What causes "Value cannot be null. (Parameter 'resource')" in microsoft/aspire?

Pass a valid IResourceWithEndpoints instance when creating the annotation; resolve the resource from the builder before constructing it.

microsoft/aspire · error · ArgumentNullException

Value cannot be null. (Parameter 'resource')

Error message

Value cannot be null. (Parameter 'resource')

What it means

AzureFrontDoorOriginAnnotation stores the resource that acts as a Front Door origin and its constructor guards against null with a standard ArgumentNullException throw-expression. This fails fast when internal code (or user code via reflection/extension APIs) constructs the annotation without a resource.

Solutions

Pass a valid IResourceWithEndpoints instance when creating the annotation; resolve the resource from the builder before constructing it.
Null-check the resource at the call site and surface a clearer error before constructing the annotation.
Update custom extension code to use WithReference-style APIs instead of constructing annotations directly.

Example fix

// before
var annotation = new AzureFrontDoorOriginAnnotation(resource!); // resource may be null

// after
if (resource is null)
{
    throw new InvalidOperationException("Origin resource was not resolved.");
}
var annotation = new AzureFrontDoorOriginAnnotation(resource);

Defensive patterns

Strategy: type-guard

Validate before calling

if (resource is null) throw new InvalidOperationException("Origin resource must be resolved before creating AzureFrontDoorOriginAnnotation.");

Type guard

static bool IsValidOriginResource(object? r) => r is IResourceWithEndpoints;

Try / catch

try { var ann = new AzureFrontDoorOriginAnnotation(resource); } catch (ArgumentNullException ex) when (ex.ParamName == "resource") { /* resolve resource from builder first */ }

Prevention

Never construct resource annotations directly; use builder extension APIs.
Null-check builder.Resource before annotation construction.
Use nullable reference types so NRT warnings surface null resources early.

When it happens

Trigger: Constructing new AzureFrontDoorOriginAnnotation(null) directly, or calling an internal helper that forwards a null resource (e.g., a generic origin API that failed to resolve an IResourceWithEndpoints).

Common situations: Writing custom Front Door extension methods that pass a possibly-null resource into the annotation; test code building annotations manually; a builder API returning null for the target resource.

Related errors

AI-assisted analysis of microsoft/aspire@25830f84bd (2026-09-16). Data as JSON: /api/errors/9c6b023659a1bbd7. Report an issue: GitHub.

Appendix: source

Thrown at src/Aspire.Hosting.Azure.FrontDoor/AzureFrontDoorOriginAnnotation.cs:16

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

using Aspire.Hosting.ApplicationModel;

namespace Aspire.Hosting.Azure;

/// <summary>
/// Represents an origin to be added to an Azure Front Door resource.
/// </summary>
internal sealed class AzureFrontDoorOriginAnnotation(IResourceWithEndpoints resource) : IResourceAnnotation
{
    /// <summary>
    /// Gets the resource for this origin.
    /// </summary>
    public IResourceWithEndpoints Resource { get; } = resource ?? throw new ArgumentNullException(nameof(resource));
}

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