Hartmut Sonnenberg - Software Developer for C# and WPF

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Biberach an der Riß, Germany

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Experience

Jan 2026 - Dec 2026

1 year

Biberach, Germany

Freelance Software Developer

Replaced conventional database access with REST APIs using HTML (MSGraph, Powerbot) in Excel, Access, and C# applications.
Designed and implemented REST interface integrations in existing Office and .NET solutions.

Jan 2025 - Present

1 year 10 months

Biberach, Germany

Freelance Software Developer

Supported and further developed the Access VBA application "Reporting Headcount Zeitwirtschaft auf SAP Basis", developed in 2019 for a globally operating engineering company.
Adapted the application again to reflect changes in the group structure.
Maintained import, analysis, and reporting features for employee and time management data.

Sep 2024 - Dec 2024

4 months

Biberach, Germany

Freelance Software Developer

Supported the Access VBA application "Reporting Headcount Zeitwirtschaft auf SAP Basis" for a globally operating engineering company.
Adapted it to reflect changes in the group structure and updated the evaluation logic for headcount and time management metrics.

Jun 2024 - Present

2 years 5 months

Biberach, Germany

Freelance Software Developer

Replaced conventional database access with REST APIs using HTML (MSGraph, Powerbot) in Excel, Access, and C# applications.
Migrated existing Office solutions to modern API-based data access.

Jan 2024 - Present

2 years 10 months

Biberach, Germany

Freelance Software Developer

Syneco Trading GmbH

Maintained, enhanced, and developed new Excel VBA tools.
Implemented and optimized Oracle 19 database access.
Adapted and extended data access layers (SQL, PL/SQL) in existing reporting and analysis tools.

Oct 2023 - Present

3 years 1 month

Biberach, Germany

Freelance Software Developer

Syneco Trading GmbH

Maintenance, further development and new development of Excel VBA tools.
Implementation and optimization of Oracle 19 database access.
Ongoing expansion of reporting and analysis functions for trading and energy data.

Apr 2023 - Sep 2023

6 months

Biberach, Germany

Freelance Software Developer

Syneco Trading GmbH

Migration of extensive Excel VBA applications from 32-bit to 64-bit.
Adaptation of data access libraries to Oracle 19c.
Expansion of database logic using Stored Procedures to improve performance and functionality.

Jan 2023 - Feb 2023

2 months

Biberach, Germany

Freelance Software Developer

Further development of a SAP-based time management reporting application for a global machine manufacturer (manufacturer of plastics processing machines).
Analysis of the age structure of employees based on time data.
Calculation of a health rate for each age group based on time management data.

Jun 2022 - Dec 2022

7 months

Biberach, Germany

Freelance Software Developer

New development of a C# and WPF client application (Visual Studio 2022, SQL Server 2019) for medical administration software.
Design of the database, creation of tables, Stored Procedures and functions in SQL Server.
Development of the C#/WPF frontend.
Delivery of the application as a COM DLL, as requested by the client.

Feb 2022 - May 2022

4 months

Biberach, Germany

Freelance Software Developer

Updates to meet changing business and legal requirements for an Excel VBA price comparison application that has been maintained for an energy company for several years.
Maintenance of the calculation logic and updates to reporting functionality.

Feb 2022 - Apr 2022

3 months

Biberach, Germany

Freelance Software Developer

Customization and further development of a Visual Basic 6 DATEV XML online module.
Continued development, started in the second half of 2021, to digitally transfer booking data to DATEV.

Jan 2022 - Jan 2022

1 month

Biberach, Germany

Freelance Software Developer

Support for the Access VBA application "Reporting Headcount Zeitwirtschaft auf SAP Basis" for a global mechanical engineering company.
Adaptation of the application to changes in the group structure.

Sep 2021 - Dec 2021

4 months

Biberach, Germany

Freelance Software Developer

Development of a Visual Basic 6 application for a mid-sized company (automotive) to automatically prepare booking data (incoming and outgoing invoices, cash transactions) from an ERP system (MariaDB).
Transformation of the data into DATEV-compatible XML files for online transfer to DATEV servers.
Goal: Eliminate manual booking entry by the accounting firm providing support.

Jul 2021 - Aug 2021

2 months

Biberach, Germany

Freelance Software Developer

Development of an Excel VBA application for time recording.
Import and analysis of employee master data and time data.
Creation of tabular reports and charts on flexitime balances, overtime, and vacation days.
Generation of emails to cost center managers with relevant reports.

Apr 2021 - Jun 2021

3 months

Biberach, Germany

Freelance Software Developer

Further development and customization of an ERP/CRM system developed in VB6 for a mid-sized client.
Maintenance and expansion of the application, with MariaDB as the backend.
Ongoing support with a monthly workload of approximately 20%.

Jan 2021 - Sep 2021

9 months

Biberach, Germany

Freelance Software Developer

Developed and modernized an Excel VBA analysis tool for medical substance analyses and intolerances.
Redesigned an existing customer information system in Excel VBA with a modern interface (Ribbon bar, callback functions).
Refactored heavily outdated code and redesigned the architecture.
Implemented multilingual support for nutrients, email texts, and other content.
Added numerous new analysis and reporting features.

