LuckyPennySoftware/MediatR · error · ArgumentNullException

Open behavior type can not be null.

Error message

Open behavior type can not be null.

What it means

OpenBehavior is a MediatR registration entity that wraps a Type flagged as an open-generic IPipelineBehavior<,>. Its constructor validation rejects a null type with ArgumentNullException before any reflection occurs, so registration fails fast with a clear cause rather than NullReferenceException inside GetInterfaces.

Source

Thrown at src/MediatR/Entities/OpenBehavior.cs:43

    /// <summary>
    /// The type of the open behavior.
    /// </summary>
    public Type OpenBehaviorType { get; }

    /// <summary>
    /// The service lifetime of the open behavior.
    /// </summary>
    public ServiceLifetime ServiceLifetime { get; }

    /// <summary>
    /// Validates whether the specified type implements the <see cref="IPipelineBehavior{TRequest, TResponse}"/> interface.
    /// </summary>
    /// <param name="openBehaviorType">The type to validate.</param>
    /// <exception cref="InvalidOperationException">Thrown if the type does not implement <see cref="IPipelineBehavior{TRequest, TResponse}"/>.</exception>
    /// <exception cref="ArgumentNullException">Thrown if <paramref name="openBehaviorType"/> is null.</exception>
    private static void ValidatePipelineBehaviorType(Type openBehaviorType)
    {
        if (openBehaviorType == null) throw new ArgumentNullException($"Open behavior type can not be null.");

        bool isPipelineBehavior = openBehaviorType.GetInterfaces()
            .Any(i => i.IsGenericType && i.GetGenericTypeDefinition() == typeof(IPipelineBehavior<,>));

        if (!isPipelineBehavior)
        {
            throw new InvalidOperationException($"The type \"{openBehaviorType.Name}\" must implement IPipelineBehavior<,> interface.");
        }
    }
}

View on GitHub (pinned to 916ef1b3d6)

Solutions

1. Confirm the Type argument you pass to AddOpenBehavior / the OpenBehavior constructor is non-null.
2. If you build the Type from a string via Type.GetType, validate the result is non-null before registering (a null return means the assembly-qualified name could not be resolved).
3. Register the behavior by typeof(...) directly to avoid nulls from runtime type resolution.

Example fix

// before
var t = Type.GetType(config.BehaviorTypeName); // null if unresolvable
configuration.AddOpenBehavior(t);
// after
var t = Type.GetType(config.BehaviorTypeName)
    ?? throw new InvalidOperationException($"Cannot resolve {config.BehaviorTypeName}");
configuration.AddOpenBehavior(t);

if (openBehaviorType is null)
    throw new ArgumentNullException(nameof(openBehaviorType), "Cannot register a null open behavior type.");

static bool IsValidOpenBehaviorType(Type? t) => t is not null;

When it happens

Trigger: Constructing `new OpenBehavior(null, serviceLifetime)` — i.e. the AddOpenBehavior plumbing passed a null Type through, typically because the caller passed null as the open behavior type.

Common situations: Calling `configuration.AddOpenBehavior(null)` or a DI/registration helper that resolves the type dynamically and returns null (e.g. Type.GetType returning null for an unresolvable assembly-qualified name).

Related errors

• The type "{openBehaviorType.Name}" must implement IPipelineB
• {openBehaviorType.Name} must implement {typeof(IPipelineBeha
• {openBehaviorType.Name} must be generic
• {implementationType.Name} must implement {typeof(IPipelineBe
• Could not create wrapper type for {requestType}

AI-assisted analysis of LuckyPennySoftware/MediatR@916ef1b3d6 (2026-08-13). Data as JSON: /api/errors/801ba1a6d7aee12f. Report an issue: GitHub.