Option Explicit On is a recommended good practice with Visual Basic .Net. It helps you as the developer to produce cleaner, more stable, more bug-free, more maintainable code. In some cases it may also help you write programs with better performance too!
with ref to https://support.microsoft.com/en-in/kb/311329#bookmark-3 option strict is also can be used instead of option explicit. Option strict inherits option explicit.
How to switch it on?
It is on by default, but you can have an extra layer of protection by placing
Option Explicit On at the top of the code file. The option will apply to the whole document.
You can switch it on via the menu in Visual Studio:
Project > [Project] Properties > Compile Tab > Option Explicit
On in the drop-down menu. The option will apply to the whole document.
All new projects
You can switch it On by default for all new Projects by selecting:
Tools > Options > Projects and Solutions > VB defaults > Option Explicit
On in the drop-down menu.
What is it?
It forces you to explicitly declare all variables.
What is the difference between explicitly declaring and implicitly declaring a variable?
Explicitly declaring a variable:
Implicitly declaring a variable:
Therefore, you should always have
Option Explicit On as you could misspel a variable during assignment, which cause your program to behave unexpectedly.