将Base64字符串转换为GIF图像

Question

For a side project I am trying to make a simple executable that takes 2 arguments:

1. Text file path (contains a Base64 encoded animated GIF) (example base.txt)
2. Name of the GIF to export (example hello.gif)

The way I am thinking it should all go is this:

1. It opens and reads the data from the file
2. It takes the data and decodes the Base64 string
3. It then saves the data as an animated GIF

Now of course there are some intermediate steps that need to happen but I don't think I need to cover that at the moment.

So in the end I am left with the following C# file which should do what I want.

using System;
using System.Drawing;
using System.Text;
using System.IO;

namespace Base642Img
{
    class MainClass
    {
        public static void Main (string[] args)
        {
            if (args.Length == 2) {
                byte[] buffer;
                String imageData;
                FileStream fileStream = new FileStream (args [0], FileMode.Open, FileAccess.Read);

                try {
                    int length = (int)fileStream.Length;
                    buffer = new byte[length];
                    int count;
                    int sum = 0;

                    while ((count = fileStream.Read (buffer, sum, length - sum)) > 0) {
                        sum += count;
                    }
                } finally {
                    fileStream.Close();
                }

                try{
                    imageData = Encoding.UTF8.GetString (buffer, 0, buffer.Length);
                    imageData = imageData.Replace (System.Environment.NewLine, "");

                    byte[] imageBytes = System.Convert.FromBase64String (imageData);

                    MemoryStream ms = new MemoryStream (imageBytes, 0, imageBytes.Length);
                    ms.Write (imageBytes, 0, imageBytes.Length);

                    Image image = Image.FromStream (ms, true);
                    image.Save (args [1], System.Drawing.Imaging.ImageFormat.Gif);
                } catch (Exception e) {
                    Console.WriteLine (e.ToString ());
                }
            } else {
                Console.WriteLine ("Incorrect number of arguments");
            }
        }
    }
}

This builds perfectly fine in MonoDevelop. But when I go to run the file (given the two arguments) it spits out the following exception:

System.FormatException: Invalid length.
  at (wrapper managed-to-native) System.Convert:InternalFromBase64String (string,bool)
  at System.Convert.FromBase64String (System.String s) [0x00000] in <filename unknown>:0 
  at Base642Img.MainClass.Main (System.String[] args) [0x00000] in <filename unknown>:0 

I have absolutely no idea what this is saying and all of my searches on Google have turned up blank so I now turn to Stack Overflow hoping that someone can help me with this.

Answer 1

Convert.FromBase64String requires that the input string be padded at the end with up to two = characters so that its total length (ignoring white-space characters) is a multiple of four. Apparently, the program that created your base 64-encoded file omitted the trailing padding.

You need to append the missing padding characters yourself as necessary:

// using System.Collections.Generic;
// using System.Linq;

string imageData = File.ReadAllText(args[0]);
HashSet<char> whiteSpace = new HashSet<char> { '\t', '\n', '\r', ' ' };
int length = imageData.Count(c => !whiteSpace.Contains(c));
if (length % 4 != 0)
    imageData += new string('=', 4 - length % 4); // Pad length to multiple of 4.
byte[] imageBytes = Convert.FromBase64String(imageData);
MemoryStream ms = new MemoryStream(imageBytes);
Image image = Image.FromStream(ms, true);
image.Save(args[1], System.Drawing.Imaging.ImageFormat.Gif);

Note: Since Convert.FromBase64String ignores white-space characters, it's unnecessary to remove new-line characters. (My thanks to EZI for pointing this out.)