namespace Project.Models
{
    // ...
    public class MyMembershipProvider : MembershipProvider
    {
        public override bool ChangePassword(string username, string oldPassword, string newPassword)
        {
            throw new NotImplementedException();
        }

        public override bool ChangePasswordQuestionAndAnswer(string username, string password, string newPasswordQuestion, string newPasswordAnswer)
        {
            throw new NotImplementedException();
        }

        public override MembershipUser CreateUser(string username, string password, string email, string passwordQuestion, string passwordAnswer, bool isApproved, object providerUserKey, out MembershipCreateStatus status)
        {
            throw new NotImplementedException();
        }

        public override bool DeleteUser(string username, bool deleteAllRelatedData)
        {
            throw new NotImplementedException();
        }
    // ...
}

            string myConnectionString = ConfigurationManager.ConnectionStrings["Default"].ConnectionString;
            NpgsqlConnection myNpgsqlConnection = new NpgsqlConnection(myConnectionString);

        public override bool ChangePassword(string username, string oldPassword, string newPassword)
        {
            
            string myConnectionString = ConfigurationManager.ConnectionStrings["Default"].ConnectionString;
            int count = 0;
            NpgsqlConnection myNpgsqlConnection = new NpgsqlConnection(myConnectionString);
            NpgsqlCommand myNpgsqlCmd = new NpgsqlCommand("aspnet_Membership_CreateUser", myNpgsqlConnection);
            myNpgsqlCmd.CommandType = CommandType.StoredProcedure;
            myNpgsqlCmd.Parameters.AddWithValue("@UserName", username);
            myNpgsqlCmd.Parameters.AddWithValue("@OldPassword", oldPassword);
            myNpgsqlCmd.Parameters.AddWithValue("@NewPassword", newPassword);
            count = myNpgsqlCmd.ExecuteNonQuery();
            myNpgsqlConnection.Close();
            return count == 1;
            //throw new NotImplementedException();
        }