microsoft/aspire · error · ArgumentNullException

Object value cannot be null (ArgumentNullException…

Error message

Object value cannot be null (ArgumentNullException: configure)

What it means

The AzureAppServiceWebsiteCustomizationAnnotation constructor wraps a delegate invoked when the WebSite bicep resource is built. It immediately null-checks the delegate and throws ArgumentNullException ('configure') when null is passed, so bad usage fails at annotation construction rather than deep in publishing.

Solutions

Provide a non-null Action<AzureResourceInfrastructure, WebSite> when creating the annotation.
Check for null before constructing/adding the annotation, or skip adding it when no customization is needed.
Fix factory/dependency-injection code so the callback resolves to a real method group or lambda.

Example fix

// before
resource.WithAnnotation(new AzureAppServiceWebsiteCustomizationAnnotation(null!));
// after
resource.WithAnnotation(new AzureAppServiceWebsiteCustomizationAnnotation((infra, site) =>
{
    site.Tags.Add("env", "prod");
}));

Defensive patterns

Strategy: validation

Validate before calling

if (configure is null)
    throw new ArgumentNullException(nameof(configure), "Provide a customization action before creating AzureAppServiceWebsiteCustomizationAnnotation.");
resource.WithAnnotation(new AzureAppServiceWebsiteCustomizationAnnotation(configure));

Type guard

if (configure is not null)
    resource.WithAnnotation(new AzureAppServiceWebsiteCustomizationAnnotation(configure));

Try / catch

try { resource.WithAnnotation(new AzureAppServiceWebsiteCustomizationAnnotation(configure)); }
catch (ArgumentNullException ex) when (ex.ParamName == "configure")
{
    logger.LogError(ex, "Website customization callback was null; skipping annotation.");
}

Prevention

Never pass null delegates to annotation constructors; use lambdas inline.
Null-check callbacks obtained from factories or configuration before adding the annotation.
Favor skipping the annotation entirely over adding one with a no-op or null callback.

When it happens

Trigger: Passing null as the configure delegate, e.g. calling WithAnnotation<AzureAppServiceWebsiteCustomizationAnnotation>(default) or constructing the annotation via a factory/reflection path that supplies a null Action<AzureResourceInfrastructure, WebSite>.

Common situations: Conditional callback selection where a local variable is null; generic annotation-adding helpers that pass through null; typos invoking the wrong overload or a variable that was never assigned.

Related errors

AI-assisted analysis of microsoft/aspire@25830f84bd (2026-09-16). Data as JSON: /api/errors/0ee33b67d75e1539. Report an issue: GitHub.

Appendix: source

Thrown at src/Aspire.Hosting.Azure.AppService/AzureAppServiceWebsiteCustomizationAnnotation.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 Aspire.Hosting.ApplicationModel;
using Azure.Provisioning.AppService;

namespace Aspire.Hosting.Azure;

/// <summary>
/// Represents an annotation for customizing an Azure Web App.
/// </summary>
public sealed class AzureAppServiceWebsiteCustomizationAnnotation(Action<AzureResourceInfrastructure, WebSite> configure)
    : IResourceAnnotation
{
    /// <summary>
    /// Gets the configuration action for customizing the Azure Web App.
    /// </summary>
    public Action<AzureResourceInfrastructure, WebSite> Configure { get; } = configure ?? throw new ArgumentNullException(nameof(configure));
}

View on GitHub (pinned to 25830f84bd)