1 public class HomeController : Controller
 2 {
 3     private List<User> users = new List<User>
 4     {
 5         new User { Id = 1, Username = "DoloresAbernathy" },
 6         new User { Id = 2, Username = "MaeveMillay" },
 7         new User { Id = 3, Username = "BernardLowe" },
 8         new User { Id = 4, Username = "ManInBlack" }
 9     };
10 
11     public HomeController()
12     {
13     }
14 }

 1 public IActionResult Csv()
 2 {
 3     var builder = new StringBuilder();
 4     builder.AppendLine("Id,Username");
 5     foreach (var user in users)
 6     {
 7         builder.AppendLine($"{user.Id},{user.Username}");
 8     }
 9 
10     return File(Encoding.UTF8.GetBytes(builder.ToString()), "text/csv", "users.csv");
11 }

Install-Package ClosedXML

 1 public IActionResult Excel()
 2 {
 3     using (var workbook = new XLWorkbook())
 4     {
 5         var worksheet = workbook.Worksheets.Add("Users");
 6         var currentRow = 1;
 7         worksheet.Cell(currentRow, 1).Value = "Id";
 8         worksheet.Cell(currentRow, 2).Value = "Username";
 9         foreach (var user in users)
10         {
11             currentRow++;
12             worksheet.Cell(currentRow, 1).Value = user.Id;
13             worksheet.Cell(currentRow, 2).Value = user.Username;
14         }
15 
16         using (var stream = new MemoryStream())
17         {
18             workbook.SaveAs(stream);
19             var content = stream.ToArray();
20 
21             return File(
22                 content,
23                 "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet",
24                 "users.xlsx");
25         }
26     }
27 }