method for converting datatable to object

You might benefit from the helper class in this post: http://lozanotek.com/blog/archive/2007/05/09/Converting_Custom_Collections_To_and_From_DataTable.aspx

public class CollectionHelper
{
    private CollectionHelper()
    {
    }

    public static DataTable ConvertTo<T>(IList<T> list)
    {
        DataTable table = CreateTable<T>();
        Type entityType = typeof(T);
        PropertyDescriptorCollection properties = TypeDescriptor.GetProperties(entityType);

        foreach (T item in list)
        {
            DataRow row = table.NewRow();

            foreach (PropertyDescriptor prop in properties)
            {
                row[prop.Name] = prop.GetValue(item);
            }

            table.Rows.Add(row);
        }

        return table;
    }

    public static IList<T> ConvertTo<T>(IList<DataRow> rows)
    {
        IList<T> list = null;

        if (rows != null)
        {
            list = new List<T>();

            foreach (DataRow row in rows)
            {
                T item = CreateItem<T>(row);
                list.Add(item);
            }
        }

        return list;
    }

    public static IList<T> ConvertTo<T>(DataTable table)
    {
        if (table == null)
        {
            return null;
        }

        List<DataRow> rows = new List<DataRow>();

        foreach (DataRow row in table.Rows)
        {
            rows.Add(row);
        }

        return ConvertTo<T>(rows);
    }

    public static T CreateItem<T>(DataRow row)
    {
        T obj = default(T);
        if (row != null)
        {
            obj = Activator.CreateInstance<T>();

            foreach (DataColumn column in row.Table.Columns)
            {
                PropertyInfo prop = obj.GetType().GetProperty(column.ColumnName);
                try
                {
                    object value = row[column.ColumnName];
                    prop.SetValue(obj, value, null);
                }
                catch
                {
                    // You can log something here
                    throw;
                }
            }
        }

        return obj;
    }

    public static DataTable CreateTable<T>()
    {
        Type entityType = typeof(T);
        DataTable table = new DataTable(entityType.Name);
        PropertyDescriptorCollection properties = TypeDescriptor.GetProperties(entityType);

        foreach (PropertyDescriptor prop in properties)
        {
            table.Columns.Add(prop.Name, prop.PropertyType);
        }

        return table;
    }
}
To see the full code in action, check this sample out:

public class MyClass
{
    public static void Main()
    {
        List<Customer> customers = new List<Customer>();

        for (int i = 0; i < 10; i++)
        {
            Customer c = new Customer();
            c.Id = i;
            c.Name = "Customer " + i.ToString();

            customers.Add(c);
        }

        DataTable table = CollectionHelper.ConvertTo<Customer>(customers);

        foreach (DataRow row in table.Rows)
        {
            Console.WriteLine("Customer");
            Console.WriteLine("---------------");

            foreach (DataColumn column in table.Columns)
            {
                object value = row[column.ColumnName];
                Console.WriteLine("{0}: {1}", column.ColumnName, value);
            }

            Console.WriteLine();
        }

        RL();
    }

    #region Helper methods

    private static void WL(object text, params object[] args)
    {
        Console.WriteLine(text.ToString(), args);
    }

    private static void RL()
    {
        Console.ReadLine();
    }

    private static void Break()
    {
        System.Diagnostics.Debugger.Break();
    }

    #endregion
}

if you know the data type you expect; you can perfectly do this for strings:

string username = dt_user.rows[0][0].ToString();

and this for ints:

int userID = int.Parse(dt_user.rows[0][0].ToString());

Although that's still ugly. I would write a couple extension methods as so:

public static string ObjectToString(this object theObject)
{
     if(!string.IsNullOrEmpty(theObject))
        return theObject.ToString();
     return string.Empty;
}

public static int ObjectToInt(this object theObject)
{
     int result = 0;
     if(!string.IsNullOrEmpty(theObject) && int.TryParse(theObject, out result))
     {
        return result;
     }
     return -1;
}

And then I would call it like this:

string username = dt_user.rows[0][0].ObjectToString();
int userid = dt_user.rows[0][0].ObjectToInt();

One more suggestion: Don't access column names by using the index if you don't have to; you can perfectly say dt_user.rows[0]["username"].ToString();

And that will guard you from unexpected results/errors in case someone changes the position of a particular column in your result set.