ASCX User Control → Blazor Component Migration

Overview

ASP.NET Web Forms User Controls (.ascx files) are reusable markup fragments with code-behind. They map directly to Blazor .razor components with .razor.cs code-behind files.

The webforms-to-blazor CLI tool handles L1 conversion automatically. This skill guides L2 repair for common patterns that require contextual understanding.

How the CLI Handles ASCX Files

Critical Rules

1. Preserve the Code-Behind

ASCX code-behind files contain business logic that should compile unchanged. The CLI:

• Strips System.Web.UI.* usings
• Adds using BlazorWebFormsComponents.CustomControls;
• Preserves UserControl base class (now mapped to BWFC's UserControl)
• Injects @inherits UserControl into the .razor file

Do NOT rewrite code-behind logic. Fix compile errors by adding missing shims or stubs.

2. Properties Become Parameters

Web Forms exposes ASCX properties to parent pages. In Blazor, these become [Parameter] properties:

// Web Forms (worked without attribute)
public string Title { get; set; }

// Blazor (requires [Parameter])
[Parameter]
public string Title { get; set; }

The CLI's ParameterAttributeTransform handles this automatically for public properties.

3. Events Become EventCallback

ASCX controls that raise events to parent pages:

// Web Forms
public event EventHandler ItemSelected;
protected void OnItemSelected(EventArgs e) => ItemSelected?.Invoke(this, e);

// Blazor
[Parameter]
public EventCallback<EventArgs> ItemSelected { get; set; }
protected async Task OnItemSelected(EventArgs e) => await ItemSelected.InvokeAsync(e);

4. Tag Prefix Registration Is Eliminated

Web Forms requires tag prefix registration in Web.config or <%@ Register %>:

<!-- Web.config -->
<add tagPrefix="uc" src="~/Controls/StatusPanel.ascx" tagName="StatusPanel" />

<!-- Usage -->
<uc:StatusPanel runat="server" Title="Dashboard" />

In Blazor, components are referenced directly by name (no prefix, no registration):

<StatusPanel Title="Dashboard" />

The CLI strips <%@ Register %> directives and removes tag prefixes automatically.

5. Partial Class Base Must Match

The .razor file and .razor.cs file form a partial class. Their base class must agree:

@* In the .razor file *@
@inherits UserControl

// In the .razor.cs file
public partial class MyControl : UserControl { }

If the CLI misses the @inherits directive, you'll get CS0263. Add it manually.

Common L2 Repair Patterns

Pattern: ASCX with LoadControl/Dynamic Loading

Web Forms can load user controls dynamically:

var ctrl = (StatusPanel)LoadControl("~/Controls/StatusPanel.ascx");
ctrl.Title = "Dynamic";
PlaceHolder1.Controls.Add(ctrl);

Blazor equivalent: Use RenderFragment or DynamicComponent:

<DynamicComponent Type="typeof(StatusPanel)"
                  Parameters="@(new Dictionary<string, object> { ["Title"] = "Dynamic" })" />

Pattern: ASCX with FindControl

Code-behind that uses FindControl to locate child controls works unchanged through the BWFC runtime:

// This works in Blazor via BWFC's FindControl runtime
var lbl = (Label)FindControl("lblStatus");
lbl.Text = "Updated";

Pattern: ASCX Exposing ChildControl Properties

Web Forms controls often wrap and expose child control properties:

// Web Forms pattern
public string StatusText
{
    get { return lblStatus.Text; }
    set { lblStatus.Text = value; }
}

In Blazor, this still works if lblStatus is resolved via FindControl or @ref. The BWFC runtime supports both patterns.

Pattern: Page_Load in User Controls

User control lifecycle methods auto-wire through BWFC's virtual methods:

// Works unchanged — BaseWebFormsComponent provides virtual Page_Load
protected override void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
        BindData();
    }
}

File Structure After Migration

// Before (Web Forms)
Controls/
  StatusPanel.ascx          ← Markup
  StatusPanel.ascx.cs       ← Code-behind
  StatusPanel.ascx.designer.cs  ← Auto-generated (discarded)

// After (Blazor)
Components/Controls/        (or wherever the CLI places them)
  StatusPanel.razor         ← Converted markup + @inherits UserControl
  StatusPanel.razor.cs      ← Preserved code-behind (usings updated)

Checklist

For each ASCX file in the migration:

• .razor file has @inherits UserControl (or appropriate base)
• .razor file has @using BlazorWebFormsComponents.CustomControls
• Code-behind compiles with updated usings
• Public properties have [Parameter] attribute
• Events converted to EventCallback<T>
• Page_Load / Page_Init overrides work (virtual methods)
• FindControl calls resolve at runtime
• Parent pages reference component by name (no tag prefix)
• Component renders expected HTML output

Reference

• Custom WebControl Migration Guide
• FindControl Migration Guide
• User Controls Migration Guide
• BWFC CustomControls namespace