Download the entire source code of .NET Framework as a .zip file. The archive includes a solution and project files that can be opened in Visual Studio. This solution will not build because it is missing crucial components such as resources, XAML files, etc. but it will be sufficient to browse the source code inside Visual Studio.

.NET 4.5.1 (50 MB) .NET 4.5.2 (55 MB) .NET 4.6 (58 MB)