The input is not a valid Base-64 string as it contains a non-base 64 character

I have a REST service that reads a file and sends it to another console application after converting it to Byte array and then to Base64 string. This part works, but when the same stream is received at the application, it gets manipulated and is no longer a valid Base64 string. Some junk characters are getting introduced into the stream.

The exception received when converting the stream back to Byte is

> The input is not a valid Base-64 string as it contains a non-base 64 > character, more than two padding characters, or a non-white space > character among the padding characters

At Service:

[WebGet(UriTemplate = "ReadFile/Convert", ResponseFormat = WebMessageFormat.Json)]  
public string ExportToExcel()
  {
      string filetoexport = "D:\\SomeFile.xls";
      byte[] data = File.ReadAllBytes(filetoexport);
      var s = Convert.ToBase64String(data);
      return s;
  }

At Application:

       var client = new RestClient("http://localhost:56877/User/");
       var request = new RestRequest("ReadFile/Convert", RestSharp.Method.GET);
       request.AddHeader("Accept", "application/Json");
       request.AddHeader("Content-Type", "application/Json");
       request.OnBeforeDeserialization = resp => {resp.ContentType =    "application/Json";};
       var result = client.Execute(request);
       byte[] d = Convert.FromBase64String(result.Content); 

Check if your image data contains some header information at the beginning:

imageCode = "data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAMgAAABkC...

This will cause the above error.

Just remove everything in front of and including the first comma, and you good to go.

imageCode = "iVBORw0KGgoAAAANSUhEUgAAAMgAAABkC...

Very possibly it's getting converted to a modified Base64, where the + and / characters are changed to - and _. See http://en.wikipedia.org/wiki/Base64#Implementations_and_history

If that's the case, you need to change it back:

string converted = base64String.Replace('-', '+');
converted = converted.Replace('_', '/');

We can remove unnecessary string input in front of the value.

string convert = hdnImage.Replace("data:image/png;base64,", String.Empty);

byte[] image64 = Convert.FromBase64String(convert);

Remove the unnecessary string through Regex

Regex regex=new Regex(@"^[\w/\:.-]+;base64,");
base64File=regex.Replace(base64File,string.Empty);

Since you're returning a string as JSON, that string will include the opening and closing quotes in the raw response. So your response should probably look like:

"abc123XYZ=="

or whatever...You can try confirming this with Fiddler.

My guess is that the result.Content is the raw string, including the quotes. If that's the case, then result.Content will need to be deserialized before you can use it.

Just in case you don't know the type of uploaded image, and you just you need to remove its base64 header:

 var imageParts = model.ImageAsString.Split(',').ToList<string>();
 //Exclude the header from base64 by taking second element in List.
 byte[] Image = Convert.FromBase64String(imageParts[1]);

Probably the string would be like this data:image/jpeg;base64,/9j/4QN8RXh... First split for / and get the second token.

var StrAfterSlash = Face.Split('/')[1];

Then Split for ; and get the first token which will be the format. In my case it's jpeg.

var ImageFormat =StrAfterSlash.Split(';')[0];

Then remove the line data:image/jpeg;base64, for the collected format

CleanFaceData=Face.Replace($"data:image/{ImageFormat };base64,",string.Empty);

I arranged a similar context as you described and I faced the same error. I managed to get it working by removing the " from the beginning and the end of the content and by replacing \/ with /.

Here is the code snippet:

var result = client.Execute(request);
var response = result.Content
    .Substring(1, result.Content.Length - 2)
    .Replace(@"\/","/");
byte[] d = Convert.FromBase64String(response);

As an alternative, you might consider using XML for the response format:

[WebGet(UriTemplate = "ReadFile/Convert", ResponseFormat = WebMessageFormat.Xml)]  
public string ExportToExcel() { //... }

On the client side:

request.AddHeader("Accept", "application/xml");
request.AddHeader("Content-Type", "application/xml");
request.OnBeforeDeserialization = resp => { resp.ContentType = "application/xml"; };

var result = client.Execute(request);
var doc = new System.Xml.XmlDocument();
doc.LoadXml(result.Content);
var xml = doc.InnerText;
byte[] d = Convert.FromBase64String(xml);

var spl = item.Split('/')[1];
var format =spl.Split(';')[0];           
stringconvert=item.Replace($"data:image/{format};base64,",String.Empty);

As Alex Filipovici mentioned the issue was a wrong encoding. The file I read in was UTF-8-BOM and threw the above error on Convert.FromBase64String(). Changing to UTF-8 did work without problems.

And some times it started with double quotes, most of the times when you call API from dotNetCore 2 for getting file

string string64 = string64.Replace(@"""", string.Empty);
byte[] bytes = Convert.ToBase64String(string64);

I get this error because a field was varbinary in sqlserver table instead of varchar.

please check if there is no == as a postfix, just add == chars at last of string

// "........V/XeAeH/wALVWKtD8lz/AAAAABJRU5ErkJggg"
"........V/XeAeH/wALVWKtD8lz/AAAAABJRU5ErkJggg=="    /* yes */