Add BL0008 diagnostic article (#36361)

Author: guardrex

aspnetcore/diagnostics/bl0008.md

@@ -0,0 +1,90 @@
+---
+title: "BL0008: Component parameters should be auto properties"
+ai-usage: ai-assisted
+description: "Learn about analysis rule BL0008: Component parameters should be auto properties"
+author: guardrex
+monikerRange: '>= aspnetcore-8.0'
+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 non-default property initializer.
+
+## Rule description
+
+The property initializer can be overwritten with `null` during form submission, causing the `EditForm` to fail with the following exception:
+
+> :::no-loc text="InvalidOperationException: EditForm requires either a Model parameter, or an EditContext parameter, please provide one of these.":::
+
+The analyzer warns developers when this pattern is detected, while safely ignoring default value initializers (`null`, `null!`, `default`, `default!`) that don't cause the same issue.
+
+Consider the following `Input` form model with a property initializer:
+
+```csharp
+[SupplyParameterFromForm]
+public InputModel Input { get; set; } = new();
+```
+
+The analyzer reports the following warning, where the `{COMPONENT}` placeholder is the component type name:
+
+> :::no-loc text="Property '{COMPONENT}.Input' has [SupplyParameterFromForm] and a property initializer. This can be overwritten with null during form posts.":::
+
+<span aria-hidden="true">✔️</span> Safe patterns that are ignored by analyzer:
+
+```csharp
+[SupplyParameterFromForm]
+public InputModel Input { get; set; } = default!;
+
+[SupplyParameterFromForm]
+public InputModel Input { get; set; } = null!;
+```
+
+## 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 `Input` model for the form is decorated with `[SupplyParameterFromForm]` and has a property initializer:
+
+```razor
+<EditForm Model="Input" OnSubmit="Submit" FormName="Starship1">
+    ...
+</EditForm>
+
+@code {
+    [SupplyParameterFromForm]
+    private InputModel Input { get; set; } = new();
+
+    private void Submit()
+    {
+        ...
+    }
+
+    public class InputModel
+    {
+        public string? Id { get; set; }
+    }
+}
+```
+
+To fix the violation, the initialization code for the `Input` 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 `Input` a [nullable reference type (NRT)](xref:migration/50-to-60#nullable-reference-types-nrts-and-net-compiler-null-state-static-analysis):
+
+```csharp
+[SupplyParameterFromForm]
+private InputModel? Input { get; set; }
+
+protected override void OnInitialized() => Input ??= new();
+```
+
+## When to suppress warnings
+
+Do not suppress a warning from this rule.

aspnetcore/diagnostics/code-analysis.md

@@ -50,6 +50,7 @@ Diagnostic ID:
 * [BL0005](xref:diagnostics/bl0005)
 * [BL0006](xref:diagnostics/bl0006)
 * [BL0007](xref:diagnostics/bl0007)
+* [BL0008](xref:diagnostics/bl0008)
 * [MVC1000](xref:diagnostics/mvc1000)
 * [MVC1001](xref:diagnostics/mvc1001)
 * [MVC1002](xref:diagnostics/mvc1002)

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