I came across an issue after updating my Visual Studio version to 15.5.3 which would cause my solution to throw an exception whenever it tried to access the database.
System.IO.FileLoadException: 'Could not load file or assembly 'System.Data.SqlClient, Version=126.96.36.199, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'. The located assembly's manifest definition does not match the assembly reference. (Exception from HRESULT: 0x80131040)'
The fix for this is nice and easy, you just need to install the NuGet (V4.4.2) package System.Data.SqlClient.