microsoft/aspire · error · ArgumentNullException

ArgumentNullException for parameter 'resource' (resource is…

Error message

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

What it means

ConnectionStringRedirectAnnotation redirects connection-string resolution from one resource to another; it requires a non-null target resource implementing IResourceWithConnectionString. Passing null throws ArgumentNullException for parameter 'resource'.

Solutions

Pass the actual resource, e.g. new ConnectionStringRedirectAnnotation(postgres)
Ensure the referenced resource is created before adding the annotation and implements IResourceWithConnectionString

Example fix

// before
builder.Annotations.Add(new ConnectionStringRedirectAnnotation(null));
// after
var db = builder.AddPostgres("db");
builder.Annotations.Add(new ConnectionStringRedirectAnnotation(db.Resource));

Defensive patterns

Strategy: validation

Validate before calling

if (resource is null) throw new InvalidOperationException("Target resource must be created before adding ConnectionStringRedirectAnnotation.");

Type guard

bool HasConnectionString(IResource? r) => r is IResourceWithConnectionString;

Prevention

Create the referenced resource before adding the annotation
Confirm the resource implements IResourceWithConnectionString
Avoid casts to IResourceWithConnectionString without a null check

When it happens

Trigger: new ConnectionStringRedirectAnnotation(null) or passing a resource reference that is null (e.g. referencing a resource variable assigned later or a failed cast).

Common situations: Wiring annotations before the referenced resource is created; referencing a resource that does not implement IResourceWithConnectionString and being stored as null after a cast pattern.

Related errors

AI-assisted analysis of microsoft/aspire@25830f84bd (2026-09-16). Data as JSON: /api/errors/ba3772c0d5405119. Report an issue: GitHub.

Appendix: source

Thrown at src/Aspire.Hosting/ApplicationModel/ConnectionStringRedirectAnnotation.cs:15

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

namespace Aspire.Hosting.ApplicationModel;

/// <summary>
/// Links to a resource that implements <see cref="IResourceWithConnectionString"/> that can be used by the containing resource to acquire a connection string.
/// </summary>
/// <param name="resource">Resource that </param>
public class ConnectionStringRedirectAnnotation(IResourceWithConnectionString resource) : IResourceAnnotation
{
    /// <summary>
    /// Callback to acquire connection string.
    /// </summary>
    public IResourceWithConnectionString Resource { get; } = resource ?? throw new ArgumentNullException(nameof(resource));
}

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