Some variables don’t change value during the execution of a program. These variables are constants that appear many times in your code.
 
The rules for naming variables also apply to naming constants. The constant’s value is a literal value or a simple expression composed of numeric or string constants and operators. You can’t use functions in declaring constants.

Const pi As Double = 3.14159265358979
Console.WriteLine(pi)

Whenever you changed this variable, it will give you error.

Output

3.14159265358979