Blazor code-behind: document partial class approach, retire base-class-only guidance

[Copilot]

The Code-behind section only described the old base class + @inherits model, which was superseded when Blazor added partial class support. Using the base class pattern causes IDEs to surface both the component and its base class as separate entries in IntelliSense/component lists.

Changes

• Preferred pattern (new): partial class with the same name as the component — no @inherits, no Base suffix, members can be private
• Alternative pattern (retained): base class model documented as a secondary option, with a callout about the IDE duplication issue

Preferred partial class pattern

Counter.razor — no @inherits needed:

<h1>Counter</h1>
<p>Current count: @currentCount</p>
<button @onclick="IncrementCount">Click me</button>

Counter.razor.cs — partial, same name, private members work:

public partial class Counter : ComponentBase
{
    private int currentCount = 0;

    private void IncrementCount()
    {
        currentCount++;
    }
}

The Razor compiler already emits a partial class for the component, so both parts merge at compile time without any explicit inheritance wiring.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Code-Behind Syntax</issue_title>
<issue_description>
2 little details on that page. For code-behind, it recommends to add "Base" to the code-behind file and use @inherit.

By doing so, whenever we use the component and type <MyComponent, it appears double, the base class and the actual class.

It should mention that you can have the code-behind with the same name (without Base), mark it as "partial", and you no longer need @inherit, and it connects the file correctly. This avoids having double-components in the list.

Also for Event Handlers, it could be good to mention that @on... syntax is only for built-in events, and that custom events do not use that syntax. Should custom events start with "On" by convention or not? The syntax is

<MyComponent Processing="Component_Processing" />

---

Document Details

⚠ Do not edit this section. It is required for docs.microsoft.com ➟ GitHub issue linking.

• ID: 60c195c0-99f6-64f5-66a3-4e69109dc8ee
• Version Independent ID: 24368808-4b90-f75a-e887-eb72a1c85f5b
• Content: Build reusable UI components with Blazor
• Content Source: docs/architecture/blazor-for-web-forms-developers/components.md
• Product: dotnet-architecture
• Technology: blazor
• GitHub Login: @danroth27
• Microsoft Alias: daroth</issue_description>

<agent_instructions>@danroth27's comment is the correct cause for this issue. Add information on partial class support.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

@danroth27 It sounds like the content wasn't updated after we added partial class support and is using the old base class model for code-behind files.

• Fixes Code-Behind Syntax #20877

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Internal previews

📄 File	🔗 Preview link
docs/architecture/blazor-for-web-forms-developers/components.md	Build reusable UI components with Blazor

[BillWagner]

This LGTM, and is ready for final review.

[Copilot]

Pull request overview

This PR updates the Blazor Web Forms migration guidance to reflect the modern code-behind pattern for Razor components, documenting the preferred partial class approach and demoting the older base-class-only (@inherits) guidance.

Changes:

• Document the preferred code-behind approach using a partial class with the same name as the component (no @inherits, private members allowed).
• Retain the base class + @inherits approach as an alternative, and add a callout about IDE duplication in IntelliSense/component lists.
• Update article metadata (ms.date, and add ai-usage).

[danroth27]

LGTM