how to get data from npgsql to dropdownlist c#

You can use Dapper ORM, it is very efficient to way to access data from Postgre Sql.

Check it,

http://dapper-tutorial.net/dapper

//Connection String for Postgre

var connectionString = "Username=****;Password=****;Host=*Server Address*;Port=*Port*;Database=*Database Name*;Pooling=true;";
var conn = new NpgsqlConnection(connectionString);

//Binding Process

In Dapper Orm you get many methods, you can use Query Method to execute your Query and return the list and bind that list to your Dropdown.