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.

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.
Defensive patterns

Strategy: try-catch

Validate before calling

// 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 / catch

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

Prevention

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

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

Appendix: 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)