"By the 2000s full mastery of the Windows API had become impractical, while individual subsystems could still be mastered."
Sinofsky argues that platform mastery can let one or two engineers complete an entire project, but by the 2000s whole-platform mastery of Windows had become impossible; at best engineers could master subsystems such as networking, DirectX, or the file system, while the added .NET, data-access, graphics, and networking APIs compounded the complexity - Hardcore Software, pp. 1486-1487, Kindle edition.
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