Reorder SharePoint list fields from code

There’s no function in the SharePoint API to reorder the fields of a SharePoint list. You have to make use of the ProcessBatchDate from the SPWeb class.

/// 

/// This function reorders the fields in the specified list
/// programmatically as specified by the firstFields parameter.
/// The fields not included in the firstFields list will be
/// added to the list.
/// 
/// The SPList object to update
/// A generic list of SPField containing the
/// first fields in the order to be displayed.
public void ReorderField(SPList list, List firstFields)
{
  List fields = new List();

  for (int i=0; i<firstFields.Count; i++)
  {
    fields.Add(firstFields[i]);
  }

  foreach (SPField field in list.Fields)
  {
    if (!fields.Contains(field))
    {
      fields.Add(field);
    }
  }

  StringBuilder sb = new StringBuilder();

  XmlTextWriter xmlWriter = new XmlTextWriter(new StringWriter(sb));
  xmlWriter.Formatting = Formatting.Indented;

  xmlWriter.WriteStartElement("Fields");

  for (int i=0; i< fields.Count; i++)
  {
    xmlWriter.WriteStartElement("Field");
    xmlWriter.WriteAttributeString("Name", fields[i].InternalName);
    xmlWriter.WriteEndElement();
  }

  xmlWriter.WriteEndElement();
  xmlWriter.Flush();

  using (SPWeb web = list.ParentWeb)
  {
    ReorderFields(web, list, sb.ToString());
  }
}

/// 

/// This function reorders the fields in the specified list
/// programmatically as specified by the xmlFieldsOrdered parameter
/// 
/// The SPWeb object containing the list
/// This function reorders the fields in the
/// specified list
/// A string in XML-format specifying the
/// field order by the location within a xml-tree
private void ReorderFields(SPWeb web, SPList list,
  string xmlFieldsOrdered)
{
  try
  {
    string fpRPCMethod = @"
	
	{0}
	REORDERFIELDS
	{1}
	{2}
	";

      // relookup list version in order to be able to update it
      list = web.Lists[list.ID];

      int currentVersion = list.Version;

      string version = currentVersion.ToString();
      string RpcCall = string.Format(fpRPCMethod, list.ID,
        SPHttpUtility.HtmlEncode(xmlFieldsOrdered), version);

      web.AllowUnsafeUpdates = true;
      web.ProcessBatchData(RpcCall);
    }
    catch (System.Net.WebException err)
    {
      // TODO: Log this exception according your exception
      // handling policies.
      Console.WriteLine("WARNING:" + err.Message);
    }
}