.NET SqlConnection and SqlCommand

I use SqlConnection and SqlCommand classes in my project with Dapper ORM but I've got a strange problem. When I use SqlCommand for inserting a row in a db table it always work correct and when I select updated data form tables everything is fine but after close the application changes I made isn't commited like data have been saved in some cache. For example I created a simple table with only two columns (UserId (PK), and UserName (Unique)) and even in that case changes aren't saved. I'm using the following code for inserting a row:

using (SqlConnection c = new SqlConnection(Settings.Default.UsersConnectionString))
        {
            c.Open();


            using (SqlCommand d = new SqlCommand())
            {
                d.CommandText = "INSERT INTO Users (UserName) VALUES ('SomeName')";
                d.CommandType = System.Data.CommandType.Text;
                d.Connection = c;
                int t = d.ExecuteNonQuery();
            }


            c.Close();
        }