MaterialDesignInXAML/MaterialDesignInXamlToolkit · error · ArgumentNullException

session

Error message

session

What it means

ArgumentNullException(nameof(session)) in DialogOpenedEventArgs constructor (DialogOpenedEventArgs.cs:7). The args expose Session for interaction during the opened callback, so a null session is illegal. Reachable only when library/user code constructs DialogOpenedEventArgs with null.

Source

Thrown at src/MaterialDesignThemes.Wpf/DialogOpenedEventArgs.cs:7

namespace MaterialDesignThemes.Wpf;

public class DialogOpenedEventArgs : RoutedEventArgs
{
    public DialogOpenedEventArgs(DialogSession session, RoutedEvent routedEvent)
    {
        if (session == null) throw new ArgumentNullException(nameof(session));

        Session = session;
        RoutedEvent = routedEvent;
    }

    /// <summary>
    /// Allows interaction with the current dialog session.
    /// </summary>
    public DialogSession Session { get; }
}

View on GitHub (pinned to 98edec3a0b)

Solutions

  1. Pass a valid DialogSession when constructing DialogOpenedEventArgs; reuse DialogHost.CurrentSession.
  2. Subscribe to DialogOpened instead of constructing the args yourself.
  3. In tests, drive a real DialogHost so the library builds the args.
Defensive patterns

Strategy: validation

Validate before calling

DialogSession? session = dialogHost.CurrentSession;
if (session is null)
    throw new InvalidOperationException("Cannot build DialogOpenedEventArgs without a session.");

var args = new DialogOpenedEventArgs(session, DialogHost.DialogOpenedEvent);

Type guard

static bool TryBuildOpenedArgs(DialogHost host, out DialogOpenedEventArgs args)
{
    args = null!;
    if (host.CurrentSession is null) return false;
    args = new DialogOpenedEventArgs(host.CurrentSession, DialogHost.DialogOpenedEvent);
    return true;
}

Prevention

When it happens

Trigger: Calling 'new DialogOpenedEventArgs(null, routedEvent)' directly, or a custom build raising DialogOpened with a null session.

Common situations: Test doubles/fakes; reflection instantiation; library forks raising the routed event manually.

Related errors

AI-assisted analysis of MaterialDesignInXAML/MaterialDesignInXamlToolkit@98edec3a0b (2026-08-13). Data as JSON: /api/errors/0367706b375f37c3. Report an issue: GitHub.