Completion of rewriting a large invoicing program project (Windows) in C#.net

Budget: 10000 PLN Deadline: 14 days

I would treat the amount of 10,000 PLN as !!the first stage of entry and stabilization!! for 14 days, rather than a promise to close the entire missing 30 percent without insight into the code. With a program for invoices, KSeF, JPK, databases, and DevExpress, such a budget makes sense as a controlled stage - code audit, gap mapping, critical fixes, comparative tests of the old and new versions, and a plan to close everything.

First, I would approach the project from the perspective of behavioral compliance - screen by screen, scenario by scenario, with a list of differences between the old application and the C#/.NET version. I can work with WPF or WinForms using DevExpress, organize the missing modules, create regression tests, and use artificial intelligence tools to analyze a large codebase, but accounting decisions need to be validated on hard cases, because paper will accept anything, but accounting software will not =)

> Question 1 - is the old version in Visual Basic 6.0 or another technology?
> Question 2 - do you have a list of modules with missing functions and errors, or do we need to recreate it first by comparing the two versions?

On your side, a repository, startup instructions, a test database, the old working version, sample documents, and an indication of the most important user paths will be needed. It makes the most sense to start with a short technical entry, because otherwise, it’s easy to lead the migration to a false end, and then discover differences in taxes, prints, or exports.

> Similar experience Ingello > https://business.ingello.com/platforma - a complex corporate platform with modules and roles
> https://business.ingello.com/forma-bpm - business processes, forms, and operational logic
> https://business.ingello.com/prime-eva - production, records, integrations, and systematic data organization

More about the approach to such projects - https://systems-fl.ingello.com Expand

Budget: 10000 PLN Deadline: 30 days

Good morning!

You are looking for a programmer to complete the migration of a large accounting program (VAT Invoice) from old technology to C# / .NET (WPF / DevExpress) and ensure that the new version fully matches the logic of the old one.

My profile is advanced engineering and migration of legacy systems. I have experience in creating desktop applications (C# / WPF), and most importantly — my strongest asset is what you expect: advanced use of AI tools.

I do not use AI just for "autocomplete." I work in continuous pair programming with advanced AI agents (Cursor, custom RAG solutions). Because of this:

I quickly analyze and understand "spaghetti code" in old technologies (e.g., VB6).
Completing someone else's code (the last 30% of the project, which is usually the most difficult) takes me significantly less time. With AI, I rapidly write test scripts that compare the output of the old version with the new one (to ensure identical tax-accounting logic).
I have experience in building complex billing and invoicing systems (logistics and transportation applications), so the domain of invoicing is not unfamiliar to me.

I would love to learn more about the architecture of this project (how the code that is 70% ready is organized). Feel free to contact me via messages. Work through Safe. Expand

Budget: 10000 PLN Deadline: 10 days

Good day. The work is generally clear. I will skillfully transfer the project from Basic to C#\WPF, preserving all current functionality and capabilities, as well as the presentation appearance. I am ready to start right here and now. If you are interested in my candidacy, I look forward to discussing the details of the work in private messages. Have a nice day.

Budget: 10000 PLN Deadline: 25 days

Hello! I have relevant experience that will allow me to effectively contribute to your project.

Why I am suitable:

Stack: I actively work with WPF and DevExpress. I am currently in the process of developing a complex desktop project on WPF using these same libraries.

Migration and reverse: I have experience working with legacy code and restoring functionality through analysis and decompilation. I understand how important it is to maintain 100% identity of logic when migrating a project to .NET.

Working with AI: I use artificial intelligence tools daily for code analysis, writing unit tests, and optimizing refactoring, which significantly speeds up migration.
I am ready to join the project, conduct an analysis of the structure, and take on part of the tasks related to migrating functionality.

Could you let me know if you have a well-established unit testing system to compare the results of the old and new versions, or will I need to refine it? Expand

Budget: 10000 PLN Deadline: 1 day

Good day
I have extensive experience in C# .net development
Write to me

Budget: 10000 PLN Deadline: 14 days

Good morning,

Thank you for posting the offer.

