Convert DataRowCollection to DataRow[]

DataRow[] rows = dt.Select();

Assuming you still have access to the datatable.

For the sake of completeness, this is another way (with Linq), and it preserve the row ordering:

DataRow[] rows = dt.Rows.Cast<DataRow>().ToArray()

This is kind of obvious, but:

DataRowCollection.CopyTo(DataRow[] array, Int32 offset) ?

It seems like no matter what, you're going to have to iterate the collection (CopyTo iterates the internal DataRowTree elements).

I suppose you could use reflection to access the non-public tree, but at a significant cost.

If you have not access to the containing table, you may use the extension method:

public static class DataRowCollectionExtensions
{
    public static IEnumerable<DataRow> AsEnumerable(this DataRowCollection source)
    {
        return source.Cast<DataRow>();
    }
}

And thereafter:

DataRow[] dataRows = dataRowCollection.AsEnumerable().ToArray();

But if you have access to the containing table, it's better to access rows using DataTable's AsEnumerable extension method (Description, Source):

DataRow[] dataRows = dataTable.AsEnumerable().ToArray();

Either way you may use DataTable's Select method with several overloads (Description, Source):

DataRow[] dataRows = dataTable.Select();