lepoco/wpfui · error · ArgumentException

{nameof(parameter)} is not type: {typeof(TEnum)}

Error message

{nameof(parameter)} is not type: {typeof(TEnum)}

What it means

The second guard in EnumToBoolConverter<TEnum>.Convert: the ConverterParameter must also be the same TEnum as the generic type. It throws ArgumentException when the XAML supplies a parameter of a different type (or omits it, leaving it null). The converter needs both operands to be the same enum to perform an EqualityComparer comparison.

Source

Thrown at src/Wpf.Ui/Converters/EnumToBoolConverter.cs:22

// All Rights Reserved.

using System.Windows.Data;

namespace Wpf.Ui.Converters;

internal class EnumToBoolConverter<TEnum> : IValueConverter
    where TEnum : Enum
{
    public object Convert(object? value, Type targetType, object? parameter, CultureInfo culture)
    {
        if (value is not TEnum valueEnum)
        {
            throw new ArgumentException($"{nameof(value)} is not type: {typeof(TEnum)}");
        }

        if (parameter is not TEnum parameterEnum)
        {
            throw new ArgumentException($"{nameof(parameter)} is not type: {typeof(TEnum)}");
        }

        return EqualityComparer<TEnum>.Default.Equals(valueEnum, parameterEnum);
    }

    public object ConvertBack(object? value, Type targetType, object? parameter, CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}

View on GitHub (pinned to ffebacd610)

Solutions

  1. Supply ConverterParameter using x:Static referencing a member of the exact TEnum type.
  2. Ensure the same EnumToBoolConverter<TEnum> specialisation is used for both value and parameter.
  3. Do not pass string literals as ConverterParameter to this converter.

Example fix

<!-- before -->
<Button IsDefault="{Binding DefaultButton, Converter={StaticResource EnumToBoolConverter}, ConverterParameter=Primary}" />

<!-- after -->
<Button IsDefault="{Binding DefaultButton, Converter={StaticResource EnumToBoolConverter}, ConverterParameter={x:Static ui:ContentDialogButton.Primary}}" />
Defensive patterns

Strategy: type-guard

Validate before calling

if (parameter is not TEnum)
{
    throw new ArgumentException($"ConverterParameter must be {typeof(TEnum)}.", nameof(parameter));
}

Type guard

static bool IsValidParameter(object? p) => p is TEnum;

Try / catch

try { return converter.Convert(value, targetType, parameter, culture); }
catch (ArgumentException ex) when (ex.Message.Contains("parameter") && ex.Message.Contains("is not type"))
{
    _logger.LogError(ex, "EnumToBoolConverter needs an x:Static ConverterParameter");
    return DependencyProperty.UnsetValue;
}

Prevention

When it happens

Trigger: XAML supplies ConverterParameter as a string, an int, a different enum, or leaves it unset; binding uses x:Static pointing at the wrong enum type.

Common situations: Typing ConverterParameter="Primary" as a literal string instead of {x:Static ...}; copying a converter usage and forgetting to update the x:Static reference; mismatched enum types between value and parameter.

Related errors

AI-assisted analysis of lepoco/wpfui@ffebacd610 (2026-08-13). Data as JSON: /api/errors/4b13d4b7382029bf. Report an issue: GitHub.