app = new Microsoft.Office.Interop.Excel.Application();
workbook = app.Workbooks.Add(1);
worksheet = (Microsoft.Office.Interop.Excel.Worksheet)workbook.Sheets[1];

//Change all cells' alignment to center
worksheet.Cells.Style.HorizontalAlignment = Microsoft.Office.Interop.Excel.XlHAlign.xlHAlignCenter;

//But then this line changes every cell style back to left alignment
worksheet.Cells[y + 1, x + 2].Style.HorizontalAlignment = Microsoft.Office.Interop.Excel.XlHAlign.xlHAlignLeft;

// fill in the starting and ending range programmatically this is just an example.
string startRange = "A1";
string endRange = "A1";
Excel.Range currentRange = (Excel.Range)excelWorksheet.get_Range(startRange , endRange );
currentRange.Style.HorizontalAlignment = Microsoft.Office.Interop.Excel.XlHAlign.xlHAlignLeft;

worksheet.get_Range("A1","A14").Cells.HorizontalAlignment =
                 Microsoft.Office.Interop.Excel.XlHAlign.xlHAlignLeft;

var range = worksheet.Range[string.Format("A{0}:C{0}", rowIndex++)];
range.Merge();
range.Value = "some value";

var style = workbook.AddStyle();
style.HorizontalAlignment = Microsoft.Office.Interop.Excel.XlHAlign.xlHAlignLeft;

range.Style = style.Name;