The Engineering Lead’s Guide to Migrating Silverlight Apps Before They Break

Your Silverlight application is a ticking clock. Since Microsoft officially ended support in October 2021, the infrastructure supporting these apps—specifically Internet Explorer 11 and the Silverlight browser plugin—has been systematically dismantled. For the Engineering Lead, this isn't just a technical debt issue; it’s a business continuity crisis. If your core enterprise workflows still rely on a

file, you are one browser update away from a total system blackout.

The traditional path of manual "archaeology"—digging through undocumented C# and XAML to reconstruct business logic—is why 70% of legacy rewrites fail or exceed their timelines.

TL;DR: Stop manual rewrites; use Visual Reverse Engineering with Replay to extract Silverlight workflows into documented React components, reducing migration timelines from 18 months to a few weeks.

The Silverlight Migration Trap: Why Traditional Rewrites Fail#

Most Engineering Leads approach a Silverlight migration by assigning a team of developers to read the original source code and "re-implement" it in React or Angular. This is a mistake. Silverlight applications are often 10-15 years old. The original architects are gone, the documentation is non-existent (67% of legacy systems lack any useful documentation), and the business logic is tightly coupled with deprecated patterns like WCF RIA Services.

When you attempt a "Big Bang" rewrite, you aren't just building a new UI; you are trying to rediscover business rules hidden in thousands of lines of XAML. This process typically takes 40 hours per screen when done manually.

⚠️ Warning: Relying on "IE Mode" in Microsoft Edge is a temporary band-aid, not a strategy. It does not solve the underlying security vulnerabilities or the inability to hire developers who want to work on a dead platform.

The Engineering Lead’s Strategy for Visual Reverse Engineering#

Visual Reverse Engineering shifts the focus from the code to the workflow. Instead of reading 15-year-old C#, you record the application as it is used by an actual human. Replay captures these interactions and transforms the visual state and underlying data structures into modern, production-ready React code.

Step 1: Workflow Inventory and Prioritization#

Don't migrate everything at once. Use a "Value vs. Complexity" matrix. Most Silverlight apps are bloated with features that are no longer used.

1. •Identify high-traffic workflows: Record the paths users take 80% of the time.
2. •Map Data Dependencies: Identify which WCF or SOAP services the Silverlight app calls.
3. •Audit Technical Debt: Use Replay’s Technical Debt Audit feature to see which parts of the legacy UI are redundant.

Step 2: Recording the "Source of Truth"#

In a Silverlight app, the UI state is often the only place where the current business logic is accurately represented. By using Replay to record a user performing a task—such as processing an insurance claim or managing a manufacturing floor—you create a "Video as Source of Truth."

Replay doesn't just record pixels; it records the DOM (or the Silverlight equivalent via accessibility layers), the network calls, and the state transitions.

💡 Pro Tip: Record "edge case" workflows specifically. These are the scenarios that developers usually miss during manual rewrites, leading to post-launch bugs.

Step 3: Automated Extraction to React#

Once the workflow is recorded, Replay’s AI Automation Suite analyzes the recording to generate React components that mirror the legacy functionality but utilize modern best practices (Tailwind CSS, TypeScript, and Functional Components).

typescript
// Example: A complex Silverlight DataGrid extracted into a Modern React Component
// Generated by Replay Visual Reverse Engineering

import React, { useState, useEffect } from 'react';
import { Table, Badge, Button } from '@/components/ui'; // From your Replay Library
import { useLegacyData } from '@/hooks/useLegacyData';

interface ClaimRecord {
  id: string;
  status: 'Pending' | 'Approved' | 'Rejected';
  amount: number;
  submittedDate: string;
}

export const ClaimManagementDashboard: React.FC = () => {
  const { data, loading, error } = useLegacyData('/api/claims/v1');
  const [selectedClaim, setSelectedClaim] = useState<ClaimRecord | null>(null);

  // Business logic preserved: Validation logic extracted from Silverlight 'Submit' event
  const handleApprove = async (id: string) => {
    if (window.confirm("Confirm approval based on legacy validation rules?")) {
      await fetch(`/api/claims/${id}/approve`, { method: 'POST' });
    }
  };

  if (loading) return <div className="animate-pulse">Loading Legacy State...</div>;

  return (
    <div className="p-6 bg-slate-50 min-h-screen">
      <h1 className="text-2xl font-bold mb-4">Claims Processing</h1>
      <Table>
        <thead>
          <tr>
            <th>ID</th>
            <th>Status</th>
            <th>Amount</th>
            <th>Actions</th>
          </tr>
        </thead>
        <tbody>
          {data.map((claim: ClaimRecord) => (
            <tr key={claim.id} className="hover:bg-white transition-colors">
              <td>{claim.id}</td>
              <td>
                <Badge variant={claim.status === 'Approved' ? 'success' : 'warning'}>
                  {claim.status}
                </Badge>
              </td>
              <td>${claim.amount.toLocaleString()}</td>
              <td>
                <Button onClick={() => handleApprove(claim.id)}>Approve</Button>
              </td>
            </tr>
          ))}
        </tbody>
      </Table>
    </div>
  );
};

Step 4: API Contract Generation#

Silverlight apps often communicate with backends via complex XML-based protocols. Replay intercepts these calls during the recording phase and generates modern API contracts (OpenAPI/Swagger) and E2E tests.

