Winform Combox DataSource 之不显示 displayemember 内容

private void Form1_Load(object sender, EventArgs e)
{
IList<TLayer> tt = new List<TLayer>();
tt.Add(new TLayer("Layer1", "1"));
tt.Add(new TLayer("Layer2", "2"));
tt.Add(new TLayer("Layer3", "3"));
tt.Add(new TLayer("Layer4", "4"));
tt.Add(new TLayer("Layer5", "5"));
comboBox1.DataSource = tt;
comboBox1.DisplayMember = "NameN";
comboBox1.ValueMember = "FL";
}

public class TLayer
{
public string NameN;public string FL;public TLayer(string NameN, string FL)
{
this.NameN = NameN;
this.FL = FL;
}
}

public class TLayer
{
public string NameN { get; set; }
public string FL { get; set; }

public TLayer(string NameN, string FL)
{
this.NameN = NameN;
this.FL = FL;
}
}