Option Strict is prevents program from automatic variable conversions, that is implicit data type conversions .
Option Strict [On Off]
By default Option Strict is Off
From the following example you can understand the use of Option Strict.
VB.NET Source Code
Public Class Form1
Private Sub Button1_Click(ByVal sender As System.Object, _
ByVal e As System.EventArgs) Handles Button1.Click
Dim longNum As Long
Dim intNum As Integer
longNum = 12345
intNum = longNum
MsgBox(intNum)
End Sub
End Class
The above program is a normal vb.net program and is in default Option Strict Off . Because its Option Strict Off we can convert the value of Long to an Integer.
Take a look at the following program.
VB.NET Source Code
When you write this source code the compiler will shows the message
"Option Strict On disallows implicit conversions from 'Long' to 'Integer'"
The compiler generate error because in the program we put "Option Strict On" and prevent the program from automatic conversion.