How to switch between sqlserver and postgresql using entity framework by parameters

For swithing the database with the same DbContext, you need to make sure your database map the DbContext Structor.

For achieve this, you could try define the parameter like

{
    "DatabaseType": "MsSQL1",

    "ConnectionStrings": {
        "DefaultConnection": "Server=(localdb)\\mssqllocaldb;Database=aspnet-TestIdentity2_2-C9C6DF1A-1A9A-497A-871E-618806FC959F;Trusted_Connection=True;MultipleActiveResultSets=true",
        "MySqlConnection": "Server=(localdb)\\mssqllocaldb;Database=aspnet-MySql-2C4FD13B-62DA-4B5D-8C8C-8F9C42CD3A67;Trusted_Connection=True;MultipleActiveResultSets=true"

    },
}

Configure in Startup.cs

services.AddDbContext<ApplicationDbContext>(options =>
{
    var dbType = Configuration["DatabaseType"];
    if (dbType == "MsSQL")
    {
       //change this to your postgresql config
        options.UseSqlServer(
            Configuration.GetConnectionString("MySqlConnection"));
    }
    else
    {
        options.UseSqlServer(
            Configuration.GetConnectionString("DefaultConnection"));
    }
});