May 2020 - Aug 2020

4 months

Biberach, Germany

Freelance Software Developer

Updated the database design and stored procedures in MS SQL Server for a water management engineering firm.
Developed and expanded an Excel VBA application to analyze water management data based on new requirements (continuation of the project from 2018).

Feb 2020 - Dec 2020

11 months

Biberach, Germany

Freelance Software Developer

Further developed an Excel VBA price comparison application for an energy group to evaluate tranche contracts.
Project phases from 02/2020–05/2020 and 11/2020–12/2020.
Maintained the calculation logic and adapted it to market and contract requirements.

Dec 2019 - Aug 2020

9 months

Biberach, Germany

Freelance Software Developer

Provided support and updates for an Access VBA gas/electricity price calculation application for SMEs (small and medium-sized enterprises) for an energy group.
Further developed a price comparison application for existing and new contracts.
Maintained the user interfaces, calculation logic, and reporting.

Jun 2019 - Feb 2020

9 months

Biberach, Germany

Freelance Software Developer

Developed an Access VBA application, "Reporting Headcount Time Management Based on SAP," for a global machine manufacturer (manufacturer of plastics processing machinery).
Imported employee and time management data from SAP into an Access database.
Provided extensive analysis tools and reports (time data trends, health rates, workforce development).
Enabled exports of analyses to Excel and PDF.

May 2019 - Dec 2019

8 months

Biberach, Germany

Freelance Software Developer

Migrated a calculation program for screws from a Visual Basic 6 application to an Excel VBA application for a manufacturer in the plastics processing machinery industry.
Implemented all class definitions, properties, and methods from the original VB6 application as VBA classes.
Ensured functional equivalence and extensibility in the new environment.

Jan 2019 - Apr 2019

4 months

Biberach, Germany

Freelance Software Developer

Developed an asynchronous backup solution with VB.NET and MSForms.
Controlled MS Robocopy in a Background Worker process to run copy operations asynchronously.
Implemented inter-process communication between the user interface and the copy process.
Goal: reliably back up data files to external drives and provide status updates to the user.

Mar 2018 - Sep 2018

7 months

Biberach, Germany

Freelance Software Developer

Developed an Excel VBA analysis tool for the HR department of a manufacturer in the plastics processing machinery industry.
Analyzed all employee time data (flexitime, vacation, sick leave).
Replaced an old dBase IV application with a modern Excel-based solution with improved analysis and usability.

Jan 2018 - Dec 2018

1 year

Biberach, Germany

Freelance Software Developer

Provided support and made updates to an Access VBA gas/electricity price calculation application for SMEs for an energy group.
Further developed a price comparison application for old and new contracts in Access and Excel.
Maintained pricing logic, reports, and user interfaces.

Jan 2018 - Dec 2018

1 year

Biberach, Germany

Freelance Software Developer

Designed the database and developed the necessary Stored Procedures in MS SQL Server for a water management engineering firm.
Programmed an Excel VBA application to store and analyze rainfall and runoff data.
Retrieved measurement data from MS SQL Server through the Excel frontend and visualized it using charts.

Apr 2017 - Feb 2018

11 months

Biberach, Germany

Freelance Software Developer

Adapted a dBase IV application for the HR department of a manufacturer of machinery for plastics processing.
Revised the table design and programmed new features.
Adapted and expanded existing reports.

Jan 2017 - Dec 2017

1 year

Biberach, Germany

Freelance Software Developer

Developed an Excel VBA application to track employee goal achievement for a manufacturer of machinery for plastics processing.
Added features to export and import data from individual goal achievement files.
Supported the HR department with recording and evaluating goals.

Apr 2002 - Jul 2010

8 years 4 months

Biberach, Germany

Freelance Software Developer

Developed an Excel VBA and Oracle-based price calculation tool for an energy producer to set prices in the electricity market.
Used production costs and load profiles to provide large customers with pricing information.
Covered the entire business process, from the offer to the signed contract and electricity delivery.
Programmed the Excel interface and contributed to the data model for the Oracle database.
Implemented stored procedures to handle the business logic.

Mar 1999 - Feb 2001

2 years

Biberach, Germany

Freelance Software Developer

Programmed a booking system for a post-production company using Visual Basic 6 and an Access database as the backend.
Planned and managed post-production resources (studios, staff) using a graphical user interface.
Supported the scheduling team in making efficient use of rooms and equipment.

Apr 1997 - Nov 1999

2 years 8 months

Biberach, Germany

Freelance Software Developer

Programmed Visual Basic 6 DLLs and a Visual Basic frontend application for a reinsurance company.
Built a multi-tier architecture with an Oracle database (3-tier model).
Developed an ActiveX server frontend and several DLLs in the middle tier on a Windows Server.
Used object-oriented programming within the capabilities of Visual Basic 6.

Aug 1996 - Apr 1997

9 months

Biberach, Germany

Freelance Software Developer

Programming a booking system for a recording studio using Visual Basic 6 and an Access database as the backend.
Supporting the scheduling team with planning studios, sound engineers, and producers.
Graphically planning and blocking or allocating resources for specific production periods.

