ASP.NET download csv file as zip?

Roughly based on this, you can create a zip file while streaming it to the client;

Response.ContentType = "application/octet-stream";
Response.Headers.Add("Content-Disposition", "attachment; filename="SqlExport.zip"");

using var archive = new ZipArchive(Response.Body, ZipArchiveMode.Create);

var entry = archive.CreateEntry("SqlExport.csv");
using var entryStream = entry.Open();

entryStream.Write(csv); // write the actual content here

entryStream.Flush();

Though rather than appending to a single csv string, you should probably consider using a StreamWriter to write each snippet of text directly into the response stream. Substituting from your linked csv example;

using var sw = new StreamWriter(entryStream);

// TODO write header

foreach (DataRow row in dt.Rows)
{
    foreach (DataColumn column in dt.Columns)
    {
        //Add the Data rows.
        await sw.WriteAsync(row[column.ColumnName].ToString().Replace(",", ";") + ',');
    }
    //Add new line.
    await sw.WriteLineAsync();
}

Though that is a terrible example of a csv file. Rather than substituting ';' characters, the string should be quoted & all quotes escaped.

However Response.Body is only available in .net 5 / core. To write directly to a http response in .net 4.8 or earlier, you’ll have to write your own HttpContent. Putting everything together, including a better csv formatter;

public class ZipContent : HttpContent
{
    private DataTable dt;
    private string name;

    public ZipContent(DataTable dt, string name = null)
    {
        this.dt = dt;
        this.name = name ?? dt.TableName;
        Headers.ContentType = MediaTypeHeaderValue.Parse("application/octet-stream");
        Headers.ContentDisposition = new ContentDispositionHeaderValue("attachment")
        {
            FileName = $"{name}.zip"
        };
    }

    private string formatCsvValue(string value)
    {
        if (value == null)
            return "";
        if (value.Contains('"') || value.Contains(',') || value.Contains('r') || value.Contains('n'))
            return $""{value.Replace(""", """")}"";
        return value;
    }

    private IEnumerable<DataColumn> Columns()
    {
        // Why is this not already an IEnumerable<DataColumn>?
        foreach (DataColumn col in dt.Columns)
            yield return col;
    }

    protected override async Task SerializeToStreamAsync(Stream stream, TransportContext context)
    {
        using var archive = new ZipArchive(stream, ZipArchiveMode.Create);

        var entry = archive.CreateEntry($"{name}.csv");
        using var entryStream = entry.Open();
        using var sw = new StreamWriter(entryStream);

        await sw.WriteLineAsync(
            string.Join(",",
                Columns()
                .Select(c => formatCsvValue(c.ColumnName))
            ));

        foreach (DataRow row in dt.Rows)
        {
            await sw.WriteLineAsync(
                string.Join(",", 
                    row.ItemArray
                    .Select(o => formatCsvValue(o?.ToString()))
                ));
        }
    }

    protected override bool TryComputeLength(out long length)
    {
        length = 0;
        return false;
    }
}