, selecting the Require
Variable Declaration option.

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