C# - How do I iterate all the rows in Excel._Worksheet?

public void IterateRows(Excel.worksheet worksheet)
{
    //Get the used Range
    Excel.Range usedRange = worksheet.UsedRange;

    //Iterate the rows in the used range
    foreach(Excel.Range row in usedRange.Rows)
    {
        //Do something with the row.

        //Ex. Iterate through the row's data and put in a string array
        String[] rowData = new String[row.Columns.Count];
        for(int i = 0; i < row.Columns.Count; i++)
            rowData[i] =Convert.ToString(row.Cells[1, i + 1].Value2);
    }
}

This compiles and runs just great for me! I'm using it to extract rows with missing fields to an error log.

I presume you are actually looking for the last used row. In that case you need to write it like this:

Range UsedRange = worksheet.UsedRange;
int lastUsedRow = UsedRange.Row + UsedRange.Rows.Count - 1;

Have a look at the UsedRange property in Excel.