DavidM

The argument order of the method is important. That is: Difference(First, Second) != Difference(Second, First)

There is only the one signature for this method.
In summary the code works as follows:

  Create new empty table
  Create a DataSet and add tables.
  Get a reference to all columns in both tables
  Create a DataRelation
  Using the DataRelation add rows with no child rows.
  Return table

public static DataTable Difference(DataTable First, DataTable Second)

{

      //Create Empty Table

      DataTable table = new DataTable("Difference");

      //Must use a Dataset to make use of a DataRelation object

      using(DataSet ds = new DataSet())

      {

            //Add tables

            ds.Tables.AddRange(new DataTable[]{First.Copy(),Second.Copy()});

            //Get Columns for DataRelation

            DataColumn[] firstcolumns  = new DataColumn[ds.Tables[0].Columns.Count];

            for(int i = 0; i < firstcolumns.Length; i++)

            {

                  firstcolumns[i] = ds.Tables[0].Columns[i];

            }

            DataColumn[] secondcolumns = new DataColumn[ds.Tables[1].Columns.Count];

            for(int i = 0; i < secondcolumns.Length; i++)

            {

                  secondcolumns[i] = ds.Tables[1].Columns[i];

            }

            //Create DataRelation

            DataRelation r = new DataRelation(string.Empty,firstcolumns,secondcolumns,false);

            ds.Relations.Add(r);

            //Create columns for return table

            for(int i = 0; i < First.Columns.Count; i++)

            {

                  table.Columns.Add(First.Columns[i].ColumnName, First.Columns[i].DataType);

            }

            //If First Row not in Second, Add to return table.

            table.BeginLoadData();

            foreach(DataRow parentrow in ds.Tables[0].Rows)

            {

                  DataRow[] childrows = parentrow.GetChildRows(r);

                  if(childrows == null || childrows.Length == 0)

                        table.LoadDataRow(parentrow.ItemArray,true);   

            }

            table.EndLoadData();

      }

      return table;

}