stride3d/stride · error · ArgumentException

The parameter of this converter must be an instance of the

Error message

The parameter of this converter must be an instance of the {nameof(XamlMarkdown)} class.

What it means

TextToMarkdownFlowDocumentConverter optionally accepts an XamlMarkdown engine as its converter parameter to control Markdown rendering. It throws this ArgumentException from Convert (TextToMarkdownFlowDocumentConverter.cs:18) only when parameter is non-null AND not an XamlMarkdown instance, because it later casts parameter to XamlMarkdown.

Solutions

  1. Remove the ConverterParameter entirely so the default XamlMarkdown engine is used (parameter == null is allowed).
  2. Pass an actual `new XamlMarkdown()` (or a XamlMarkdown resource) as the parameter.
  3. Configure a shared XamlMarkdown as a StaticResource and reference it via ConverterParameter="{StaticResource md}".
  4. Guard with `parameter is XamlMarkdown or null` before calling Convert.

Example fix

// before
<TextBlock ... ConverterParameter="github" /> <!-- string, throws -->
// after
<md:XamlMarkdown x:Key="Engine"/>
<TextBlock ... ConverterParameter="{StaticResource Engine}" />
Defensive patterns

Strategy: validation

Validate before calling

bool ok = parameter == null || parameter is XamlMarkdown;

Type guard

static bool HasValidEngine(object p) => p is null or XamlMarkdown;

Try / catch

try { return converter.Convert(value, targetType, param, culture); }
catch (ArgumentException) { return null; }

Prevention

When it happens

Trigger: Passing a non-XamlMarkdown, non-null ConverterParameter — e.g. a string, a boolean flag, or another IMarkdown engine implementation — into a binding using this converter.

Common situations: Developers passing configuration strings or flags via ConverterParameter out of habit; supplying a different Markdown engine type after a library refactor; copy-paste from converters that take string parameters.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

AI-assisted analysis of stride3d/stride@96fad776d2 (2026-09-14). Data as JSON: /api/errors/9b84d4c96b5d7099. Report an issue: GitHub.

Appendix: source

Thrown at sources/presentation/Stride.Core.Presentation.Wpf/ValueConverters/TextToMarkdownFlowDocumentConverter.cs:18

// Copyright (c) .NET Foundation and Contributors (https://dotnetfoundation.org/ & https://stride3d.net) and Silicon Studio Corp. (https://www.siliconstudio.co.jp)
// Distributed under the MIT license. See the LICENSE.md file in the project root for more information.
using System;
using System.Globalization;
using System.Windows.Data;
using System.Windows.Documents;

namespace Stride.Core.Presentation.ValueConverters
{
    [ValueConversion(typeof(string), typeof(FlowDocument))]
    public class TextToMarkdownFlowDocumentConverter : OneWayValueConverter<TextToMarkdownFlowDocumentConverter>
    {
        /// <inheritdoc/>
        public override object Convert(object value, Type targetType, object parameter, CultureInfo culture)
        {
            if (parameter != null && !(parameter is XamlMarkdown))
            {
                throw new ArgumentException($"The parameter of this converter must be an instance of the {nameof(XamlMarkdown)} class.");
            }

            if (value == null)
                return null;

            var engine = (XamlMarkdown)parameter ?? defaultMarkdown.Value;
            if (engine == null)
                return null;

            try
            {
                var text = value.ToString();
                return engine.Transform(text);
            }
            catch (ArgumentException) { }
            catch (FormatException) { }
            catch (InvalidOperationException) { }

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