.NET core - read connection string into Entity Framework Core

One approach can be adding a ConnectionString property in the DBContext.cs class and then setting it on the startup.cs of the API project explicitly. Add the following code in DBcontext.cs

   using System;
   using System.Data.SqlClient;

   using Microsoft.EntityFrameworkCore;

   public class DBContext 
   {
       public static void SetConnectionString(string connectionString)
       {
           if (ConnectionString == null)

           {
               ConnectionString = connectionString;
           }
           else
           {
               throw new Exception();
           }
       }
       // this part will help you to open the connection
       public static SqlConnection OpenConnection()
       {
           SqlConnection connection = new SqlConnection(ConnectionString);
           connection.Open();
           return connection;
       }

       private static string ConnectionString { get; set; }

       //add the connectionString to options

         protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
       {
           if (!optionsBuilder.IsConfigured)
           {
               optionsBuilder.UseSqlServer(ConnectionString);
           }
       }
   }

Now, in the API project add a reference of the EFData project and in the startup.cs file set the ConnectionString

 public void Configure(IApplicationBuilder app, IHostingEnvironment env)
        {
            var connectionString = this.Configuration.GetConnectionString("DBName");
            Namespace.DBContextContext.SetConnectionString(connectionString); //replace namespace with the namespace suitable for your solution

            //here goes rest of your default code
        }

This way you should be able to access the Connection in your API project.