MSBuild web download task which only downloads newer

LDownloadFile.Targets

  <UsingTask TaskName="LDownloadFile" TaskFactory="CodeTaskFactory" AssemblyFile="$(MSBuildToolsPath)\Microsoft.Build.Tasks.v12.0.dll">
    <ParameterGroup>
      <Url ParameterType="System.String" Required="true" />
      <File ParameterType="System.String" Required="false" />
      <OutputFolder ParameterType="System.String" Required="false" />
      <OnlyIfNewer ParameterType="System.Boolean" Required="false" />
    </ParameterGroup>
    <Task>
      <Using Namespace="System.Web"/>
      <Code Type="Fragment" Language="cs"><![CDATA[
        using (var client = new System.Net.WebClient())
            {
              var targetFile = (OutputFolder != null ? OutputFolder + "/" : "") + (File ?? System.IO.Path.GetFileName(new Uri(Url).LocalPath));
              var serverLastModifiedFile = targetFile+".dl";
              string serverLastModified = null;
      
              if (OnlyIfNewer) {
                using (client.OpenRead(Url)) {
                  // targetLength = Convert.ToInt64(client.ResponseHeaders["Content-Length"]);
                  serverLastModified = client.ResponseHeaders["Last-Modified"].ToString();
                }
                // var fileCreate = (new System.IO.FileInfo(targetFile)).CreationTimeUtc;

                string diskLastModified = null;
                if (System.IO.File.Exists(serverLastModifiedFile)) {
                  diskLastModified = System.IO.File.ReadAllText(serverLastModifiedFile);
                }

                if (!string.IsNullOrEmpty(diskLastModified) && diskLastModified == serverLastModified) {
                  Log.LogMessage("Skipping as already latest (Server: " + serverLastModified + ", Disk:" + diskLastModified + ") >" + targetFile);
                  return true;
                }

                Log.LogMessage("File exists but is a different (Server: " + serverLastModified + " Disk:" + diskLastModified + ") >" + targetFile);
              }

              client.DownloadFile(Url, targetFile);
              if (OnlyIfNewer) {
                System.IO.File.WriteAllText(serverLastModifiedFile, serverLastModified);
              }
              Log.LogMessage("Downloaded " + targetFile);
            }
        ]]></Code>
    </Task>
  </UsingTask>