Calling public variable/method from one class to MainForm.cs

Question

Hello,
I am trying to call:

public double lose;


public void LoseWeight()
{
        lose = weight - 400;
}


to


listBox.Items.Add("calories : " + lose); //on MainForm.cs



But for some reason, I'm getting error: The name "lose" does not exist in the current context.
I thought all public variables and methods can be shared.
Is there a special rule for getting things from class -> MainForm?

thank you :)