dotnet/aspnetcore · error · InvalidOperationException

The Router component requires a value for the parameter AppA

Error message

The Router component requires a value for the parameter AppAssembly.

What it means

Thrown in Router.SetParametersAsync. AppAssembly (marked [EditorRequired]) is the assembly the Router scans for [Route]/@page components. If it is null after parameters are set, the Router has nothing to scan and cannot build a route table, so it refuses to render.

Source

Thrown at src/Components/Components/src/Routing/Router.cs:127

        _locationAbsolute = NavigationManager.Uri;
        NavigationManager.LocationChanged += OnLocationChanged;
        NavigationManager.OnNotFound += OnNotFound;
        RoutingStateProvider = ServiceProvider.GetService<IRoutingStateProvider>();

        if (HotReloadManager.IsSupported)
        {
            HotReloadManager.Default.OnDeltaApplied += ClearRouteCaches;
        }
    }

    /// <inheritdoc />
    public async Task SetParametersAsync(ParameterView parameters)
    {
        parameters.SetParameterProperties(this);

        if (AppAssembly == null)
        {
            throw new InvalidOperationException($"The {nameof(Router)} component requires a value for the parameter {nameof(AppAssembly)}.");
        }

        // Found content is mandatory, because even though we could use something like <RouteView ...> as a
        // reasonable default, if it's not declared explicitly in the template then people will have no way
        // to discover how to customize this (e.g., to add authorization).
        if (Found == null)
        {
            throw new InvalidOperationException($"The {nameof(Router)} component requires a value for the parameter {nameof(Found)}.");
        }

        if (NotFoundPage != null)
        {
#pragma warning disable CS0618 // Type or member is obsolete
            if (NotFound != null)
            {
                throw new InvalidOperationException($"Setting {nameof(NotFound)} and {nameof(NotFoundPage)} properties simultaneously is not supported. Use either {nameof(NotFound)} or {nameof(NotFoundPage)}.");
            }
#pragma warning restore CS0618 // Type or member is obsolete

View on GitHub (pinned to 294cab2f9b)

Solutions

1. Set AppAssembly on the Router: <Router AppAssembly="@typeof(Program).Assembly">.
2. If pages live in another assembly, also pass AdditionalAssemblies.
3. For Blazor Web App with .Client project, point AppAssembly at the client project's assembly (e.g., @typeof(_Imports).Assembly or the client Program).

Example fix

<!-- before -->
<Router Found="@Found" NotFoundPage="@typeof(NotFound)">

<!-- after -->
<Router AppAssembly="@typeof(Program).Assembly"
       Found="@Found" NotFoundPage="@typeof(NotFound)">

// In App.razor, always set AppAssembly; guard against null at the markup level.
<Router AppAssembly="@(AppAssembly ?? throw new InvalidOperationException(nameof(Router.AppAssembly)))"
        Found="@Found" ...>
@code { static Assembly AppAssembly => typeof(Program).Assembly; }

When it happens

Trigger: Rendering <Router> without an AppAssembly parameter, or binding it to a null expression. The standard usage is <Router AppAssembly="@typeof(Program).Assembly" ...>.

Common situations: A hand-written App.razor where AppAssembly was deleted or never added; binding AppAssembly to an expression that evaluates to null; splitting into Client/Server projects and forgetting to set AppAssembly on the Router in one of them.

Related errors

• The Router component requires a value for the parameter Foun
• Setting NotFound and NotFoundPage properties simultaneously
• The type {NotFoundPage.FullName} does not have a Microsoft.A
• The RouteView component requires a non-null value for the pa
• The following routes are ambiguous: '{existingText}' in '{ex

AI-assisted analysis of dotnet/aspnetcore@294cab2f9b (2026-08-06). Data as JSON: /api/errors/e9ef0c0f847dd2ca. Report an issue: GitHub.