microsoft/aspire · error · ArgumentNullException

ArgumentNullException for parameter 'callback' (callback is…

Error message

ArgumentNullException for parameter 'callback' (callback is null).

What it means

CertificateTrustConfigurationCallbackAnnotation wraps a callback that customizes certificate trust for a resource. The constructor requires a non-null callback; passing null throws ArgumentNullException for parameter 'callback'.

Solutions

Pass a valid async callback, e.g. ctx => { ...; return Task.CompletedTask; }
If the annotation is conditional, only add it when the callback delegate is available
Null-check the delegate at the call site before constructing the annotation

Example fix

// before
resource.Annotations.Add(new CertificateTrustConfigurationCallbackAnnotation(null));
// after
resource.Annotations.Add(new CertificateTrustConfigurationCallbackAnnotation(context =>
{
    context.CertificateTrustConfiguration... // customize trust
    return Task.CompletedTask;
}));

Defensive patterns

Strategy: validation

Validate before calling

if (callback is null) throw new InvalidOperationException("Certificate trust callback must be configured before adding the annotation.");

Try / catch

try { ann = new CertificateTrustConfigurationCallbackAnnotation(callback); } catch (ArgumentNullException) { ann = new CertificateTrustConfigurationCallbackAnnotation(DefaultTrustCallback); }

Prevention

Pass lambda callbacks inline rather than nullable delegate variables
Only add the annotation when the callback is available
Null-check delegates sourced from DI or options

When it happens

Trigger: new CertificateTrustConfigurationCallbackAnnotation(null) or passing a variable/chain that resolves to null (e.g. a conditional expression yielding null, or a method returning a null Func).

Common situations: Building annotations dynamically where the callback delegate was never assigned; refactoring removed the lambda but left the annotation; dependency injection failed to supply the delegate.

Related errors

AI-assisted analysis of microsoft/aspire@25830f84bd (2026-09-16). Data as JSON: /api/errors/2990d7c4d7b4e5f8. Report an issue: GitHub.

Appendix: source

Thrown at src/Aspire.Hosting/ApplicationModel/CertificateTrustConfigurationCallbackAnnotation.cs:18

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.

using System.Diagnostics.CodeAnalysis;
using System.Security.Cryptography.X509Certificates;

namespace Aspire.Hosting.ApplicationModel;

/// <summary>
/// An annotation that indicates a resource wants to manage how custom certificate trust is configured.
/// </summary>
/// <param name="callback">The callback used to customize certificate trust for the resource.</param>
public sealed class CertificateTrustConfigurationCallbackAnnotation(Func<CertificateTrustConfigurationCallbackAnnotationContext, Task> callback) : IResourceAnnotation
{
    /// <summary>
    /// Gets the callback to invoke to populate or modify the certificate authority collection.
    /// </summary>
    public Func<CertificateTrustConfigurationCallbackAnnotationContext, Task> Callback { get; } = callback ?? throw new ArgumentNullException(nameof(callback));
}

/// <summary>
/// Context provided to a <see cref="CertificateTrustConfigurationCallbackAnnotation"/> callback.
/// </summary>
public sealed class CertificateTrustConfigurationCallbackAnnotationContext
{
    /// <summary>
    /// Gets the <see cref="DistributedApplicationExecutionContext"/> for this session.
    /// </summary>
    public required DistributedApplicationExecutionContext ExecutionContext { get; init; }

    /// <summary>
    /// Gets the resource to which the annotation is applied.
    /// </summary>
    public required IResource Resource { get; init; }

    /// <summary>

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