Context: Companies adopt many software reuse practices, such as software product line, reuse verbatim, and systematic reuse, to improve their internal software development and maximize the benefits. Contemporary software engineering (SE) practices, such as microservices and InnerSource, influence internal software reuse.

Objective: In this thesis, we aim to improve internal software reuse in the context of contemporary SE practices. To do that, we want to 1) understand the state-of-the-art and the state-of-the-practice of software reuse costs and benefits and the challenges that companies are currently facing and 2) identify interventions to improve internal software reuse. 

Methods: We conducted a systematic literature review to understand the state-of-the-art of software reuse costs and benefits. We performed two exploratory case studies to understand the state-of-the-practice of software reuse costs and benefits, challenges, and improvement areas in the context of contemporary SE practices.  We performed another follow-up improving case study to investigate the medium-sized case company's readiness of adopting InnerSource for software reuse.

Results: Existing literature reported more software reuse benefits than costs. The most reported software reuse benefits are better product quality and improved productivity. Verbatim reuse and systematic reuse result in more reuse benefits. Most of the included primary studies are of moderate quality, with only four having high quality. Practitioners think that software reuse costs in developing reusable assets will be paid off when developers start to reuse them. Challenges in software reuse in the context of contemporary SE practices differ between medium-sized and large-sized companies. Both of the companies perceive that InnerSource can help improve internal software reuse. Asking practitioners about both current and desired InnerSource reuse status helps identify the needed InnerSource improvements, thus helping companies succeed in adopting InnerSource for reuse.  

Conclusion: Both existing literature and our two case studies investigating software reuse in the context of contemporary SE practices showed that software reuse improves quality and productivity and has costs in developing and integrating reusable assets. However, the overall benefits outweigh the costs. Both case companies faced challenges in improving their internal reuse, where the most common challenges were about developing and maintaining reusable assets. The results showed that InnerSource helps develop and maintain reusable assets and further improves internal software reuse.

Context: Producers of reusable software assets frequently encounter a high volume of feature requests and bug reports from consumers. InnerSource (IS) offers a potential solution through Participatory Reuse (PR), where consumers participate in the development and maintenance of reusable assets.

Objective: This thesis aims to understand, organize, and improve PR by: 1) understanding the state of the art and practice regarding the costs, benefits, and challenges of software reuse; 2) organizing existing knowledge on PR; and 3) supporting companies in understanding and improving their PR practices.

Methods: This thesis used a mixed-methods approach (an SLR, a case survey, and four case studies) to investigate PR. The SLR explored the reported costs and benefits of software reuse. An exploratory case study assessed reuse practices at a medium-sized company, followed by an improving case study on its readiness for PR. The case survey synthesized existing knowledge into a PR catalog and taxonomy, which was validated and refined through expert evaluations and two additional case studies.

Results: The results of SLR and exploratory case study showed that improved product quality and productivity are the primary benefits of software reuse, though not without associated costs and challenges. The results of two case studies with the same company demonstrated that IS, particularly PR, can help address reuse challenges such as discoverability and ownership of reusable assets. We developed and tested an instrument to assess the company's readiness to adopt PR, identifying areas for improvement and potential solutions. To organize the PR body of knowledge, we developed a PR catalog and taxonomy. The catalog consolidated PR challenges, solutions, and lessons from industrial cases, while the taxonomy provides a mechanism to characterize PR. Finally, we developed a checklist based on the taxonomy for practitioners to assess their current PR practices and identify desired changes.

Conclusion: This thesis advances the field of PR by proposing and validating interventions to improve and characterize PR. The proposed readiness instrument helped the case company to reflect on its current PR practices and identify the areas for improvement. The PR catalog was found to be valuable by experts for providing a clear mapping from PR challenges to the associated solutions and lessons. With the help of two case studies, this thesis demonstrates the utility of the PR taxonomy and its associated checklist in characterizing PR and identifying areas for improvement.