Q:

How to write a linq query to search for a value in dataset and retrieve its row index

I have a Dataset ds in which i have two columns ITEM_ID and ITEM_VALUE. Now a linq query has to be written to search for a item id value and retrieve the row index of that value. i just don't have an idea need some help

A:

I think you mean DataTable, DataSet does not contain columns, it contains DataTables. So maybe this is what you want:
var rowIndex = yourDataTable.AsEnumerable()
                            .Select((row,index)=>new{row,index})
                            .Where(x=>object.Equals(x.row["ITEM_ID"],yourItemID))
                            .Select(x=>x.index).FirstOrDefault();

