Best practices for serializing objects to a custom string format for use in an output file

Here is a generic fashion for creating CSV from a list of objects, using reflection:

public static string ToCsv<T>(string separator, IEnumerable<T> objectlist)
{
    Type t = typeof(T);
    FieldInfo[] fields = t.GetFields();

    string header = String.Join(separator, fields.Select(f => f.Name).ToArray());

    StringBuilder csvdata = new StringBuilder();
    csvdata.AppendLine(header);

    foreach (var o in objectlist) 
        csvdata.AppendLine(ToCsvFields(separator, fields, o));

    return csvdata.ToString();
}

public static string ToCsvFields(string separator, FieldInfo[] fields, object o)
{
    StringBuilder linie = new StringBuilder();

    foreach (var f in fields)
    {
        if (linie.Length > 0)
            linie.Append(separator);

        var x = f.GetValue(o);

        if (x != null)
            linie.Append(x.ToString());
    }

    return linie.ToString();
}

Many variations can be made, such as writing out directly to a file in ToCsv(), or replacing the StringBuilder with an IEnumerable and yield statements.