AvaloniaUI/Avalonia · error · ExpressionParseException

Unexpected data

Error message

Unexpected data 

What it means

Error "Unexpected data " thrown in AvaloniaUI/Avalonia.

Source

Thrown at src/Avalonia.Base/Rendering/Composition/Expressions/ExpressionParser.cs:18

using System;
using System.Collections.Generic;
using System.Diagnostics.CodeAnalysis;
using System.Linq;

// ReSharper disable StringLiteralTypo

namespace Avalonia.Rendering.Composition.Expressions
{
    internal class ExpressionParser
    {
        public static Expression Parse(ReadOnlySpan<char> s)
        {
            var p = new TokenParser(s);
            var parsed = ParseTillTerminator(ref p, "", false, false, out _);
            p.SkipWhitespace();
            if (p.Length != 0)
                throw new ExpressionParseException("Unexpected data ", p.Position);
            return parsed;
        }

        private static ReadOnlySpan<char> Dot => ".".AsSpan();
        static bool TryParseAtomic(ref TokenParser parser, 
            [MaybeNullWhen(returnValue: false)] out Expression expr)
        {
            // We can parse keywords, parameter names and constants
            expr = null;
            if (parser.TryParseKeywordLowerCase(ExpressionKeywords.StartingValue))
                expr = new KeywordExpression(ExpressionKeyword.StartingValue);
            else if(parser.TryParseKeywordLowerCase(ExpressionKeywords.CurrentValue))
                expr = new KeywordExpression(ExpressionKeyword.CurrentValue);
            else if(parser.TryParseKeywordLowerCase(ExpressionKeywords.FinalValue))
                expr = new KeywordExpression(ExpressionKeyword.FinalValue);
            else if(parser.TryParseKeywordLowerCase(ExpressionKeywords.Pi))
                expr = new KeywordExpression(ExpressionKeyword.Pi);
            else if(parser.TryParseKeywordLowerCase(ExpressionKeywords.True))

View on GitHub (pinned to 11c5427268)

Solutions

1. Fix the composition expression syntax: remove the trailing or unexpected extra tokens (e.g. an unmatched operator or closing parenthesis) from the expression string.
2. Validate the expression with a known-good sample from the Avalonia composition animation docs before using it in StartAnimation.

When it happens

Trigger: Raised by the composition expression parser when extra tokens remain after a complete expression was parsed. Defend by validating expression strings before assigning them to composition animations and by rejecting trailing characters after the top-level expression.

Common situations: See trigger scenarios.

AI-assisted analysis of AvaloniaUI/Avalonia@11c5427268 (2026-08-13). Data as JSON: /api/errors/6c4e509f64ac4657. Report an issue: GitHub.