What causes "The base64 content is invalid." in d2phap/ImageGlass?

Strip all whitespace and newlines from the content before calling: content = new string(content.Where(c => !char.IsWhiteSpace(c)).ToArray()).

d2phap/ImageGlass · error · FormatException

The base64 content is invalid.

Error message

The base64 content is invalid.

What it means

Thrown by BHelper.ConvertBase64ToBytes when the input does not match the data-URI regex (^data:(?<type>image/[a-z+\-]*);base64,)?(?<data>[a-zA-Z0-9+/=]+)$ (case-insensitive, anchored). The MIME prefix is optional, so a bare base64 string is accepted, but any byte outside the base64 alphabet — interior whitespace, newlines, URL-safe chars '-', '_', or a missing/invalid data: prefix — fails the match and raises FormatException. A prior ArgumentNullException is thrown only when the content is null/whitespace, so this error specifically means 'non-empty but malformed'.

Source

Thrown at v9/Components/ImageGlass.Base/BHelper/Photoing.cs:432

    /// Converts base64 string to byte array, returns MIME type and raw data in byte array.
    /// </summary>
    /// <param name="content">Base64 string</param>
    /// <returns></returns>
    public static (string MimeType, byte[] ByteData) ConvertBase64ToBytes(string? content)
    {
        if (string.IsNullOrWhiteSpace(content))
        {
            throw new ArgumentNullException(nameof(content));
        }

        // data:image/svg-xml;base64,xxxxxxxx
        // type is optional
        var base64DataUri = Base64DataUriRegex();

        var match = base64DataUri.Match(content);
        if (!match.Success)
        {
            throw new FormatException("The base64 content is invalid.");
        }


        var base64Data = match.Groups["data"].Value;
        var byteData = Convert.FromBase64String(base64Data);
        var mimeType = match.Groups["type"].Value.ToLowerInvariant();

        if (mimeType.Length == 0)
        {
            // use default PNG MIME type
            mimeType = "image/png";
        }

        return (mimeType, byteData);
    }


    /// <summary>

View on GitHub (pinned to 4a3c4fecef)

Solutions

Strip all whitespace and newlines from the content before calling: content = new string(content.Where(c => !char.IsWhiteSpace(c)).ToArray()).
If you have URL-safe base64, convert it first: replace '-' with '+' and '_' with '/' before passing.
Ensure the prefix, if present, is exactly 'data:image/<sub>;base64,' and that the data is standard-alphabet base64 with valid padding.
Validate against the same regex (or try Convert.FromBase64String in a sandbox) before calling to produce a clearer upstream error.

Example fix

// before
var (mime, bytes) = BHelper.ConvertBase64ToBytes(raw);

// after: sanitize whitespace + url-safe -> standard base64
var cleaned = new string(raw.Where(c => !char.IsWhiteSpace(c)).ToArray())
    .Replace('-', '+').Replace('_', '/');
var (mime, bytes) = BHelper.ConvertBase64ToBytes(cleaned);

Defensive patterns

Strategy: validation

Validate before calling

// Sanitize to standard base64 with no whitespace before converting.
var cleaned = new string(content.Where(c => !char.IsWhiteSpace(c)).ToArray())
    .Replace('-', '+').Replace('_', '/');
if (!System.Text.RegularExpressions.Regex.IsMatch(
        cleaned,
        @"(^data:image/[a-z+\-]*;base64,)?[a-zA-Z0-9+/=]+$",
        RegexOptions.IgnoreCase))
    throw new FormatException("Content is not valid base64 / data URI.");

Try / catch

try { var (mime, bytes) = BHelper.ConvertBase64ToBytes(cleaned); }
catch (FormatException) { /* not a base64 data URI */ }

Prevention

Strip whitespace/newlines from base64 before passing.
Convert URL-safe base64 ('-','_') to standard ('+','/') first.
Ensure any 'data:' prefix uses ';base64,' not ';utf8,'.

When it happens

Trigger: Passing a base64 data URI with interior whitespace or line breaks; using URL-safe base64 ('-','_') instead of standard ('+','/'); supplying a 'data:image/svg+xml;utf8,' (non-base64) URI; truncating the data portion; including a BOM or stray quote characters.

Common situations: Reading base64 from a JSON/theme file that was pretty-printed with wrapped lines; copy-paste introducing spaces; SVG encoded as 'utf8' instead of 'base64'; a theme/clipboard payload encoded by a tool that emits URL-safe base64.

Related errors

AI-assisted analysis of d2phap/ImageGlass@4a3c4fecef (2026-08-13). Data as JSON: /api/errors/3524facfe9868da3. Report an issue: GitHub.