ASP.NET Core change EF connection string when user logs in

Create a DbContext factory

public static class DbContextFactory
{
    public static Dictionary<string, string> ConnectionStrings { get; set; }

    public static void SetConnectionString(Dictionary<string, string> connStrs)
    {
        ConnectionStrings = connStrs;
    }

    public static MyDbContext Create(string connid)
    {
        if (!string.IsNullOrEmpty(connid))
        {
            var connStr = ConnectionStrings[connid];
            var optionsBuilder = new DbContextOptionsBuilder<MyDbContext>();
            optionsBuilder.UseSqlServer(connStr);
            return new MyDbContext(optionsBuilder.Options);
        }
        else
        {
            throw new ArgumentNullException("ConnectionId");
        }
    }
}

Intialize DbContext factory

In startup.cs

public void Configure()
{
  Dictionary<string, string> connStrs = new Dictionary<string, string>();
  connStrs.Add("DB1", Configuration["Data:DB1Connection:ConnectionString"]);
  connStrs.Add("DB2", Configuration["Data:DB2Connection:ConnectionString"]);
  DbContextFactory.SetConnectionString(connStrs);
}

Usage

var dbContext= DbContextFactory.Create("DB1");