Private declarations are private to the type, not the object

void Main()
{
    var test1 = new Test();
    var test2 = new Test();

    Test.IncrementMemberStatic(test1);
    Test.IncrementMemberStatic(test2);
    test1.IncrementMember(test1);
    test1.IncrementMember(test2);

    Console.WriteLine("test1.Member = {0}", test1.Member);
    Console.WriteLine("test2.Member = {0}", test2.Member);
}

// Define other methods and classes here
public class Test
{
    private int _Member = 0;

    public int Member { get { return _Member; } }

    public static void IncrementMemberStatic(Test test)
    {
        test._Member++;
    }

    public void IncrementMember(Test test)
    {
        test._Member++;
    }

}