为什么不调用重载方法呢？

我已经对类tt的Equals方法进行了重载，但是调试运行后发现它硬是不进重载方法，只调用了Object的Equals方法，为什么呢？

public class RequestQueue<T>
{
    private LinkedList<T> items = new LinkedList<T>();
    private int _count;
    public int Count
    {
        get
        {
            return this._count;
        }
    }
    public RequestQueue(int i)
    {
        this._count = i;
    }

    public void EnQueue(T item)
    {
        if (this.items.Count == this._count)
        {
            items.RemoveFirst();
            items.AddLast(item);
        }
        else
        {
            items.AddLast(item);
        }
    }

    public int Test(T item)
    {
        return items.Count(s => s.Equals(item));
    }
}
public class tt
{
    public string Im { get; set; }
    public string UA { get; set; }
    public override bool Equals(object obj)
    {
        tt a = obj as tt;
        if (a.Im == this.Im && a.UA == this.UA)
            return true;
        return false;
        //return base.Equals(obj);
    }
    public override int GetHashCode()
    {
        return this.ToString().GetHashCode();
    }
}

 protected void Page_Load(object sender, EventArgs e)
    {
        
        RequestQueue<tt> a = new RequestQueue<tt>(100);
        tt va = new tt() { Im = "1", UA = "2" };
        tt vb = new tt() { Im = "1", UA = "3" };
        tt vc = new tt() { Im = "1", UA = "2" };
        Response.Write(a.Test(va));
    }