💰 ROI Insight: Manual screen reconstruction takes ~40 hours. Replay reduces this to ~4 hours. For an enterprise app with 50 screens, that is a saving of 1,800 engineering hours—roughly $225,000 in labor costs alone.

Bridging the Documentation Gap#

The greatest risk in any migration is the "Black Box" effect. You know what the app does, but you don't know how it does it. Replay’s Blueprints feature creates a living architecture map of your legacy system.

1. •Flows: Visualizes how a user moves from Screen A to Screen B.
2. •Library: Automatically identifies recurring UI patterns (buttons, grids, modals) and organizes them into a standardized Design System.
3. •Technical Debt Audit: Flags deprecated logic and suggests modern alternatives (e.g., replacing a custom Silverlight validation engine with Zod or Yup).

typescript
// Replay-Generated API Contract for Legacy WCF Service
// This allows your backend team to build the shim/wrapper immediately

/**
 * @description Extracted from Silverlight 'GetOrderDetails' workflow
 * @service OrderProcessingService.svc
 */
export interface LegacyOrderResponse {
  OrderGUID: string; // Mapping: string
  Total_Amount: number; // Mapping: decimal
  Is_Taxable: boolean; // Mapping: bool
  LineItems: Array<{
    SKU: string;
    Quantity: number;
    UnitPrice: number;
  }>;
}

Addressing the Security and Compliance Hurdles#

For Engineering Leads in Financial Services, Healthcare (HIPAA), or Government, "cloud-only" tools are often a non-starter. Legacy modernization involves sensitive data.

Replay is built for regulated environments:

• •SOC2 Type II Compliant
• •HIPAA-ready for healthcare data extraction
• •On-Premise Deployment: Run the entire extraction engine within your own VPC or air-gapped environment.
• •PII Masking: Automatically redact sensitive user information during the recording and extraction phase.

📝 Note: When recording Silverlight apps in highly regulated sectors, Replay’s masking engine ensures that no actual PII (Personally Identifiable Information) leaves your secure environment, even if you are using the AI suite for code generation.

The Cost of Inaction: A $3.6 Trillion Problem#

Global technical debt is estimated at $3.6 trillion. Silverlight applications represent a significant portion of this in the enterprise sector. Every month you delay migration, the "knowledge rot" increases. Developers who understand the original system retire, and the cost of hiring specialized contractors to maintain the legacy environment skyrockets.

By moving from an 18-month "Big Bang" rewrite to a 4-week "Visual Extraction" with Replay, you don't just save money—you eliminate the risk of a catastrophic failure.

Implementation Checklist for Engineering Leads:#

• • Identify the "Ticking Clock": When does your current browser support for Silverlight officially end in your organization?
• • Run a Pilot: Select one high-complexity screen and use Replay to extract it. Compare the output to a manual rewrite attempt.
• • Define the Stack: Target React/TypeScript with a modern component library (like Shadcn UI or MUI).
• • Automate Testing: Use Replay’s generated E2E tests to ensure the new React app behaves exactly like the Silverlight original.

Frequently Asked Questions#

How does Replay handle complex Silverlight business logic?#

Replay records the inputs, outputs, and state changes of every user action. While it doesn't "read" the C# source code directly, it observes the results of the logic. It then generates React functional components that replicate these state transitions. For complex backend logic, Replay generates the API contracts required to keep the logic on the server while modernizing the client.

Can Replay migrate Silverlight to platforms other than React?#

While Replay is optimized for React (the industry standard for enterprise modernization), the extracted Blueprints and API contracts can be used to accelerate development in Angular, Vue, or even Flutter.

What is the average time savings?#

We see an average of 70% time savings. A project that would typically take 18 months of manual engineering can often be completed in 3 to 4 months, with the initial "Visual Extraction" of the UI taking only days or weeks.

Does Replay require access to our source code?#

No. Replay works by observing the application at runtime. This is particularly useful for legacy systems where the source code might be lost, incomplete, or impossible to build on modern machines.

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Ready to modernize without rewriting? Book a pilot with Replay - see your legacy screen extracted live during the call.