现在来看看使这一切发生的代码。我必须承认设计时的体验有一些怪癖。可能是由于我对自定义控件的编写不太熟练（其实这两个都是我的第一次尝试），所以我非常希望输入以下内容。在我当前正在开发的基于 WCAB/WCSF 的项目中，我在 Visual Studio 设计视图中看到类似的错误，但我不知道为什么。网站编译完毕，一切都可以在线运行。
Visual Studio 中的设计时错误示例 http://www.egil.dk/wp-content/styleplaceholder-error.jpg
以下是StylePlaceHolder控件的代码：
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Linq;
using System.Security.Permissions;
using System.Web;
using System.Web.UI;
using System.Web.UI.HtmlControls;
[assembly: TagPrefix("Assimilated.Extensions.Web.Controls", "ass")]
namespace Assimilated.WebControls.Stylesheet
{
[AspNetHostingPermission(SecurityAction.Demand, Level = AspNetHostingPermissionLevel.Minimal)]
[AspNetHostingPermission(SecurityAction.InheritanceDemand, Level = AspNetHostingPermissionLevel.Minimal)]
[DefaultProperty("SkinID")]
[ToolboxData("<{0}:StylePlaceHolder runat=\"server\" SkinID=\"ThemeStyles\"></{0}:StylePlaceHolder>")]
[ParseChildren(true, "Styles")]
[Themeable(true)]
[PersistChildren(false)]
public class StylePlaceHolder : Control
{
private List<Style> _styles;
[Browsable(true)]
[Category("Behavior")]
[DefaultValue("ThemeStyles")]
public override string SkinID { get; set; }
[Browsable(false)]
public List<Style> Styles
{
get
{
if (_styles == null)
_styles = new List<Style>();
return _styles;
}
}
protected override void CreateChildControls()
{
if (_styles == null)
return;
// add child controls
Styles.ForEach(Controls.Add);
}
protected override void OnLoad(EventArgs e)
{
base.OnLoad(e);
// get notified when page has finished its load stage
Page.LoadComplete += Page_LoadComplete;
}
void Page_LoadComplete(object sender, EventArgs e)
{
// only remove if the page is actually using themes
if (!string.IsNullOrEmpty(Page.StyleSheetTheme) || !string.IsNullOrEmpty(Page.Theme))
{
// Make sure only to remove style sheets from the added by
// the runtime form the current theme.
var themePath = string.Format("~/App_Themes/{0}",
!string.IsNullOrEmpty(Page.StyleSheetTheme)
? Page.StyleSheetTheme
: Page.Theme);
// find all existing stylesheets in header
var removeCandidate = Page.Header.Controls.OfType<HtmlLink>()
.Where(link => link.Href.StartsWith(themePath)).ToList();
// remove the automatically added style sheets
removeCandidate.ForEach(Page.Header.Controls.Remove);
}
}
protected override void AddParsedSubObject(object obj)
{
// only add Style controls
if (obj is Style)
base.AddParsedSubObject(obj);
}
}
}
以及样式控件的代码：
using System.ComponentModel;
using System.Security.Permissions;
using System.Web;
using System.Web.UI;
[assembly: TagPrefix("Assimilated.Extensions.Web.Controls", "ass")]
namespace Assimilated.WebControls.Stylesheet
{
[AspNetHostingPermission(SecurityAction.Demand, Level = AspNetHostingPermissionLevel.Minimal)]
[AspNetHostingPermission(SecurityAction.InheritanceDemand, Level = AspNetHostingPermissionLevel.Minimal)]
[DefaultProperty("CssUrl")]
[ParseChildren(true, "InlineStyle")]
[PersistChildren(false)]
[ToolboxData("<{0}:Style runat=\"server\"></{0}:Style>")]
[Themeable(true)]
public class Style : Control
{
public Style()
{
// set default value... for some reason the DefaultValue attribute do
// not set this as I would have expected.
TargetMedia = "All";
}
#region Properties
[Browsable(true)]
[Category("Style sheet")]
[DefaultValue("")]
[Description("The url to the style sheet.")]
[UrlProperty("*.css")]
public string CssUrl
{
get; set;
}
[Browsable(true)]
[Category("Style sheet")]
[DefaultValue("All")]
[Description("The target media(s) of the style sheet. See http://www.w3.org/TR/REC-CSS2/media.html for more information.")]
public string TargetMedia
{
get; set;
}
[Browsable(true)]
[Category("Style sheet")]
[DefaultValue(EmbedType.Link)]
[Description("Specify how to embed the style sheet on the page.")]
public EmbedType Type
{
get; set;
}
[Browsable(false)]
[PersistenceMode(PersistenceMode.InnerDefaultProperty)]
public string InlineStyle
{
get; set;
}
[Browsable(true)]
[Category("Conditional comment")]
[DefaultValue("")]
[Description("Specifies a conditional comment expression to wrap the style sheet in. See http://msdn.microsoft.com/en-us/library/ms537512.aspx")]
public string ConditionalCommentExpression
{
get; set;
}
[Browsable(true)]
[Category("Conditional comment")]
[DefaultValue(CommentType.DownlevelHidden)]
[Description("Whether to reveal the conditional comment expression to downlevel browsers. Default is to hide. See http://msdn.microsoft.com/en-us/library/ms537512.aspx")]
public CommentType ConditionalCommentType
{
get; set;
}
[Browsable(true)]
[Category("Behavior")]
public override string SkinID { get; set; }
#endregion
protected override void Render(HtmlTextWriter writer)
{
// add empty line to make output pretty
writer.WriteLine();
// prints out begin condition comment tag
if (!string.IsNullOrEmpty(ConditionalCommentExpression))
writer.WriteLine(ConditionalCommentType == CommentType.DownlevelRevealed ? "<!{0}>" : "<!--{0}>",
ConditionalCommentExpression);
if (!string.IsNullOrEmpty(CssUrl))
{
// add shared attribute
writer.AddAttribute(HtmlTextWriterAttribute.Type, "text/css");
// render either import or link tag
if (Type == EmbedType.Link)
{
// <link href=\"{0}\" type=\"text/css\" rel=\"stylesheet\" media=\"{1}\" />
writer.AddAttribute(HtmlTextWriterAttribute.Href, ResolveUrl(CssUrl));
writer.AddAttribute(HtmlTextWriterAttribute.Rel, "stylesheet");
writer.AddAttribute("media", TargetMedia);
writer.RenderBeginTag(HtmlTextWriterTag.Link);
writer.RenderEndTag();
}
else
{
// <style type="text/css">@import "modern.css" screen;</style>
writer.RenderBeginTag(HtmlTextWriterTag.Style);
writer.Write("@import \"{0}\" {1};", ResolveUrl(CssUrl), TargetMedia);
writer.RenderEndTag();
}
}
if(!string.IsNullOrEmpty(InlineStyle))
{
// <style type="text/css">... inline style ... </style>
writer.AddAttribute(HtmlTextWriterAttribute.Type, "text/css");
writer.RenderBeginTag(HtmlTextWriterTag.Style);
writer.Write(InlineStyle);
writer.RenderEndTag();
}
// prints out end condition comment tag
if (!string.IsNullOrEmpty(ConditionalCommentExpression))
{
// add empty line to make output pretty
writer.WriteLine();
writer.WriteLine(ConditionalCommentType == CommentType.DownlevelRevealed ? "<![endif]>" : "<![endif]-->");
}
}
}
public enum EmbedType
{
Link = 0,
Import = 1,
}
public enum CommentType
{
DownlevelHidden = 0,
DownlevelRevealed = 1
}
}
