How to use CsvHelper to generate a CSV file in .NET 10 MVC

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Generating CSV files is a common requirement in web applications for data export. CsvHelper is the industry-standard library for reading and writing CSV files in .NET. In this post, we’ll walk through a simple example of how to use CsvHelper in a .NET 10 MVC project to generate and download a spreadsheet.

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1. Install CsvHelper

First, you need to add the CsvHelper NuGet package to your project. You can do this via the .NET CLI:

dotnet add package CsvHelper

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2. Define Your Model

Create a simple class that represents the data you want to export.

public class Product
{
    public int Id { get; set; }
    public string Name { get; set; }
    public decimal Price { get; set; }
    public DateTime CreatedAt { get; set; }
}

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3. Create the MVC Controller

In your controller, you’ll create an action that generates the CSV data and returns it as a file. We’ll use a MemoryStream so that we don’t have to save a temporary file on the server.

using CsvHelper;
using Microsoft.AspNetCore.Mvc;
using System.Globalization;
using System.IO;
using System.Text;

public class ExportController : Controller
{
    public IActionResult DownloadProducts()
    {
        // 1. Mock data (in a real app, this comes from a database)
        var products = new List<Product>
        {
            new Product { Id = 1, Name = "Laptop", Price = 999.99m, CreatedAt = DateTime.Now },
            new Product { Id = 2, Name = "Mouse", Price = 25.50m, CreatedAt = DateTime.Now },
            new Product { Id = 3, Name = "Keyboard", Price = 75.00m, CreatedAt = DateTime.Now }
        };

        // 2. Setup the stream and writers
        using (var memoryStream = new MemoryStream())
        {
            using (var writer = new StreamWriter(memoryStream, Encoding.UTF8))
            using (var csv = new CsvWriter(writer, CultureInfo.InvariantCulture))
            {
                // 3. Write records to the CSV
                csv.WriteRecords(products);
                
                // 4. Ensure all data is flushed to the stream
                writer.Flush();
            }
            
            // 5. Return the file to the browser
            // ToArray() is safe for simple exports; the browser will prompt for download
            return File(memoryStream.ToArray(), "text/csv", "products_export.csv");
        }
    }
}

4. Create the View

Finally, add a button to your Razor view (e.g., Index.cshtml) using ASP.NET Core Tag Helpers to trigger the download action.

<div class="text-center">
    <h3>Data Export</h3>
    <p>Click the button below to download the product list as a CSV file.</p>
    
    <a asp-controller="Export" 
       asp-action="DownloadProducts" 
       class="btn btn-primary">
        <i class="fas fa-file-csv"></i> Download CSV Report
    </a>
</div>

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5. How it Works

• MemoryStream: We use a memory stream to avoid writing a physical file to the server’s disk.
• StreamWriter: Bridges the gap between the CsvWriter and the MemoryStream.
• CsvWriter: The core component of CsvHelper that handles the formatting and data serialization.
• File(): A built-in MVC method that returns a FileContentResult, prompting the browser to download the data as a file with the specified MIME type (text/csv).

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6. Advanced: Customizing the Output

If you need to change column headers or ignore certain properties, you can use a ClassMap.

public sealed class ProductMap : ClassMap<Product>
{
    public ProductMap()
    {
        Map(m => m.Id).Name("Product ID");
        Map(m => m.Name).Name("Product Name");
        Map(m => m.Price).Format("C"); // Format as currency
        Map(m => m.CreatedAt).Ignore(); // Don't include this in the CSV
    }
}

Then register it in your controller before writing records:

csv.Context.RegisterClassMap<ProductMap>();
csv.WriteRecords(products);

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7. Conclusion

Using CsvHelper in .NET 10 MVC is straightforward and highly efficient. By combining MemoryStream with CsvHelper’s CsvWriter, you can provide seamless data export functionality to your users with minimal code.

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References

• CsvHelper Documentation
• NuGet: CsvHelper