如何用linq减去两个数据表

Question

Is there any way to subtract two datatables in order to have rows of the first datatable that are not in the second one?

I have tried .Except() method like below but it does not work for me.

dt1.AsEnumerable().Except(dt2.AsEnumerable()).CopyToDataTable();

I think i have mistake in using this method but i could not find that?

Answer 1

You can create your own Comparer using IEquailtyComparer

 public class CustomDataRowComparer : IEqualityComparer<DataRow>
    {
        public bool Equals(DataRow x, DataRow y)
        {
            for (int i = 0; i < x.Table.Columns.Count; i++)
            {
                if (x[i].ToString() != y[i].ToString())
                {
                    return false;
                }

            }
            return true;
        }

        public int GetHashCode(DataRow obj)
        {
            return obj.ToString().GetHashCode();
        }
    }

Later you can call it like:

CustomDataRowComparer myDRComparer = new CustomDataRowComparer();
var result2 = dt1.AsEnumerable().Except(dt2.AsEnumerable(),myDRComparer).CopyToDataTable();

Answer 2

Except will not work because the references are different. Each "copy" of a data row in memory is a different object, thus the equality comparison being made in Except will always return false.

You need to compare with something comparable, like row IDs. A couple ideas how to do this:

var rowsInFirstNotInSecond = dt1.Rows.Where(r1=>!dt2.Rows.Any(r2=>r1.ID == r2.ID)); 

Or:

var rowsInFirstNotInSecond = dt1.Rows.Where(r1=>dt2.FindById(r1.ID) == null);

Answer 3

I have used the following code to subtract two datatables from each other :

var query =    
    from row1 in dt1    
    where !(from row2 in dt2    
            select row2.ID)    
           .Contains(row1.ID)    
    select row1;

this code returns exactly what i want...

Answer 4

This Code works for sure:

var rows =dtFirst.AsEnumerable().Except(dtSecond.AsEnumerable(), DataRowComparer.Default);
 DataTable result = null;
 if (rows.Count() != 0)
      result = rows.CopyToDataTable();