AvaloniaUI/Avalonia · error · InvalidCastException

Could not convert '{s}' to {typeof(T)}.

Error message

Could not convert '{s}' to {typeof(T)}.

What it means

InvalidCastException thrown by AvaloniaListConverter.ConvertFrom when TypeUtilities.TryConvert fails to convert one of the comma-separated string tokens to the target element type T. The converter splits the input string on ',' and converts each segment; any unconvertible token aborts the whole conversion.

Source

Thrown at src/Avalonia.Base/Collections/AvaloniaListConverter.cs:38

        public override object? ConvertFrom(ITypeDescriptorContext? context, CultureInfo? culture, object? value)
        {
            if (value is not string stringValue)
                return null;

            var result = new AvaloniaList<T>();

            // TODO: Use StringTokenizer here.
            var values = stringValue.Split(',');

            foreach (var s in values)
            {
                if (TypeUtilities.TryConvert(typeof(T), s, culture, out var v))
                {
                    result.Add((T)v!);
                }
                else
                {
                    throw new InvalidCastException($"Could not convert '{s}' to {typeof(T)}.");
                }
            }

            return result;
        }
    }
}

View on GitHub (pinned to 11c5427268)

Solutions

1. Ensure every comma-separated token is a valid representation of T (correct format, locale, enum name).
2. Pre-validate/trim tokens before conversion; strip empty segments from trailing commas.
3. Provide a TypeConverter for T or register a custom TypeUtilities converter if the default cannot parse your format.
4. Catch InvalidCastException at the binding/error boundary and surface a validation message to the user.

Example fix

// before (XAML): <local:MyList>1,2,foo</local:MyList>  <!-- AvaloniaList<int> -->

// after
<local:MyList>1,2,3</local:MyList>

var tokens = stringValue.Split(',', StringSplitOptions.RemoveEmptyEntries);
foreach (var t in tokens)
    if (!TypeUtilities.TryConvert(typeof(T), t.Trim(), culture, out _))
        throw new FormatException($"Invalid token '{t}' for {typeof(T)}.");

try { var list = (AvaloniaList<T>)converter.ConvertFromString(str); }
catch (InvalidCastException ex) { /* report invalid token to user/validation */ }

When it happens

Trigger: XAML/markup or TypeDescriptor code converting a string like '1,2,foo' to an AvaloniaList<int>; passing a value that cannot be parsed by TypeUtilities for T (e.g. non-numeric text for an int list, malformed enums for an enum list).

Common situations: User-authored or data-bound string content feeding a TypeConverter for an AvaloniaList<T>; locale/format mismatches (e.g. decimal separator, enum casing); trailing commas or stray whitespace tokens.

Related errors

• Invalid Cue string "{value}"
• Easing "{e}" was not found in {Namespace} namespace.
• IterationCount can't be a negative number.
• dictionary
• Reset called on collection without reset handler.

AI-assisted analysis of AvaloniaUI/Avalonia@11c5427268 (2026-08-13). Data as JSON: /api/errors/c047fa3ee6988817. Report an issue: GitHub.