string result = System.Text.Encoding.UTF8.GetString(byteArray);

There're at least four different ways doing this conversion.

1. Encoding's GetString
   , but you won't be able to get the original bytes back if those bytes have non-ASCII characters.

2. BitConverter.ToString
   The output is a "-" delimited string, but there's no .NET built-in method to convert the string back to byte array.

3. Convert.ToBase64String
   You can easily convert the output string back to byte array by using Convert.FromBase64String.
   Note: The output string could contain '+', '/' and '='. If you want to use the string in a URL, you need to explicitly encode it.

4. HttpServerUtility.UrlTokenEncode
   You can easily convert the output string back to byte array by using HttpServerUtility.UrlTokenDecode. The output string is already URL friendly! The downside is it needs System.Web assembly if your project is not a web project.

A full example:

byte[] bytes = { 130, 200, 234, 23 }; // A byte array contains non-ASCII (or non-readable) characters

string s1 = Encoding.UTF8.GetString(bytes); // ���
byte[] decBytes1 = Encoding.UTF8.GetBytes(s1);  // decBytes1.Length == 10 !!
// decBytes1 not same as bytes
// Using UTF-8 or other Encoding object will get similar results

string s2 = BitConverter.ToString(bytes);   // 82-C8-EA-17
String[] tempAry = s2.Split('-');
byte[] decBytes2 = new byte[tempAry.Length];
for (int i = 0; i < tempAry.Length; i++)
    decBytes2[i] = Convert.ToByte(tempAry[i], 16);
// decBytes2 same as bytes

string s3 = Convert.ToBase64String(bytes);  // gsjqFw==
byte[] decByte3 = Convert.FromBase64String(s3);
// decByte3 same as bytes

string s4 = HttpServerUtility.UrlTokenEncode(bytes);    // gsjqFw2
byte[] decBytes4 = HttpServerUtility.UrlTokenDecode(s4);
// decBytes4 same as bytes

A general solution to convert from byte array to string when you don't know the encoding:

static string BytesToStringConverted(byte[] bytes)
{
    using (var stream = new MemoryStream(bytes))
    {
        using (var streamReader = new StreamReader(stream))
        {
            return streamReader.ReadToEnd();
        }
    }
}

I saw some answers at this post and it's possible to be considered completed base knowledge, because I have a several approaches in C# Programming to resolve the same problem. The only thing that is necessary to be considered is about a difference between pure UTF-8 and UTF-8 with a BOM.

Last week, at my job, I needed to develop one functionality that outputs CSV files with a BOM and other CSV files with pure UTF-8 (without a BOM). Each CSV file encoding type will be consumed by different non-standardized APIs. One API reads UTF-8 with a BOM and the other API reads without a BOM. I needed to research the references about this concept, reading the "What's the difference between UTF-8 and UTF-8 without BOM?" Stack Overflow question, and the Wikipedia article "Byte order mark" to build my approach.

Finally, my C# Programming for both UTF-8 encoding types (with BOM and pure) needed to be similar to this example below:

// For UTF-8 with BOM, equals shared by Zanoni (at top)
string result = System.Text.Encoding.UTF8.GetString(byteArray);

//for Pure UTF-8 (without B.O.M.)
string result = (new UTF8Encoding(false)).GetString(byteArray);