export DataTable To Excel

static DataTable GetTable()
{
DataTable table = new DataTable(); // New data table.
table.Columns.Add("Dosage", typeof(int)); // Add five columns.
table.Columns.Add("Drug", typeof(string));
table.Columns.Add("Patient", typeof(string));
table.Columns.Add("Date", typeof(DateTime));

table.Rows.Add(15, "Abilify", "xxx", DateTime.Now); // Add five data rows.
table.Rows.Add(40, "Accupril", "yyy", DateTime.Now);
table.Rows.Add(40, "Accutane", "zzz", DateTime.Now);
table.Rows.Add(20, "Aciphex", "zyy", DateTime.Now);
table.Rows.Add(45, "Actos", "xxxy", DateTime.Now);

return table; // Return reference.
}


private void exportDataTableToExcel(DataTable dt, string filePath)
{
// Excel file Path
string myFile = filePath;

//System.Data.DataRow dr = default(System.Data.DataRow);

int colIndex = 0;
int rowIndex = 0;

// Open the file and write the headers
StreamWriter fs = new StreamWriter(myFile, false);

fs.WriteLine("<? xml version=\"1.0\"?>");
fs.WriteLine("<?mso-application progid=\"Excel.Sheet\"?>");
fs.WriteLine("<ss:Workbook xmlns:ss=\"urn:schemas-microsoft-com:office:spreadsheet\">");

// Create the styles for the worksheet
fs.WriteLine(" <ss:Styles>");
// Style for the column headers
fs.WriteLine(" <ss:Style ss:ID=\"1\">");
fs.WriteLine(" <ss:Font ss:Bold=\"1\" ss:Color=\"#FFFFFF\"/>");
fs.WriteLine(" <ss:Alignment ss:Horizontal=\"Center\" ss:Vertical=\"Center\" " + "ss:WrapText=\"1\"/>");
fs.WriteLine(" <ss:Interior ss:Color=\"#254117\" ss:Pattern=\"Solid\"/>");
fs.WriteLine(" </ss:Style>");
// Style for the column information
fs.WriteLine(" <ss:Style ss:ID=\"2\">");
fs.WriteLine(" <ss:Alignment ss:Vertical=\"Center\" ss:WrapText=\"1\"/>");
fs.WriteLine(" </ss:Style>");
// Style for the column headers
fs.WriteLine(" <ss:Style ss:ID=\"3\">");
fs.WriteLine(" <ss:Font ss:Bold=\"1\" ss:Color=\"#FFFFFF\"/>");
fs.WriteLine(" <ss:Alignment ss:Horizontal=\"Center\" ss:Vertical=\"Center\" " + "ss:WrapText=\"1\"/>");
fs.WriteLine(" <ss:Interior ss:Color=\"#736AFF\" ss:Pattern=\"Solid\"/>");
fs.WriteLine(" </ss:Style>");
fs.WriteLine(" </ss:Styles>");

// Write the worksheet contents
fs.WriteLine("<ss:Worksheet ss:Name=\"Sheet1\">");
fs.WriteLine(" <ss:Table>");

fs.WriteLine(" <ss:Row>");
foreach (DataColumn dc in dt.Columns)
{
fs.WriteLine(string.Format(" <ss:Cell ss:StyleID=\"1\">" + "<ss:Data ss:Type=\"String\">{0}</ss:Data></ss:Cell>", dc.ColumnName ));
}

fs.WriteLine(" </ss:Row>");

object cellText = null;

// Write contents for each cell
foreach (DataRow dr in dt.Rows)
{
rowIndex = rowIndex + 1;
colIndex = 0;
fs.WriteLine(" <ss:Row>");
foreach (DataColumn dc in dt.Columns)
{
cellText = dr[dc];
// Check for null cell and change it to empty to avoid error
if (cellText == null ) cellText = "";
fs.WriteLine(string.Format(" <ss:Cell ss:StyleID=\"2\">" +
"<ss:Data ss:Type=\"String\">{0}</ss:Data></ss:Cell>", cellText));
colIndex = colIndex + 1;
}
fs.WriteLine(" </ss:Row>");
}

fs.WriteLine(" <ss:Row>");
fs.WriteLine(" </ss:Row>");

// Close up the document
fs.WriteLine(" </ss:Table>");
fs.WriteLine("</ss:Worksheet>");
fs.WriteLine("</ss:Workbook>");
fs.Close();
}

Example:
exportDataTableToExcel (GetTable(),"C:\\PatientDetails.xls");
will write the content of data table with Formatting Styles.