Nunit once per assembly setup teardown

gistfile1.cs

using NUnit.Framework;
using TestingClasses;

namespace Tests
{
    [SetUpFixture]
    public class BaseContext
    {
        private A _a;
        [SetUp]
        public void setup()
        {
            _a = new A();
        }

        [TearDown]
        public void tearDown()
        {
            _a = null;
        }
    }
}

namespace Tests.n1
{
    public class Test1 
    {
        [Test]
        public void it_should_be_true()
        {
            Assert.AreEqual(1, A.InstanceCounter);
        }
    }

    public class Test2 
    {
        [Test]
        public void it_should_be_true()
        {
            Assert.AreEqual(1, A.InstanceCounter);
        }
    }
}
namespace Tests.n2
{
    public class Test1
    {
        [Test]
        public void it_should_be_true()
        {
            Assert.AreEqual(1, A.InstanceCounter);
        }
    }

    public class Test2
    {
        [Test]
        public void it_should_be_true()
        {
            Assert.AreEqual(1, A.InstanceCounter);
        }
    }
}

namespace TestingClasses
{
    public class A
    {
        public static int InstanceCounter = 0;

        public A()
        {
            InstanceCounter++;
        }
    }
}