https://msdn.microsoft.com
https://www.cnblogs.com (Chinese, highly recommended)
https://www.codeplex.com (source code, highly recommended)
https://www.codeproject.com (English, highly recommended)
https://code.msdn.microsoft.com/ (recommended)
https://www.asp.net
https://www.gotdotnet.com
https://www.windowsforms.com
https://www.c-sharpcorner.com