I have some mscorlib issues in VS 2017 community when trying to build by C# projct

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C#

.NET-Core

Hello,

This is my first project in VS 2017.

I have a lot of errors like these:

Error   CS7069  Reference to type 'ICloneable' claims it is defined in 
'mscorlib', but it could not be found
Error   CS7069  Reference to type 'Component' claims it is defined in 
'System', but it could not be found ClassLibrary3
Error   CS7069  Reference to type 'MarshalByRefObject' claims it is defined 
in 'mscorlib', but it could not be found
Error   CS7069  Reference to type 'MarshalByRefObject' claims it is defined 
in 'mscorlib', but it could not be found
Error   CS7069  Reference to type 'MarshalByRefObject' claims it is defined 
in 'mscorlib', but it could not be found

I'm trying to create sql connection like this:

using (SqlConnection conn = new SqlConnection(connstr)) {..}

What I have tried:

Tried to add references to mscorlib of the targeted .Net version.
Tried to use this solution NuGet Gallery | Microsoft.NETCore.Portable.Compatibility 1.0.1[^]