使用 AssociateControlID 呈现动态标签 WebControl 时引发异常

Question

调用在运行时动态构建 HTML 元素并将呈现的元素作为字符串返回的 Web 服务时出现异常。

RenderControl()方法调用出现如下错误：

System.Web.HttpException：找不到与标签“lblCertificate”关联的 ID 为“txtCertificate”的控件。

StringWriter stringWriter = new StringWriter();
HtmlTextWriter writer = new HtmlTextWriter(stringWriter);

// Create System.Web.UI.WebControls.Panel (container)
Panel pnlFRow1 = new Panel();
pnlFRow1.CssClass = "f-row";

// Create System.Web.UI.WebControls.Textbox
TextBox txtCertificate = new TextBox();
txtCertificate.ID = "txtCertificate";
txtCertificate.CssClass = "f-input f-input-full";

// Create System.Web.UI.WebControls.Label
Label lblCertificate = new Label();
lblCertificate.ID = "lblCertificate";
lblCertificate.CssClass = "f-label f-label-full";
lblCertificate.AssociatedControlID = txtCertificate.ID;
lblCertificate.Text = "Certificate:";
Panel pnlCertificate = new Panel();
pnlCertificate.CssClass = "f-label f-label-full";

// Binding child controls to parent controls
pnlFRow1.Controls.Add(lblCertificate);
pnlFRow1.Controls.Add(pnlCertificate);
pnlCertificate.Controls.Add(txtCertificate);

// Render control
pnlContent.RenderControl(writer);

// Return rendered HTML
return writer.InnerWriter.ToString();

我尝试将pnlFRow1.Controls.Add(lblCertificate); 行放在pnlCertificate.Controls.Add(txtCertificate); 行之后，认为这可能是顺序很重要的问题，但这会导致相同的错误。

AssociatedControlID 属性是必须具备的，以便将Label WebControl 呈现为实际的<label> 元素，并且必须在输入控件之前显示。

Answer 1

一种方法是继承 Label 类，并将 RenderBeginTag 方法替换为您更喜欢的方法。如果您嵌套 Label 类，它应该保留在本地上下文中并阻止它泄漏到应用程序的其余部分

例如：

public partial class _Default : Page 
{
        public class Label : System.Web.UI.WebControls.Label {
        public string For = null;
        public override void RenderBeginTag(HtmlTextWriter writer) {
            AddAttributeIfExists(writer, "ID", ID);
            AddAttributeIfExists(writer, "For", For);
            writer.RenderBeginTag("Label");
        }
        private void AddAttributeIfExists(HtmlTextWriter writer, string name, string value) {
            if (!string.IsNullOrWhiteSpace(value))
                writer.AddAttribute(name, value);
        }
    }

    protected void Page_Load(object sender, EventArgs e) {
        StringWriter stringWriter = new StringWriter();
        HtmlTextWriter writer = new HtmlTextWriter(stringWriter);

        Panel test = new Panel() { ID = "PanelTest" };

        TextBox txtCertificate = new TextBox() { ID = "txtCertificate" };
        Label lblCertificate = new Label() { ID = "lblCertificate", Text = "Certificate", For = txtCertificate.ClientID };

        test.Controls.Add(lblCertificate);
        test.Controls.Add(txtCertificate);

        test.RenderControl(writer);
        string teststring = writer.InnerWriter.ToString();
    }
}

注意：您现在可以使用自定义的For 属性，而不是使用AssociatedControlId，它会做同样的事情。另请注意 Label 类是如何嵌套在您的页面类中的，因此它不会干扰其他页面。

不幸的是，正如我们在此答案的第一个版本中发现的那样，如果您尝试在没有 AssociatedControlId 的情况下使用 .Attributes["for"]，.NET 会将 <Label> 转换为 <span>，所以上面的示例应该得到围绕这个。