Question

I have the following datatable – Let’s say called MyTable:

Col1:    Col2:    Col3:    Col4:
1        abc      def      <null>
2        abc      def      ghi
1        abc      def      <null>
3        abc      def      <null>
1        abc      def      <null>

And I’m trying to get the distinct rows:

Col1:    Col2:    Col3:    Col4:
1        abc      def      <null>
2        abc      def      ghi
3        abc      def      <null>

I tried the following LINQ statement:

MyTable = (From dr As DataRow In MyTable Select dr).Distinct.CopyToDataTable

But it’s returning the original datatable with the repeated rows back to me.

What am I doing wrong AND how can I get my desired output??

Answer 1

Distinct relies on the objects all implementing IEquatable and/or having sensible implementations of GetHashCode and Equals. The DataRow class…doesn’t. It doesn’t check that the value of each column is equal in the Equals method, it just uses the default implementation, which is to say that it checks that the references are equal, and that’s not what you want.

You can provide your own IEqualityComparer, since you can’t change the methods in DataRow.

The following should work, if you provide an instance of it to to Distinct:

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

    public int GetHashCode(DataRow obj)
    {
        unchecked
        {
            int output = 23;
            for (int i = 0; i < obj.Table.Columns.Count; i++)
            {
                output += 19 * obj[i].GetHashCode();
            }
            return output;
        }
    }
}