MaterialDesignInXAML/MaterialDesignInXamlToolkit · error · ArgumentNullException

canBuild

Error message

canBuild

What it means

HintProxyBuilder is the internal registration record behind HintProxyFabric.RegisterBuilder. Its constructor rejects a null canBuild predicate (and separately a null build delegate) so that Get() can never dereference a null Func when walking the builder list. The exception surfaces at registration time, not later when a SmartHint tries to resolve a control.

Source

Thrown at src/MaterialDesignThemes.Wpf/HintProxyFabric.cs:12

namespace MaterialDesignThemes.Wpf;

public static partial class HintProxyFabric
{
    private sealed class HintProxyBuilder
    {
        private readonly Func<Control?, bool> _canBuild;
        private readonly Func<Control, IHintProxy> _build;

        public HintProxyBuilder(Func<Control?, bool> canBuild, Func<Control, IHintProxy> build)
        {
            _canBuild = canBuild ?? throw new ArgumentNullException(nameof(canBuild));
            _build = build ?? throw new ArgumentNullException(nameof(build));
        }

        public bool CanBuild(Control? control) => _canBuild(control);
        public IHintProxy? Build(Control control) => _build(control);
    }

    private static readonly List<HintProxyBuilder> Builders = new();

    static HintProxyFabric()
    {
        Builders.Add(new HintProxyBuilder(c => c is ComboBox, c => new ComboBoxHintProxy((ComboBox)c)));
        Builders.Add(new HintProxyBuilder(c => c is TextBox, c => new TextBoxHintProxy((TextBox)c)));
        Builders.Add(new HintProxyBuilder(c => c is RichTextBox, c => new RichTextBoxHintProxy((RichTextBox)c)));
        Builders.Add(new HintProxyBuilder(c => c is PasswordBox, c => new PasswordBoxHintProxy((PasswordBox)c)));
    }

    public static void RegisterBuilder(Func<Control?, bool> canBuild, Func<Control, IHintProxy> build)

View on GitHub (pinned to 98edec3a0b)

Solutions

1. Pass a non-null Func<Control?, bool> for canBuild and a non-null Func<Control, IHintProxy> for build.
2. If you have no filtering predicate, supply c => c is YourControl (or _ => true) for canBuild.
3. Register the builder once at application startup, after the control type is known.

Example fix

// before
HintProxyFabric.RegisterBuilder(null, c => new MyHintProxy(c));
// after
HintProxyFabric.RegisterBuilder(c => c is MyControl, c => new MyHintProxy((MyControl)c));

if (canBuild is null) throw new ArgumentNullException(nameof(canBuild));
if (build is null) throw new ArgumentNullException(nameof(build));
HintProxyFabric.RegisterBuilder(canBuild, build);

When it happens

Trigger: Calling HintProxyFabric.RegisterBuilder(null, build) or RegisterBuilder(canBuild, null), i.e. passing a null Func for either the matcher or the builder.

Common situations: Registering a custom hint control (e.g. a third-party text box) into SmartHint and accidentally passing a null lambda, often after a refactor that removed the predicate.

Related errors

• textBox
• textBox
• content
• session
• session

AI-assisted analysis of MaterialDesignInXAML/MaterialDesignInXamlToolkit@98edec3a0b (2026-08-13). Data as JSON: /api/errors/169fcb072e4c452a. Report an issue: GitHub.