ASP.NET - How to include CSS only if it isn't already included?

问题

I use the code bellow to dynamically include a CSS file:

HtmlHead head = (HtmlHead)Page.Header;
HtmlLink link = new HtmlLink();
link.Attributes.Add("href", Page.ResolveClientUrl("~/App_Themes/Default/StyleSheet.css"));
link.Attributes.Add("type", "text/css");
link.Attributes.Add("rel", "stylesheet");
head.Controls.Add(link);

The problem is: I want to do it only once, and only if it isn't alrealy included in the page.

How do I verify if it is already included?

Edit:

Answers telling me to include in page load using !IsPostBack won't solve my problem, as this code will be inside a Web User Control and my page may have a lot of the same user control.

For example, I use the code below to do it with javascript:

if (!Page.ClientScript.IsClientScriptIncludeRegistered("jsScript"))
{
    Page.ClientScript.RegisterClientScriptInclude("jsScript", ResolveUrl("~/Utilities/myScript.js"));
}

回答1:

Did it...

the code I used is as follows:

        Boolean cssAlrealyIncluded = false;
        HtmlLink linkAtual;
        foreach (Control ctrl in Page.Header.Controls)
        {
            if (ctrl.GetType() == typeof(HtmlLink))
            {
                linkAtual = (HtmlLink)ctrl;

                if (linkAtual.Attributes["href"].Contains("datePicker.css"))
                {
                    cssAlrealyIncluded = true;
                }
            }
        }

        if (!cssAlrealyIncluded)
        {
            HtmlLink link = new HtmlLink();
            link.Attributes.Add("href", ResolveUrl("~/Utilities/datePickerRsx/datePicker.css"));
            link.Attributes.Add("type", "text/css");
            link.Attributes.Add("rel", "stylesheet");
            Page.Header.Controls.Add(link);
        }

回答2:

Why not in your user control, add a value to HttpContext.Current.Items indicating that the stylesheet has already been included? This will prevent you from needing to look at every header control for every instance of the user control.

回答3:

In most cases you shouldn't care if CSS gets included more than once. It's generally not a problem.

EDIT: Order only matters if you need to be able to override css styles in subsequent style sheets.

In ASP.NET, you can include the CSS in your masterpage (assuming you have one) and then it will be guaranteed to only be included once. Since masterpages are available programmatically (even from user controls), you could even write some properties (or methods) that allow you to control which CSS externals to include when.

来源：https://stackoverflow.com/questions/2366605/asp-net-how-to-include-css-only-if-it-isnt-already-included

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