如何从编码的 base64 UTF 字符串中删除 BOM？

Question

I have a file encoded in base64 using openssl base64 -in en -out en1 in a command line in MacOS and I am reading this file using the following code:

string fileContent = File.ReadAllText(Path.Combine(AppContext.BaseDirectory, MConst.BASE_DIR, "en1"));
var b1 = Convert.FromBase64String(fileContent);
var str1 = System.Text.Encoding.UTF8.GetString(b1);

The string I am getting has a ? before the actual file content. I am not sure what's causing this, any help will be appreciated.

Example Input:

import pandas
import json

Encoded file example:

77u/DQppbXBvcnQgY29ubmVjdG9yX2FwaQ0KaW1wb3J0IGpzb24NCg0K

Output based on the C# code:

?import pandas
import json

Answer 1

Normally, when you read UTF (with BOM) from a text file, the decoding is handled for you behind the scene. For example, both of the following lines will read UTF text correctly regardless of whether or not the text file has a BOM:

File.ReadAllText(path, Encoding.UTF8);
File.ReadAllText(path); // UTF8 is the default.

The problem is that you're dealing with UTF text that has been encoded to a Base64 string. So, ReadAllText() can no longer handle the BOM for you. You can either do it yourself by (checking and) removing the first 3 bytes from the byte array or delegate that job to a StreamReader, which is exactly what ReadAllText() does :

var bytes = Convert.FromBase64String(fileContent);
string finalString = null;

using (var ms = new MemoryStream(bytes))
using (var reader = new StreamReader(ms))  // Or:
// using (var reader = new StreamReader(ms, Encoding.UTF8))
{
    finalString = reader.ReadToEnd();
}
// Proceed to using finalString.