C# - The IndexOf() Method Returns -1 All The Time

i have two data tables with the same structure

the first one has one row of the second one

the second one has set of rows

what i want is to get the row which comes next to the row of the first data table in the second data table.

the first data table his name is :: temp

the second data table his name is :: dt

i do the following:

DataTable temp = new DataTable();
temp = dt.Clone();
DataColumn[] keyColumn = new DataColumn[1];
keyColumn[0] = temp.Columns["photoId"];
temp.PrimaryKey = keyColumn;
temp = (DataTable)(Session["currentImage"]);
DataRow[] drr = new DataRow[1];
index = dt.Rows.IndexOf(temp.Rows[0]);

but the index always comes with one value = -1

although the temp.rows[0] its contents changed all the time

when i write dt.Rows.IndexOf(dt.Rows[1]) for examble i get 1

but this is not what i want to do ,, what i want to do exactly is to get the datarow next to the datarow of the first dataTable in the second dataTable