Modify contents of static files - asp.net

Question

how can I modify the response before it is sent to the client when I use Microsoft.Owin.StaticFiles?
FileServerOptions options = new FileServerOptions();
options.FileSystem = new Microsoft.Owin.FileSystems.PhysicalFileSystem(Path.Combine(Environment.CurrentDirectory, "Content/"));
options.DefaultFilesOptions.DefaultFileNames = new string[] { "index.htm", "index.html" };
options.StaticFileOptions.OnPrepareResponse = (r) =>
{
r.OwinContext.Response.WriteAsync("test");
};
options.EnableDefaultFiles = true;
app.UseFileServer(options);
"test" is never written into the response. I tried to use another middleware which waits until the StaticFiles Middleware is executed:
app.Use((ctx, next) =>
{
return next().ContinueWith(task =>
{
return ctx.Response.WriteAsync("Hello World!");
});
});
FileServerOptions options = new FileServerOptions();
options.FileSystem = new Microsoft.Owin.FileSystems.PhysicalFileSystem(Path.Combine(Environment.CurrentDirectory, "Content/"));
options.DefaultFilesOptions.DefaultFileNames = new string[] { "index.htm", "index.html" };
options.EnableDefaultFiles = true;
app.UseFileServer(options);
But this didn't work. How can I modify the response?

Answer 1

On prepare response is not meant to modify the content of a static file.
You are only allowed to add the header.
I needed to pass some variable that change to a static web page and I got around it by using
On prepare response and passed the variables as cookies for the page.
This works nicely for a few variables but if you want to change a page significantly you are better of using mvc components.
appBuilder.UseStaticFiles(new StaticFileOptions()
{
RequestPath = new PathString(baseUrl),
FileSystem = new PhysicalFileSystem(staticFilesLocation),
ContentTypeProvider = new JsonContentTypeProvider(),
OnPrepareResponse = r => r.OwinContext.Response.Cookies.Append("baseUrl",_webhostUrl)
});