Share a variable between two classes

Question

I have two different .cs windows in just 1 project. Each one runs a different part of my program. But now I need to use a variable (i) of mainwindow.cs in Form.cs. This variable

Answer 1

If you declare your variable without an access modifier as you have done, it is implicitly private and thus only accessible within the class where it is declared (MainWindow in this case). You can add an access modifier:

internal float i;

This will allow you to access i from other classes within your assembly, like Form1.

See MSDN for more information on access modifiers: https://msdn.microsoft.com/en-us/library/wxh6fsc7.aspx

You should almost never expose fields like i outside of a class, though; instead what you want to do is use a property:

private float i;

public float I
{
    get { return i; }
    set { i = value; }
}

Better yet, you could make use of an auto-implemented property so you don't even need to have a backing field (i):

public float I { get; set; }

There are many reasons why exposing properties rather than fields is better. Here is one source on the topic (it's centered around VB, but the theories should be the same).

Addendum: please consider proper naming conventions for variables. i is not a good name for your variable.

Answer 2

your default access modifier is private if you don't mention it . Make it public

public float i {get; set;}