Jan 1993 - Present

33 years 10 months

Biberach, Germany

Freelance Software Developer Visual Basic, VBA, C# and WPF

Self-employed

Developing custom software for reinsurers and companies in the advertising and film industries (recording studios and post-production companies).
Many years of development work for an energy company, a mechanical engineering company, and a water management engineering firm serving residential areas.
In recent years, additional projects for companies in the automotive and energy industries, as well as in the medical software field.
Areas of focus: C# and WPF (since 2012), databases (MS Access, MS SQL Server, Oracle, MariaDB), standard SQL, MS Transact SQL, Visual Basic 5.0/6.0 (since 1995), VBA for Microsoft Office (Excel and Access, since 1994).
Additional skills: Integrating REST APIs into VBA applications, including Auth1.0 and Auth2.0, programmed email sending via MS Graph sendMail, Oracle PL/SQL, XML, XAML, HTML, CSS, Python.
Good knowledge of the .NET Framework and Windows APIs.
Many years of ongoing collaboration with several companies, with a high level of reliability in scheduling and communication with clients.

Industry experience

See where this freelancer has spent most of their professional time.

Experienced in Manufacturing, Energy, Healthcare, Automotive, Utilities, and Media and Entertainment.

Manufacturing

Energy

Healthcare

Automotive

Utilities

Media and Entertainment

Business area experience

See which departments and functions this freelancer has contributed to most.

Experienced in Information Technology, Product Development, Business Intelligence, Human Resources, Operations, and Finance.

Information Technology

Product Development

Business Intelligence

Human Resources

Operations

Finance

Summary

Since 1993, I have developed custom software for reinsurers and companies in the advertising and film industries, such as sound studios and post-production companies. I have also carried out a wide range of development work over many years for an energy group, an engineering company, and a civil engineering firm working in water management in residential areas.

Clients get a highly experienced and dedicated developer who picks things up quickly. Meeting realistic deadlines is a given for me, as is communicating with clients. Long-term collaboration with several companies is also a hallmark of my work.

Skills

Areas Of Focus

C# And Wpf Software Development Since 2012
Database Software Development: Ms Access, Ms Sql Server, Oracle, And Mariadb
Standard Sql, Ms Transact Sql
Visual Basic 5.0 And 6.0 Software Development Since 1995
Vba Software Development For Microsoft Office (Excel And Access) Since 1994

Additional Skills

Experience Integrating Rest Apis Into Vba Applications, Including Auth1.0, Auth 2.0
Programmed Email Sending Using Ms Graph Sendmail Instead Of Outlook (Classic)
Oracle Pl Sql, Xml, Xaml, Html, Css, Python
Good Knowledge Of The .Net Framework And Windows Apis

Languages

German

Native

English

Intermediate

Statistics

Experience

Total positions 32

Experience in Manufacturing 34 y

Avg length 2 y 1 m

Longest experience 33 y 9 m

Global experience

Countries worked in 1 (Germany)

Primary country Germany

Expertise

Recent roles Freelance Software Developer, Freelance Software Developer Visual Basic, VBA, C# and WPF

Main industries Manufacturing, Energy, Healthcare

Main business areas Information Technology, Product Development, Business Intelligence

Member since

February 2026

Last update

October 2026

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Frequently asked questions

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Hartmut is based in Biberach an der Riß, Germany and can operate in on-site, hybrid, and remote work models.

Hartmut speaks the following languages: German (Native), English (Intermediate).

Hartmut has at least 34 years of experience. During this time, Hartmut has worked in at least 2 different roles and for 2 different companies. The average length of individual experience is 1 year and 1 month. Note that Hartmut may not have shared all experience and actually has more experience.

Based on recent experience, Hartmut would be well-suited for roles such as: Freelance Software Developer, Freelance Software Developer Visual Basic, VBA, C# and WPF.

Hartmut's most recent position is Freelance Software Developer.

In recent years, Hartmut has worked for Syneco Trading GmbH and Self-employed.

Hartmut is most experienced in industries like Manufacturing, Energy, and Healthcare. Hartmut also has some experience in Automotive, Utilities, and Media and Entertainment.

Hartmut is most experienced in business areas like Information Technology, Product Development, and Business Intelligence. Hartmut also has some experience in Human Resources, Operations, and Finance.

Hartmut has recently worked in industries like Manufacturing, Energy, and Healthcare.

Hartmut has recently worked in business areas like Information Technology, Product Development, and Business Intelligence.

The availability of Hartmut needs to be confirmed.

Daily rate distribution

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<€720 €800-880 €880-960 €960+

The rates shown represent the typical market range for freelancers in this position based on recent contracts on our platform.

Average rates for similar positions

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Daily rate avg. 853 €

The average daily rate is the mean of all daily rates from recent contracts of comparable freelancers on our platform.

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Median rate 860 €

The median daily rate is the middle value of all daily rates — half of comparable freelancers charge less, half charge more. Unlike the average, it is barely affected by outliers.

Calculated based on our freelancers’ daily rates as of 29 Oct 2026. Actual rates may vary depending on seniority level, experience, skill specialization, project complexity, and engagement length.