The implementation of the UNION operator is equivalent to the TSQL expresion UNION ALL
WARNING*This means that duplicates can appear

The First table is used to construct the new DataTables columns, so in effect, the columns in the Second table only need to have the same Data Types and not both Name and Data Type.

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

  Create new empty table
  Add columns to empty table.
   Loop through First table and add rows
   Loop through Second table and add rows
   Return Table.

public static DataTable Union (DataTable First, DataTable Second)

{

      //Result table

      DataTable table = new DataTable("Union");

      //Build new columns

      DataColumn[] newcolumns = new DataColumn[First.Columns.Count];

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

      {

            newcolumns[i] = new DataColumn(First.Columns[i].ColumnName, First.Columns[i].DataType);

      }

      //add new columns to result table

      table.Columns.AddRange(newcolumns);

      table.BeginLoadData();

      //Load data from first table

      foreach(DataRow row in First.Rows)

      {

            table.LoadDataRow(row.ItemArray,true);

      }

      //Load data from second table

      foreach(DataRow row in Second.Rows)

      {

            table.LoadDataRow(row.ItemArray,true);

      }

      table.EndLoadData();

      return table;

}