There is the String.Join-method designed for this.
var mystring = String.Join(" OR ", idsArr);
This will result in the following string:
export_sdf_id=3746 OR export_sdf_id=3806 OR export_sdf_id=23 OR export_sdf_id=6458 OR export_sdf_id=3740 OR export_sdf_id=3739 OR export_sdf_id=3742
Note that the brackets are omited as they are not needed for your query.
1
solved How to create string from array of strings [duplicate]