Declare a pubilc variable

public static class GlobalVar
    {
        /// <summary>
        /// Global variable that is constant.
        /// </summary>
        public const string GlobalString = "Important Text";

        /// <summary>
        /// Static value protected by access routine.
        /// </summary>
        static int _globalValue;

        /// <summary>
        /// Access routine for global variable.
        /// </summary>
        public static int GlobalValue
        {
            get
            {
                return _globalValue;
            }
            set
            {
                _globalValue = value;
            }
        }

        /// <summary>
        /// Global static field.
        /// </summary>
        public static bool GlobalBoolean;
    }

public class MyForm : Form
{
	public void foo()
	{
		Data = "balbla";
	}
	public string Data;
}

public void bar()
{
	MyForm form1 = new MyForm();
	form1.foo();
	
	MyForm form2 = new MyForm();
	Console.WriteLine(form2.Data);
}

public static string Data;