MVC Convert Base64 String to Image, but ... System.FormatException

Question

My controller is getting an uploaded image in the request object in this code:

[HttpPost]
public string Upload()
{
    string fileName = Request.Form["FileName"];
    string description = Request.Form["Description"];
    string image = Request.Form["Image"];

    return fileName;
}

The value of image (at least the beginning of it) looks a lot like this:

data:image/jpeg;base64,/9j/4AAQSkZJRgABAgEAYABgAAD/7gAOQWRvYmUAZAAAAAAB/...

I tried to convert using the following:

byte[] bImage = Convert.FromBase64String(image);

However, that gives the System.FormatException: "The input is not a valid Base-64 string as it contains a non-base 64 character, more than two padding characters, or an illegal character among the padding characters."

I get the feeling that the problem is that at least the start of the string isn't base64, but for all I know none of it is. Do I need to parse the string before decoding it? Am I missing something completely different?

Answer 1

It looks like you may just be able to strip out the "data:image/jpeg;base64," part from the start. For example:

const string ExpectedImagePrefix = "data:image/jpeg;base64,";
...
if (image.StartsWith(ExpectedImagePrefix))
{
    string base64 = image.Substring(ExpectedImagePrefix.Length);
    byte[] data = Convert.FromBase64String(base64);
    // Use the data
}
else
{
    // Not in the expected format
}

Of course you may want to make this somewhat less JPEG-specific, but I'd try that as a very first pass.