Is the source code for Microsoft .net CLR and C# open? If yes, where can I download/browse it?
You can use Reflector to view them.
Or, if you have Resharper 5 installed, you can download the source code for viewing purpose.
(source: jetbrains.com)
But no, it is not open source, despite that you can download and view the source code.
for BCL you can check here (codeplex)
and for the CLR it self here (githup)
While I'm not intimately familiar with the licensing of Microsoft's available source code, there is at least one implementation of the CLR and C# compiler that is open source -- Novell's Mono.
http://www.mono-project.com/
Part of .NET Core is now open-source, starting with .NET Core 5.0.
Here is the project site: http://www.dotnetfoundation.org/netcore5
Here is the code: https://github.com/dotnet/corefx
You can have a look at the official one: Microsoft Reference Source.
You can browse the source code in Visual Studio as well, while debugging. To enable this feature, follow the steps described here: Configure Visual Studio 2013 for debugging .NET framework
You can download the source code of .NET with the help of Netmassdownloader