Select Duplicate Values in a Column in a dataset

There is no direct method to obtain the duplicate values in a column in a dataset.

.Net does not have any method to determine the duplicate values in a column of a DataSet. 
The following code snippet selects the duplicate columnvalues in a specified column, and gives back 
the column values in an Arraylist. This collection can be modified to any other new collection.

CodeSnippet: 
Please  Note:
dataset, tablename, "ID" are only examples. You can change it however the need be.
   
To use declare a Arraylist as follow:
ArrayList duplicates = SelectDuplicates(dataSet.Tables["tablename"].Copy(),"ID");






private ArrayList SelectDuplicates(DataTable table, string columnName)
{
    ArrayList Duplicates = new ArrayList();

   object LastValue = table.Rows[0][columnName]; 

   for (int iValue = 1; iValue< table.Rows.Count-1 ; iValue++)
   {
      if (ColumnEqual(LastValue, table.Rows[iValue][columnName]))
      {
         Duplicates.Add(LastValue);
      }
      
      LastValue = table.Rows[iValue][columnName];
   }

   return Duplicates;

}


private bool ColumnEqual(object A, object B)
{
// Compares two values to see if they are equal. Also compares DBNULL.Value.
// Note: If your DataTable contains object fields, then you must extend this
// function to handle them in a meaningful way if you intend to group on them.

if ( A == DBNull.Value && B == DBNull.Value ) // both are DBNull.Value
return true; 


if ( A == DBNull.Value || B == DBNull.Value ) // only one is DBNull.Value
return false; 

return ( A.Equals(B) ); // value type standard comparison

}