AvaloniaUI/Avalonia · error · FormatException

Bad format specifier.

Error message

Bad format specifier.

What it means

Thrown by ThrowFormatException_BadFormatSpecifier() in the Pooled collections ThrowHelper. This is a reserved/internal throw helper for malformed format specifiers during string formatting inside the pooled-array utilities. In the current Avalonia codebase this helper has no call sites — it is vestigial dead code ported from .NET's internal collections, so it is effectively unreachable through any public API.

Source

Thrown at src/Avalonia.Base/Collections/Pooled/ThrowHelper.cs:336

            throw new InvalidOperationException("No value provided.");
        }

        [DoesNotReturn]
        internal static void ThrowInvalidOperationException_ConcurrentOperationsNotSupported()
        {
            throw new InvalidOperationException("Concurrent operations are not supported.");
        }

        [DoesNotReturn]
        internal static void ThrowInvalidOperationException_HandleIsNotInitialized()
        {
            throw new InvalidOperationException("Handle is not initialized.");
        }

        [DoesNotReturn]
        internal static void ThrowFormatException_BadFormatSpecifier()
        {
            throw new FormatException("Bad format specifier.");
        }

        private static ArgumentException GetArgumentException(ExceptionResource resource)
        {
            return new ArgumentException(GetResourceString(resource));
        }

        private static InvalidOperationException GetInvalidOperationException(ExceptionResource resource)
        {
            return new InvalidOperationException(GetResourceString(resource));
        }

        private static ArgumentException GetWrongKeyTypeArgumentException(object? key, Type targetType)
        {
            return new ArgumentException($"Wrong key type. Expected {targetType}, got: '{key}'.", nameof(key));
        }

        private static ArgumentException GetWrongValueTypeArgumentException(object? value, Type targetType)

View on GitHub (pinned to 11c5427268)

Solutions

1. Confirm the exception originates from Avalonia.Base Pooled code and not your own FormatException; check the full stack trace for the actual caller.
2. If it appears in a fork, audit any new format-parsing code that calls ThrowFormatException_BadFormatSpecifier and validate the specifier before calling.
3. In a stock build this should never fire — file an issue against Avalonia if it does, including the stack trace.

// No public caller exists; if you maintain a fork that wires this helper,
// validate the format specifier before delegating to it.
if (!IsValidSpecifier(token)) return;

try { /* format-parsing path in forked pooled code */ }
catch (FormatException ex) when (ex.Message == "Bad format specifier.")
{
    // log and fall back to default formatting
}

When it happens

Trigger: Only reachable if a future change wires ThrowFormatException_BadFormatSpecifier() into a format-parsing path in the Pooled collections. Today there is no caller; searching the entire repo returns only the definition at ThrowHelper.cs:334.

Common situations: You will not encounter this in normal Avalonia development. If you do see it, a custom fork or unreleased build added a format-string parser that delegates to this helper and was passed an invalid specifier token.

Related errors

• Not supported format
• Invalid CornerRadius.
• Invalid BindingValueType.
• Unknown binding path element type {element.GetType().FullNam
• Unknown format specifier

AI-assisted analysis of AvaloniaUI/Avalonia@11c5427268 (2026-08-13). Data as JSON: /api/errors/588c5cfb2ba138e4. Report an issue: GitHub.