xml.LoadData - Data at the root level is invalid. Line 1, position 1

The hidden character is probably BOM. The explanation to the problem and the solution can be found here, credits to James Schubert, based on an answer by James Brankin found here.

Though the previous answer does remove the hidden character, it also removes the whole first line. The more precise version would be:

string _byteOrderMarkUtf8 = Encoding.UTF8.GetString(Encoding.UTF8.GetPreamble());
if (xml.StartsWith(_byteOrderMarkUtf8))
{
    xml = xml.Remove(0, _byteOrderMarkUtf8.Length);
}

I encountered this problem when fetching an XSLT file from Azure blob and loading it into an XslCompiledTransform object. On my machine the file looked just fine, but after uploading it as a blob and fetching it back, the BOM character was added.

Use Load() method instead, it will solve the problem. See more

The issue here was that myString had that header line. Either there was some hidden character at the beginning of the first line or the line itself was causing the error. I sliced off the first line like so:

xml.LoadXml(myString.Substring(myString.IndexOf(Environment.NewLine)));

This solved my problem.

I Think that the problem is about encoding. That's why removing first line(with encoding byte) might solve the problem.

My solution for Data at the root level is invalid. Line 1, position 1. in XDocument.Parse(xmlString) was replacing it with XDocument.Load( new MemoryStream( xmlContentInBytes ) );

I've noticed that my xml string looked ok:

<?xml version="1.0" encoding="utf-8"?>

but in different text editor encoding it looked like this:

?<?xml version="1.0" encoding="utf-8"?>

At the end i did not need the xml string but xml byte[]. If you need to use the string you should look for "invisible" bytes in your string and play with encodings to adjust the xml content for parsing or loading.

Hope it will help

Save your file with different encoding:

File > Save file as... > Save as UTF-8 without signature.

In VS 2017 you find encoding as a dropdown next to Save button.