/// <summary>
/// MVC action result that generates the file content using a delegate that writes the content directly to the output stream.
/// </summary>
public class FileGeneratingResult : FileResult
{
    /// <summary>
    /// The delegate that will generate the file content.
    /// </summary>
    private readonly Action<System.IO.Stream> content;

    private readonly bool bufferOutput;

    /// <summary>
    /// Initializes a new instance of the <see cref="FileGeneratingResult" /> class.
    /// </summary>
    /// <param name="fileName">Name of the file.</param>
    /// <param name="contentType">Type of the content.</param>
    /// <param name="content">Delegate with Stream parameter. This is the stream to which content should be written.</param>
    /// <param name="bufferOutput">use output buffering. Set to false for large files to prevent OutOfMemoryException.</param>
    public FileGeneratingResult(string fileName,string contentType,Action<System.IO.Stream> content,bool bufferOutput=true)
        : base(contentType)
    {
        if (content == null)
            throw new ArgumentNullException("content");

        this.content = content;
        this.bufferOutput = bufferOutput;
        FileDownloadName = fileName;
    }

    /// <summary>
    /// Writes the file to the response.
    /// </summary>
    /// <param name="response">The response object.</param>
    protected override void WriteFile(System.Web.HttpResponseBase response)
    {
        response.Buffer = bufferOutput;
        content(response.OutputStream);
    }
}

控制器方法现在是这样的：

public ActionResult Export(int id)
{
    return new FileGeneratingResult(id + ".csv","text/csv",stream => this.GenerateExportFile(id,stream));
}

public void GenerateExportFile(int id,Stream stream)
{
    stream.Write(/**/);
}

请注意，如果缓冲关闭，

stream.Write(/**/);

变得非常慢解决方案是使用BufferedStream。在一种情况下，这样做的性能提高了大约100倍。看到

Unbuffered Output Very Slow