Add BL0008 diagnostic article

Author: guardrex

aspnetcore/diagnostics/bl0008.md

@@ -0,0 +1,64 @@
+---
+title: "BL0008: Component parameters should be auto properties"
+description: "Learn about analysis rule BL0008: Component parameters should be auto properties"
+author: guardrex
+monikerRange: '>= aspnetcore-3.1'
+ms.author: wpickett
+ms.date: 11/17/2025
+uid: diagnostics/bl0008
+---
+# BL0008: Component parameters should be auto properties
+
+|                                     | Value        |
+| -                                   | -            |
+| **Rule ID**                         | BL0008       |
+| **Category**                        | Usage        |
+| **Fix is breaking or non-breaking** | Non-breaking |
+
+## Cause
+
+A property has the [`[SupplyParameterFromForm]` attribute](xref:Microsoft.AspNetCore.Components.SupplyParameterFromFormAttribute) and is initialized with a property initializer. 
+
+## Rule description
+
+The property can be overwritten with `null` when the component receives parameters, causing it to lose its initial value.
+
+## How to fix violations
+
+To ensure the initialized value isn't overwritten, move the initialization to one of the [`OnInitialized{Async}` lifecycle methods](xref:blazor/components/lifecycle#component-initialization-oninitializedasync).
+
+Consider the following Razor component that generates a BL0008 code analysis warning because the `Model` for the form is decorated with `[SupplyParameterFromForm]` and has a property initializer:
+
+```razor
+<EditForm Model="Model" OnSubmit="Submit" FormName="Starship1">
+    ...
+</EditForm>
+
+@code {
+    [SupplyParameterFromForm]
+    private Starship Model { get; set; } = new();
+
+    private void Submit()
+    {
+        ...
+    }
+
+    public class Starship
+    {
+        public string? Id { get; set; }
+    }
+}
+```
+
+To fix the violation, the initialization code for the `Model` property is moved to the [`OnInitialized` lifecycle method](xref:blazor/components/lifecycle#component-initialization-oninitializedasync), leaving the property as an [automatically implemented property (*auto property*)](/dotnet/csharp/programming-guide/classes-and-structs/auto-implemented-properties). The following change also makes `Model` a [nullable reference types (NRT)](xref:migration/50-to-60#nullable-reference-types-nrts-and-net-compiler-null-state-static-analysis):
+
+```csharp
+[SupplyParameterFromForm]
+private Starship? Model { get; set; }
+
+protected override void OnInitialized() => Model ??= new();
+```
+
+## When to suppress warnings
+
+Do not suppress a warning from this rule.

aspnetcore/toc.yml

@@ -1499,6 +1499,8 @@ items:
         uid: diagnostics/bl0006
       - name: BL0007
         uid: diagnostics/bl0007
+      - name: BL0008
+        uid: diagnostics/bl0008
       - name: MVC1000
         uid: diagnostics/mvc1000
       - name: MVC1001