title: get distinct rows from datatable using Linq (distinct with mulitiple columns)
body: <p>I am trying to distinct on multiple columns  and get datarows from datatable. but getting error.</p>
  
  <pre><code> Dim query As IEnumerable(Of DataRow) = 
              (From row As DataRow In SourceTable.AsEnumerable() _
               Select row.Field(Of String)("ColumnName1"),
                      row.Field(Of String)("ColumnName2") ).Distinct()
  </code></pre>
  
  <p>below error:</p>
  
  <pre><code>Unable to cast object of type '&lt;DistinctIterator&gt;d__7a`1[System.String]' 
  to type 'System.Collections.Generic.IEnumerable`1[System.Data.DataRow]'.
  </code></pre>
  
  <p>I want another datatable with distinct row based on given columns from SourceTable.</p>