The basic Union method in ExcelInterop

BasicUnion.cs

//builds the union range form the indices
private static Range Union(IEnumerable<int> rows)
{
    Range range = null;
    foreach (int row in rows)
    {
        var currentLine = ExcelAddIn.ActiveSheet.Range["A" + row + ":" + "C" + row];
        range = range == null ? currentLine : ExcelAddIn.Application.Union(range, currentLine);
    }
    return range;
}

//and use it with all indices from 1 to count
public BuildUnionAll(int count)
{
  var rowIndices = Enumerable.Range(1, count);
  Range range = Union(rowIndices);
}