How to check if a file exists on an webserver by its URL?

You can use .NET to do a HEAD request and then look at the status of the response.

Your code would look something like this (adapted from The Lowly HTTP HEAD Request):

// create the request
HttpWebRequest request = WebRequest.Create(url) as HttpWebRequest;

// instruct the server to return headers only
request.Method = "HEAD";

// make the connection
HttpWebResponse response = request.GetResponse() as HttpWebResponse;

// get the status code
HttpStatusCode status = response.StatusCode;

Here's a list detailing the status codes that can be returned by the StatusCode enumerator.

Can we assume that you are running your web application on the same web server as you are retrieving your help pages from? If yes, then you can use the Server.MapPath method to find a path to the file on the server combined with the File.Exists method from the System.IO namespace to confirm that the file exists.

Had the same problem myself and found this question and the answers here really useful.

But the answers here use the old WebRequest-class which is a bit outdated, it has no async support for starters. So I wanted to use the more modern way of doing it with HttpClient. Here is an example with a little helper class to check if the file exist:

using System.Net.Http;
using System.Threading.Tasks;

class HttpClientHelper
{
    private static HttpClient _httpClient;

    public static async Task<bool> DoesFileExist(string url)
    {
        if (_httpClient == null)
        {
            _httpClient = new HttpClient();
        }

        using (HttpRequestMessage request = new HttpRequestMessage(HttpMethod.Head, url))
        {
            using (HttpResponseMessage response = await _httpClient.SendAsync(request))
            {
                return response.StatusCode == System.Net.HttpStatusCode.OK;
            }
        }
    }
}

Usage:

if (await HttpClientHelper.DoesFileExist("https://www.google.com/favicon.ico"))
{
    // Yes it does!  
}
else
{
    // No it doesn't!
}