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I need an advice on decoding base64. I will be doing it in c#.

The thing is, I don't know what type of format the decoding will output it may be text, XML, images or PDF. I only have the base64 encoded string.

How do you guys advice me to proceed? Any suggestions?

Many image types and pdfs include a magic number, where the first X bytes identify the file type. You should decode the string and examine the binary for these (https://asecuritysite.com/forensics/magic gives a list of them). If you still can't identify it check whether it parses as XML using an XML parser else assume it's text.

Extract the MIME type from a base64 string:

/**
 * Extract the MIME type from a base64 string
 * @param encoded Base64 string
 * @return MIME type string
 */
private static String extractMimeType(final String encoded) 
 final Pattern mime = Pattern.compile("^data:([a-zA-Z0-9]+/[a-zA-Z0-9]+).*,.*");
 final Matcher matcher = mime.matcher(encoded);
 if (!matcher.find())
 return "";
 return matcher.group(1).toLowerCase();

Usage:

final String encoded = "data:image/png;base64,iVBORw0KGgoAA...5CYII=";
extractMimeType(encoded); // "image/png"
extractMimeType("garbage"); // ""

Then you can write your byte array:

var filePath = System.IO.Path.Combine(folderPath, string.Format("pdf_0.pdf", Guid.NewGuid()));
var byteArray = Convert.FromBase64String(base64pdf);

File.WriteAllBytes(filePath, byteArray);

And open you file:

Device.OpenUri(new Uri("file://" + filePath));

Or tokenize the data since the 64 encoded data look like this "data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAoAAAAKAC" and parse that string.