public class Company
{
    public string Name { get; set; }

    public Company()
    {
        Name = "The Name";
    }
}

[TestFixture]
public class NullTest
{
    [Test]
    public void TestExtensionMethodFailWhenTheObjectIsNull()
    {
        Company company = new Company();

        Assert.AreEqual(company.IsNullObj(), true);
    }
}

public static class Extension
{
    public static bool IsNullObj(this Company obj)
    {
        return (obj == null);
    }
}