Managing Resource Dependencies

WebSharper automates the management of resource dependencies. For the purposes of WebSharper, a resource is any HTML code that can be rendered to the <head> section of a page. Most commonly, this will be a <link> tag pointing to a CSS file, or a <script> tag pointing to a JavaScript reference. Pages written with WebSharper infer their minimal necessary resource set and the correct resource ordering.

Most resources are declared as subclasses of the Core.Resources.BaseResource class (see "Declaring Resources and Dependencies" below for the different ways to declare a resource). Such a resource is declared with an associated path. The path is resolved by WebSharper as follows:

  1. If the assembly contains an embedded resource file whose name corresponds to the resource's path, and the following assembly-wide attribute is present:

    [<assembly:WebSharper.WebResource("myfile.js", "mime/type")>]
    do ()

    then WebSharper

    • extracts this file into the relevant subfolder of your application (/Scripts/WebSharper/<path> for JavaScript files, /Content/WebSharper/<path> for other resources)

    • adds the correct tag to <head> (<script> for JavaScript files, <link> for CSS files) with the corresponding path.

  2. If there is no such embedded resource file, then WebSharper simply adds the correct tag to <head> without altering the path. This means that for such resources, you should use absolute URLs, either inside the application (for example /some/path.js) or to an external website (for example //

A resource of type BaseResource can also be declared with a base path and a set of subpaths. This is useful for a library consisting of several files that need to be loaded eg. from a CDN. In this case, step 1 above is skipped, and a tag is added for each subpath by combining it with the base path.

Dependency resolution

A resource is only included by WebSharper if it is required by a client-side element. This means that each page of your website only contains the minimum set of resources that its contents need.

An assembly, a type, a module, a static member or a module let declaration can be marked as requiring a resource (see "Declaring Resources and Dependencies" below for the different ways to declare a dependency). Any page that calls this item's client-side code will have the given resource included.

A resource B can also be required by another resource A. In this case, any code that requires A will also include B. WebSharper ensures that B is located before A in the <head>.

Declaring Resources and Dependencies

In Client-side WebSharper code

To declare a resource in a WebSharper library or application, you can declare a class inheriting from BaseResource. Use the constructor with a single argument for single paths, and multiple arguments for a base path and a set of subpaths.

module Resources =

    open WebSharper.Core.Resources

    type R1() =
        inherit BaseResource("path.js")
    type R2() =
        inherit BaseResource("//",
            "file1.js", "file2.js", "file3.css")

You can also implement more complex resources (for example, resources that require a bit of inline JavaScript) by directly implementing the IResource interface. You can emit arbitrary HTML in the Render method using the provided HtmlTextWriter.

type R3() =
    interface R.IResource with
        member this.Render ctx writer = ...

A resource dependency can be declared on a type, a member or an assembly by annotating it with the Require attribute. It is parameterized by the type of the resource to require:

type MyWidget() = ...

let F x = ...


You can also use the Require attribute on a resource class to define dependency relations between resources. The dependent resource link will be inserted first by WebSharper.

A simplification: if you do not need dependencies between resources or custom Render and want to require a resource in a single code point, defining the subclass can be skipped by using BaseResource as the type and adding the path arguments on the Require attribute instead:

[<Require(typeof<BaseResource>, "path.js")>]
type MyWidget() = ...

[<Require(typeof<BaseResource>, "//",
            "file1.js", "file2.js", "file3.css")>]
let F x = ...

In general, when adding extra arguments on the Require atttribute, WebSharper will use those to use the matching constructor on the given resource type with those arguments.

In Server-side WebSharper code

Sometimes you might need to depend on a resource without having any client-side code; typically, a CSS file. In this case, you can add the web control WebSharper.Web.Require anywhere in your page. This control does not directly output any HTML at the location where you put it, but it incurs a dependency on the resource that you pass to it.

Here is an example UI.Next page with a dependency on R1 from above:

let MyPage() =
        div [
            h1 [text "This page includes the script at `path.js`."]
            Doc.WebControl(new Web.Require<R1>())

In WebSharper Interface Generator

To declare a resource in WIG, you can use one of the following functions:

  • Resource declares a BaseResource with a single path.
let R1 = Resource "ResourceClassName" "path.js"
  • Resources declares a BaseResource with a base path and multiple subpaths.
let R2 =
    Resources "ResourceClassName2"
        "//" ["file1.js"; "file2.js"; "file1.css"]

In either case, your resource must be included in the Assembly declaration. A common idiom is to create a sub-namespace called Resources and to include all resources in it:

let Assembly =
    Assembly [
        Namespace "My.Library" [
            // Library classes...
        Namespace "My.Library.Resources" [

To declare that a class or an assembly depends on a given resource, you can use one of the following functions:

  • Requires declares a dependency on resources declared in this assembly.
let C =
    Class "My.Library.C"
    |+> (* members... *)
    |> Requires [R1; R2]
let Assembly =
    Assembly [
        // ...
    |> Requires [R3]
  • RequiresExternal declares a dependency on resources declared outside of this assembly.
let C2 =
    Class "My.Library.C2"
    |+> (* members... *)
    |> RequiresExternal [typeof<Other.Library.Resources.R4>]

Resource Implementation

The resource dependency graphs are constructed for every WebSharper-processed assembly and are serialized to binary. They are stored within the assembly itself. At runtime all the graphs of all the referenced assemblies are deserialized and merged into a single graph.

WebSharper computes a hash of all generated files, and files embedded and referred to from a WebResource attribute (see above). These hashes are added as query parameters on script links generated by the Sitelets runtime to avoid browsers caching an outdated version.

Overriding Resource URLs

External resources implemented using BaseResource can have their URL overridden on a per-application basis. For example, you can force your application to use a different version of JQuery than the one used by default by WebSharper.

This configuration is done in the application configuration file.

To override a given resource URL, simply add an appSetting whose key is the fully qualified name of the resource, and whose value is the URL you want to use. For example, to tell WebSharper to use a local copy of JQuery located at the root of your application, you can add the following to your application configuration file:

In appsettings.json:

    "WebSharper.JQuery.Resources.JQuery": ""

In Web.config / App.config:

    <add key="WebSharper.JQuery.Resources.JQuery" value="" />

Note that the fully qualified name is in IL format. This means that nested types and types located inside F# modules are separated by + instead of ..

If you are a library author and have a resource declared by directly implementing the IResource interface, you can make it configurable by using the function ctx.GetSetting, which retrieves the setting with a given key from the application configuration file.

Using CDN

You can automatically point to CDN for the WebSharper core libraries. Simply set WebSharper.StdlibUseCdn to true in your application configuration file.

The links generated by WebSharper, instead of pointing to /Scripts/WebSharper/... for scripts and /Content/WebSharper/... for CSS, will point to //{assembly}/{version}/{filename}. You can configure this URL by setting the WebSharper.StdlibCdnFormat configuration setting. And finally, you can configure the CDN URL for the resources of a specific assembly (from the standard WebSharper library or not) by setting the WebSharper.CdnFormat.{assemblyname} configuration setting.