We are Stellar Logic Sp. z o.o., a company specializing in .NET technologies. On a daily basis, we carry out projects related to the development of desktop and web applications, as well as modernization and migration of systems based on older technologies.

Your project seems very interesting, and we would be happy to discuss the possibility of collaboration. We believe that our experience in the .NET ecosystem will allow us to effectively support you in completing the application migration.

We would appreciate your contact to discuss the project details and expectations regarding collaboration.

Budget: 10000 PLN Deadline: 5 days

Hello. I am ready to take on your order. I have extensive experience in the technologies you need. I have been working with C# for over 6 years. I am also skilled in using AI agents such as Claude, Codex, and Cursor, which will greatly assist in your project. I am ready to start working today.

Budget: 10000 PLN Deadline: 5 days

I am an experienced C#/.NET developer with 17 years of hands-on experience building, maintaining, and improving enterprise applications using ASP.NET MVC/Core, WebForms, SOAP services, WPF/UWP, Xamarin, IIS, SQL, Repository Pattern, Unit of Work, JavaScript, HTML, and CSS.

I have worked on many types of .NET applications, including web platforms, desktop applications, legacy system modernization, API integrations, database-driven systems, and production support. I am comfortable working with both existing legacy codebases and modern .NET Core applications, and I can quickly understand project structure, business logic, and technical issues.

I can help with backend development, bug fixing, feature implementation, database optimization, API/SOAP integration, IIS deployment, UI updates, performance improvement, and clean architecture using patterns such as Repository and Unit of Work. I will focus on writing stable, maintainable, and scalable code that fits your current system.

I am confident I can support your project professionally and deliver reliable results.

Looking forward to working with you. Thank you.
Da Pham

Budget: 10000 PLN Deadline: 20 days

We already have a practically ready workflow process for migrating large corporate applications to C#/.NET, which can be quickly adapted to your project and discussed here now - I am online =)

I would consider a budget of 10,000 PLN as a reasonable first stage, but rather not as a guarantee to complete the entire 30% scope without a code review.

I would start with an audit of the current version, comparing the old program with the new one, building a detailed list of deficiencies and regression tests for invoices, contractors, records, KSeF, and JPK.

Then we will close the modules in batches - first the consistency of logic and data, then the DevExpress interface, followed by accounting-tax case tests and preparation for deployment in the new version.

To start, we will need a repository, a working old version, the current C#/.NET version, a sample database, a list of known deficiencies, and information about the DevExpress license.
- Does the current 70% already have a written list of missing screens and regressions, or does it need to be recreated by comparing the old and new versions?

- Are the sources of the old program available, especially if there is VB6, or are we only comparing the working application and the database?

- https://business.ingello.com/forma-bpm - a similar approach to business processes, logic mapping, and stage control

- https://business.ingello.com/platforma - a larger business platform with modules, roles, and integrations

- https://business.ingello.com/prime-eva - related experience with sales data, integrations, and operational stability

Main description of our approach for FLH - https://systems-fl.ingello.com Expand

Budget: 10000 PLN Deadline: 14 days

Finishing the remaining 30% of a complex invoicing desktop app in C#/.NET WPF with DevExpress, while faithfully matching the old program's behavior down to UI details.

My approach: first I'd do a structured gap analysis, mapping old-program screens and logic against the current WPF codebase to produce a prioritized TODO list. Then I'd work module by module, using the old version as the reference spec, paying close attention to edge cases in document numbering, tax calculations, and JPK/KSeF data structures where silent regressions are most likely. Finally, integration and regression testing against real invoice/document scenarios before handoff.

The critical risk here is behavioral drift, where the new version looks right but calculates or sequences operations differently. I'd handle this by running both versions in parallel on the same test data and diffing outputs.

What database engine and ORM is the current WPF version using, and is the old program VB6 with its own proprietary data format or a standard SQL backend?

Budget: 10000 PLN Deadline: 30 days

Good day, I can help with porting software functionality on the C# .NET stack. I can also assist with analyzing the software functionality from which the porting is being done. For further discussion - feel free to write.

Budget: 10000 PLN Deadline: 10 days

Hello! I will complete your task quickly and efficiently.

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Write to me, I will start working today. I will be glad to collaborate with you! Expand