create or replace function TestFun return integer is
  Result integer;
begin
  Result := 1;
  return(Result);
end TestFun;

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data;
using System.Data.OracleClient;

public partial class _Default : System.Web.UI.Page 
{
    protected void Page_Load(object sender, EventArgs e)
    {
        OracleConnection conn = new OracleConnection("..xxxx..");
        conn.Open();
        OracleCommand cmd = conn.CreateCommand();
        cmd.CommandText = "TestFun";
        cmd.CommandType = CommandType.StoredProcedure;
        decimal res = (decimal)cmd.ExecuteScalar();
        Label1.Text = res.ToString();
    }
}

Server Error in '/' Application.
ORA-06550: line 1, column 7:
PLS-00221: 'TESTFUN' is not a procedure or is undefined
ORA-06550: line 1, column 7:
PL/SQL: Statement ignored
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.Data.OracleClient.OracleException: ORA-06550: line 1, column 7:
PLS-00221: 'TESTFUN' is not a procedure or is undefined
ORA-06550: line 1, column 7:
PL/SQL: Statement ignored

    protected void Page_Load(object sender, EventArgs e)
    {

        OracleConnection conn = new OracleConnection("xxx");
        conn.Open();
        OracleCommand cmd = conn.CreateCommand();
        cmd.CommandText = "declare pResult integer; begin pResult := TestFun; end;";
        cmd.CommandType = CommandType.Text;
        object res = cmd.ExecuteScalar();
        Label1.Text = res.ToString();
    }