Strip the byte order mark from string in C#

I recently had issues with the .NET 4 upgrade, but until then the simple answer is

String.Trim()

removes the BOM up until .NET 3.5.

However, in .NET 4 you need to change it slightly:

String.Trim(new char[]{'\uFEFF'});

That will also get rid of the byte order mark, though you may also want to remove the ZERO WIDTH SPACE (U+200B):

String.Trim(new char[]{'\uFEFF','\u200B'});

This you could also use to remove other unwanted characters.

Some further information is from String.Trim Method:

The .NET Framework 3.5 SP1 and earlier versions maintain an internal list of white-space characters that this method trims. Starting with the .NET Framework 4, the method trims all Unicode white-space characters (that is, characters that produce a true return value when they are passed to the Char.IsWhiteSpace method). Because of this change, the Trim method in the .NET Framework 3.5 SP1 and earlier versions removes two characters, ZERO WIDTH SPACE (U+200B) and ZERO WIDTH NO-BREAK SPACE (U+FEFF), that the Trim method in the .NET Framework 4 and later versions does not remove. In addition, the Trim method in the .NET Framework 3.5 SP1 and earlier versions does not trim three Unicode white-space characters: MONGOLIAN VOWEL SEPARATOR (U+180E), NARROW NO-BREAK SPACE (U+202F), and MEDIUM MATHEMATICAL SPACE (U+205F).

It's of course best if you can strip it out while still on the byte array level to avoid unwanted substrings / allocs. But if you already have a string, this is perhaps the easiest and most performant way to handle this.

Usage:

            string feed = ""; // input
            bool hadBOM = FixBOMIfNeeded(ref feed);

            var xElem = XElement.Parse(feed); // now does not fail

    /// <summary>
    /// You can get this or test it originally with: Encoding.UTF8.GetString(Encoding.UTF8.GetPreamble())[0];
    /// But no need, this way we have a constant. As these three bytes `[239, 187, 191]` (a BOM) evaluate to a single C# char.
    /// </summary>
    public const char BOMChar = (char)65279;

    public static bool FixBOMIfNeeded(ref string str)
    {
        if (string.IsNullOrEmpty(str))
            return false;

        bool hasBom = str[0] == BOMChar;
        if (hasBom)
            str = str.Substring(1);

        return hasBom;
    }

Yet another generic variation to get rid of the UTF-8 BOM preamble:

var preamble = Encoding.UTF8.GetPreamble();
if (!functionBytes.Take(preamble.Length).SequenceEqual(preamble))
    preamble = Array.Empty<Byte>();
return Encoding.UTF8.GetString(functionBytes, preamble.Length, functionBytes.Length - preamble.Length);

Use a regex replace to filter out any other characters other than the alphanumeric characters and spaces that are contained in a normal certificate thumbprint value:

certficateThumbprint = Regex.Replace(certficateThumbprint, @"[^a-zA-Z0-9\-\s*]", "");

And there you go. Voila!! It worked for me.

I solved the issue with the following code

using System.Xml.Linq;

void method()
{
    byte[] bytes = GetXmlBytes();
    XDocument doc;
    using (var stream = new MemoryStream(docBytes))
    {
        doc = XDocument.Load(stream);
    }
 }