Programmatically get the version number of a DLL

Answer by @Ben proved to be useful for me. But I needed to check the product version as it was the main increment happening in my software and followed semantic versioning.

myFileVersionInfo.ProductVersion

This method met my expectations

Update: Instead of explicitly mentioning dll path in program (as needed in production version), we can get product version using Assembly.

Assembly assembly = Assembly.GetExecutingAssembly();
FileVersionInfo fileVersionInfo =FileVersionInfo.GetVersionInfo(assembly.Location); 
string ProdVersion= fileVersionInfo.ProductVersion;

While the original question may not have been specific to a web service, here is a complete testWebService you can add to display a web service non-cached response plus the file version. We use file version instead of assembly version because we want to know a version, but with all assembly versions 1.0.0.0, the web site can be easily patched (signing and demand link still active!). Replace @Class@ with the name of the web api controller this service is embedded in. It's good for a go/nogo on a web service plus a quick version check.

  [Route("api/testWebService")]
  [AllowAnonymous]
  [HttpGet]
  public HttpResponseMessage TestWebService()
  {
      HttpResponseMessage responseMessage = Request.CreateResponse(HttpStatusCode.OK);
      string loc = Assembly.GetAssembly(typeof(@Class@)).Location;
      FileVersionInfo versionInfo = FileVersionInfo.GetVersionInfo(loc);
      responseMessage.Content = new StringContent($"<h2>The XXXXX web service GET test succeeded.</h2>{DateTime.Now}<br/><br/>File Version: {versionInfo.FileVersion}");
      responseMessage.Content.Headers.ContentType = new MediaTypeHeaderValue("text/html");
      Request.RegisterForDispose(responseMessage);
      return responseMessage;
  }

I found it also necessary to add the following to web.config under configuration to make it truly anonymous

<location path="api/testwebservice">
    <system.web>
        <authorization>
            <allow users="*" />
        </authorization>
    </system.web>
</location>