ASP.NET User Control : can't initialize a user control property using Eval("...")

I designed a user control. It contains a public property "CurrentValue". When I try to initialize the property using an Eval expression, a null value is assigned.

// In below code, label assignment is OK, RatingNull user control assignment get null

<asp:TemplateField>
    <ItemTemplate>
        <asp:Label ID="Label1" runat="server" Text='<%# Eval("Difficulty") %>'
        <uc1:RatingNull ID="RatingNull1" runat="server" CurrentValue='<%# Eval("Difficulty") %>' />
    </ItemTemplate>

If I directly assigned a value (ie, CurrentValue="5"), user control works fine.

public partial class RatingNull : System.Web.UI开发者_Go百科.UserControl
{
    private string _CurrentValue;

    public string CurrentValue
    {
        get { return _CurrentValue; }
        set { _CurrentValue = value; }
    }
    (...)
}

The following code works for me. When do you use _CurrentValue field?

UserControl

public partial class Test1 : System.Web.UI.UserControl
{

    public string CurrentValue
    {
        get { return (string)ViewState["CurrentValue"] ?? string.Empty; }
        set { ViewState["CurrentValue"] = value; }
    }

    protected override void Render(HtmlTextWriter writer)
    {
        base.Render(writer);

        writer.Write(this.CurrentValue);
    }

}

Page

protected void Page_Load(object sender, EventArgs e)
{
    var ds = new[]
    {
        new { FirstName = "F1", LastName = "L1" },
        new { FirstName = "F2", LastName = "L2" },
    };

    DataList1.DataSource = ds;
    DataList1.DataBind();
}

Html Markup

<asp:DataList ID="DataList1" runat="server">
    <ItemTemplate>
        <uc1:Test ID="Test1" runat="server" CurrentValue='<%# Eval("FirstName") %>' />
    </ItemTemplate>
</asp:DataList>