It may help to use a different constructor for Uri.
If you have the server name
string server = "http://www.myserver.com";
and have a relative Uri path to append to it, e.g.
string relativePath = "sites/files/images/picture.png"
When creating a Uri from these two I get the "format could not be determined" exception unless I use the constructor with the UriKind argument, i.e.
// this works, because the protocol is included in the string
Uri serverUri = new Uri(server);
// needs UriKind arg, or UriFormatException is thrown
Uri relativeUri = new Uri(relativePath, UriKind.Relative);
// Uri(Uri, Uri) is the preferred constructor in this case
Uri fullUri = new Uri(serverUri, relativeUri);
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