Question

I am using DataRelation to compare two DataTable and return a third table containing the rows that are not share by both original tables by using this tutorial. Below is the code.

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;    
}

Surprisingly enough when I exercised this code, it only worked on few cases and failed in others.

There was one case, I was comparing two identical table (but has different source) that both has:

• Same Column Name & DataType
• Same Number of Rows & Columns
• Same Value in Each Cells

Only to have a brand new table returned that actually has the exact same composition as the two original tables!

What did I possibly miss?

Is it possible that two identical tables (sharing features I mentioned above) can have other different properties (which doesn’t seem visible to users’ eyes)?

Or is it possible that this is actually a bad method? What are the possible alternatives?

EDITED

1. Both tables have same primitive datatype such as: System.String, System.Int32, System.DateTime
2. These code doesn’t work on all samples I tested
3. Here is a print screen of 1 sample (using DataSet Visualizer)

Answer 1

I had to write something similar once and this is the approach I used:

First this approach only works if you don’t have duplicate rows in each table.

Using primary keys..

First.PrimaryKey = firstcolumns;
Second.PrimaryKey = secondcolumns; //These throw exceptions when you have duplicate rows

Then..

foreach (DataRow dr in Second.Rows)
{
    List<Object> l = new List<Object>();

    foreach (DataColumn dc in secondcolumns) l.Add(dr[dc]);

    if (First.Rows.Find(l.ToArray()) == null) //NOT FOUND
    {
        table.Rows.Add(l.ToArray());
    }
}

foreach (DataRow dr in First.Rows)
{
    List<Object> l = new List<Object>();

    foreach (DataColumn dc in firstcolumns) l.Add(dr[dc]);

    if (Second.Rows.Find(l.ToArray()) == null) //NOT FOUND
    {
        table.Rows.Add(l.ToArray());
    }
}

Cheers,