What causes "configure or configureSlot must be provided." in microsoft/aspire?

Pass a configure lambda: b => { ... } describing the website settings you want.

microsoft/aspire · error · ArgumentException

configure or configureSlot must be provided.

Error message

configure or configureSlot must be provided.

What it means

PublishAsAzureAppServiceWebsite accepts two optional callbacks: one for the base website configuration and one for slot-specific configuration. If both are null there is nothing to configure, so the extension throws an ArgumentException rather than silently doing nothing.

Solutions

Pass a configure lambda: b => { ... } describing the website settings you want.
Pass configureSlot instead if you only need per-slot customization.
Remove the PublishAsAzureAppServiceWebsite call entirely if no customization is needed.
If configure is conditionally built, fall back to an empty lambda b => { } instead of null.

Example fix

// before
builder.PublishAsAzureAppServiceWebsite(null, null);
// after
builder.PublishAsAzureAppServiceWebsite(website => { website.Template.SetProperty("siteConfig", new { http20Enabled = true }); });

Defensive patterns

Strategy: validation

Validate before calling

if (configure is null && configureSlot is null) throw new ArgumentException("Provide configure or configureSlot before calling PublishAsAzureAppServiceWebsite.");

Try / catch

try { builder.PublishAsAzureAppServiceWebsite(configure, configureSlot); } catch (ArgumentException ex) { logger.LogError(ex, "PublishAsAzureAppServiceWebsite requires at least one callback"); }

Prevention

Never pass null for both callbacks; use an empty lambda when no customization is needed.
Wrap the extension in a helper that enforces a non-null callback.
Review fluent chains after refactors that remove lambda bodies.

When it happens

Trigger: Calling builder.PublishAsAzureAppServiceWebsite() with no arguments at all, or explicitly passing null for both configure and configureSlot, on an IComputeResource.

Common situations: Copy-pasting the call from docs and omitting the lambda; generating the call from a templating tool that leaves the configure argument unfilled; refactoring that removed the lambda body but kept a null argument.

Understand the failure class

Background: "missing required argument" and "the following required arguments were not provided": what required-argument errors mean and how to fix them — this error's family across 20 libraries.

Related errors

AI-assisted analysis of microsoft/aspire@25830f84bd (2026-09-16). Data as JSON: /api/errors/b602a6782a664a1c. Report an issue: GitHub.

Appendix: source

Thrown at src/Aspire.Hosting.Azure.AppService/AzureAppServiceComputeResourceExtensions.cs:44

    /// builder.AddProject&lt;Projects.Api&gt;("name").PublishAsAzureAppServiceWebsite((infrastructure, app) =>
    /// {
    ///     // Configure the App Service WebSite resource here
    /// });
    /// </code>
    /// </example>
    /// </remarks>
    /// <ats-remarks />
    [AspireExport]
    public static IResourceBuilder<T> PublishAsAzureAppServiceWebsite<T>(this IResourceBuilder<T> builder,
        Action<AzureResourceInfrastructure, WebSite>? configure = null,
        Action<AzureResourceInfrastructure, WebSiteSlot>? configureSlot = null)
        where T : IComputeResource
    {
        ArgumentNullException.ThrowIfNull(builder);

        if (configure == null && configureSlot == null)
        {
            throw new ArgumentException("configure or configureSlot must be provided.");
        }

        if (!builder.ApplicationBuilder.ExecutionContext.IsPublishMode)
        {
            return builder;
        }

        builder.ApplicationBuilder.AddAzureAppServiceInfrastructureCore();

        if (configure != null)
        {
            builder = builder.WithAnnotation(new AzureAppServiceWebsiteCustomizationAnnotation(configure));
        }

        if (configureSlot != null)
        {
            builder = builder.WithAnnotation(new AzureAppServiceWebsiteSlotCustomizationAnnotation(configureSlot));
        }

View on GitHub (pinned to 25830f84bd)