AvaloniaUI/Avalonia · error · ArgumentException

Unable to parse effect: {s}

Error message

Unable to parse effect: {s}

What it means

Effect.Parse throws an ArgumentException when the input does not match a supported effect grammar. Currently the parser recognizes 'blur(<radius>)' syntax; any other leading token, malformed parenthesis, non-numeric radius, or trailing characters trigger the error. Note ParseError is defined as a method that always throws, used via 'throw ParseError(s)'.

Source

Thrown at src/Avalonia.Base/Media/Effects/Effect.cs:43

            static e => (e.Sender as T)?.RaiseInvalidated(EventArgs.Empty));

        foreach (var property in properties)
        {
            property.Changed.Subscribe(invalidateObserver);
        }
    }

    /// <summary>
    /// Raises the <see cref="Invalidated"/> event.
    /// </summary>
    /// <param name="e">The event args.</param>
    protected void RaiseInvalidated(EventArgs e) => Invalidated?.Invoke(this, e);

    /// <inheritdoc />
    public event EventHandler? Invalidated;


    static Exception ParseError(string s) => throw new ArgumentException("Unable to parse effect: " + s);
    public static IEffect Parse(string s)
    {
        var span = s.AsSpan();
        var r = new TokenParser(span);
        if (r.TryConsume("blur"))
        {
            if (!r.TryConsume('(') || !r.TryParseDouble(out var radius) || !r.TryConsume(')') || !r.IsEofWithWhitespace())
                throw ParseError(s);
            return new ImmutableBlurEffect(radius);
        }

       
        if (r.TryConsume("drop-shadow"))
        {
            if (!r.TryConsume('(') || !r.TryParseDouble(out var offsetX)
                                   || !r.TryParseDouble(out var offsetY))
                throw ParseError(s);
            double blurRadius = 0;

View on GitHub (pinned to 11c5427268)

Solutions

1. Use the exact supported syntax: "blur(<number>)" e.g. "blur(5)".
2. For other effects, construct the effect object directly (e.g. new BlurEffect { Radius = 5 }) instead of parsing a string.
3. Validate the string matches the blur pattern before parsing.

Example fix

// before
var effect = Effect.Parse("dropshadow(2)");

// after
var effect = new BlurEffect { Radius = 5 };
// or parse valid syntax:
var effect = Effect.Parse("blur(5)");

IEffect effect;
if (s.StartsWith("blur(", StringComparison.Ordinal) && s.EndsWith(")"))
    effect = Effect.Parse(s);
else
    effect = null; // or construct a known effect directly

IEffect effect;
try { effect = Effect.Parse(s); }
catch (ArgumentException) { effect = null; }

When it happens

Trigger: Calling Effect.Parse(s) where s does not start with 'blur(' followed by a valid double radius and ')'. E.g. "dropshadow(5)", "blur()", "blur(abc)", "blur(5". The parser uses a TokenParser and is strict about consuming '(' double ')'.

Common situations: Trying to use WPF-style effect syntax not supported by Avalonia (e.g. drop shadow via string). Typo in the effect keyword. Missing or malformed parentheses/radius. Expecting a richer effect DSL than what exists.

Understand the failure class

• Parsing and encoding errors: unexpected token, malformed input — why parsers reject input and how to find the real culprit.

Related errors

• Invalid brush string: '{s}'.
• Unknown CacheMode: {s}
• Invalid color string: '{s}'.
• Invalid color string: '{s.ToString()}'.
• Specified family is not supported.

AI-assisted analysis of AvaloniaUI/Avalonia@11c5427268 (2026-08-13). Data as JSON: /api/errors/bff66015e020e8ff. Report an issue: GitHub.