SupplyParameterFromTempData

Summary

Provides [SupplyParameterFromTempData] attribute for Blazor SSR components to read and write TempData values, consistent with [SupplyParameterFromQuery] and [SupplyParameterFromForm] patterns.

Motivation

While TempData is accessible via the [CascadingParameter] ITempData approach, many scenarios only need simple read/write of a single value. The attribute-based approach:

• Reduces boilerplate for common use cases
• Provides consistency with existing SupplyParameterFrom* attributes
• Enables automatic two-way binding without manual TempData["key"] access

Design

Attribute

[AttributeUsage(AttributeTargets.Property, AllowMultiple = false, Inherited = true)]
public sealed class SupplyParameterFromTempDataAttribute : CascadingParameterAttributeBase
{
    /// Gets or sets the TempData key. If not specified, the property name will be used.
    public string? Name { get; set; }

    internal override bool SingleDelivery => false;
}

Framework abstractions

Two new types enable attribute-driven cascading value suppliers without building a full ICascadingValueSupplier from scratch:

CascadingParameterSubscription — an abstract base class representing an active subscription to a cascading parameter:

public abstract class CascadingParameterSubscription : IDisposable
{
    public abstract object? GetCurrentValue();
    public abstract void Dispose();
}

CascadingParameterValueProvider<TAttribute> — an internal generic ICascadingValueSupplier that manages subscriptions for a given attribute type. Consumers provide a factory Func<ComponentState, TAttribute, CascadingParameterInfo, CascadingParameterSubscription> and the provider handles CanSupplyValue, GetCurrentValue, Subscribe, and Unsubscribe by delegating to subscription instances.

TryAddCascadingValueSupplier<TAttribute> — a new public extension method on IServiceCollection that registers a CascadingParameterValueProvider<TAttribute> using TryAddEnumerable:

public static IServiceCollection TryAddCascadingValueSupplier<TAttribute>(
    this IServiceCollection serviceCollection,
    Func<IServiceProvider, Func<ComponentState, TAttribute, CascadingParameterInfo, CascadingParameterSubscription>> subscribeFactory)
    where TAttribute : CascadingParameterAttributeBase;

TempDataCascadingValueSupplier

The internal TempDataCascadingValueSupplier class bridges the cascading value provider and the underlying TempData store:

• SetRequestContext(HttpContext) — wires the supplier to the current request.
• CreateSubscription(ComponentState, SupplyParameterFromTempDataAttribute, CascadingParameterInfo) — uses reflection to create a property getter for the decorated property, registers a value callback, and returns a TempDataSubscription.
• GetValue(string, Type) — reads from TempData using Get() semantics (marks for deletion). Handles enum conversion (int → enum), type mismatches (returns null), and deserialization errors (caught, logged, returns null).
• RegisterValueCallback(string, Func<object?>) — stores a getter invoked at persist time. Throws InvalidOperationException for duplicate keys.
• PersistValues(ITempData) — iterates all registered callbacks, reads current property values, and writes them into TempData. Callback exceptions are caught, logged, and do not prevent other keys from being persisted.
• DeleteValueCallback(string) — removes a registered callback (called on component unsubscribe to prevent memory leaks).

Lifecycle

1. Initialize: TempDataCascadingValueSupplier.SetRequestContext(HttpContext) is called during EndpointHtmlRenderer.InitializeStandardComponentServicesAsync, wiring the supplier to the current request.
2. Subscribe: When a component renders, CascadingParameterValueProvider<SupplyParameterFromTempDataAttribute> calls TempDataCascadingValueSupplier.CreateSubscription(), which uses reflection to build a property getter and registers a callback via RegisterValueCallback().
3. GetValue: TempDataSubscription.GetCurrentValue() delegates to TempDataCascadingValueSupplier.GetValue(), which reads from TempData (marks for deletion). Enum values stored as int are converted to the target enum type. Type mismatches and deserialization errors are caught and logged, returning null.
4. Persist: When TempDataService.Save() is called, it invokes TempDataCascadingValueSupplier.PersistValues(tempData), which iterates all registered callbacks, reads the current property values from the components, and writes them back into TempData.
5. Unsubscribe: When a component is disposed, TempDataSubscription.Dispose() calls DeleteValueCallback() to remove the registered callback.

Implementation details

• Case-insensitive callbacks: The internal callback dictionary uses StringComparer.OrdinalIgnoreCase.
• Duplicate key guard: RegisterValueCallback throws InvalidOperationException if a callback is already registered for the same key — multiple components cannot bind to the same TempData key.
• TempData resolution: TempDataCascadingValueSupplier reads TempData from HttpContext.Items[typeof(ITempData)], the same single instance used by the cascading parameter approach.
• Type safety: Enum values stored as int are converted via Enum.ToObject. If the stored value's type is not assignable to the target type, null is returned instead of throwing.
• Error logging: Two log events — TempDataPersistFail (callback exception during persist) and TempDataDeserializeFail (exception during read).

Registration

Automatically enabled when calling AddRazorComponents():

services.TryAddScoped<TempDataCascadingValueSupplier>();
services.TryAddCascadingValueSupplier<SupplyParameterFromTempDataAttribute>(
    sp => sp.GetRequiredService<TempDataCascadingValueSupplier>().CreateSubscription);

Usage

Basic:

@code {
    [SupplyParameterFromTempData]
    public string? Message { get; set; }
}

Custom key:

@code {
    [SupplyParameterFromTempData(Name = "flash_message")]
    public string? Message { get; set; }
}

Form with redirect:

@page "/form"

<p>@Message</p>

<form method="post" @formname="myform" @onsubmit="Submit">
    <AntiforgeryToken />
    <button type="submit">Submit</button>
</form>

@code {
    [SupplyParameterFromTempData]
    public string? Message { get; set; }

    void Submit()
    {
        Message = "Success!";
        NavigationManager.NavigateTo("/form", forceLoad: true);
    }
}

Testing

• Unit tests (TempDataCascadingValueSupplierTest.cs): 17 tests covering GetValue (null cases, case-insensitivity, enum conversion, nullable enum, type mismatches, deserialization errors), RegisterValueCallback (add, duplicate key guard), PersistValues (single/multiple keys, null values, callback exceptions, case-insensitivity), and DeleteValueCallback.
• E2E tests: SupplyParameterFromTempDataReadsAndSavesValues added to both TempDataCookieTest and TempDataSessionStorageTest, verifying read/write roundtrip through the attribute.

Out of scope

• Peek() and Keep() semantics — use [CascadingParameter] ITempData for advanced control
• Custom serialization

Risks

• Callback ordering: When a component uses both TempData["key"] directly and [SupplyParameterFromTempData] for the same key, the final persisted value depends on execution order. Mitigation: Document that mixing approaches for the same key is unsupported.

Fixes #49683
Fixes #65039