RedditCsvFileResult

gistfile1.txt

// this is a reworking of the CsvFileResult code posted to the CSharp Reddit:
// https://www.reddit.com/r/csharp/comments/12sip6r/oom_on_custom_fileresult_streaming_from_queryable/

using Microsoft.AspNetCore.Mvc;
using System.Reflection;

public class CSVFileResult<T> : FileResult where T : class
{
    private readonly IQueryable<T> _data;

    public CSVFileResult(string contentType, IQueryable<T> data, string fileDownloadName) : base(contentType)
    {
        _data = data;
        FileDownloadName = fileDownloadName;
    }

    struct FieldDefinition
    {
        public FieldDefinition(PropertyInfo first, PropertyInfo[]? second)
        {
            this.First = first;
            this.Second = second;
        }
        public PropertyInfo First { get; }
        public PropertyInfo[]? Second { get; }
    }

    static bool IsPrimitive(Type type)
    {
        return
            type == typeof(string) ||
            type == typeof(int) ||
            type == typeof(DateTime);
    }

    static string Escape(string s)
    {
        // double up the quotes.
        return s.Replace("\"", "\"\"");
    }

    static async Task WriteValue(TextWriter writer, object? val)
    {
        await writer.WriteAsync('\"');
        var str = val?.ToString() ?? string.Empty;
        str = Escape(str);
        await writer.WriteAsync(str);
        await writer.WriteAsync('\"');
    }

    public async override Task ExecuteResultAsync(ActionContext context)
    {

        var response = context.HttpContext.Response;
        context.HttpContext.Response.Headers.Add("Content-Disposition", new[] { "attachment; filename=" + FileDownloadName });

        using var writer = new StreamWriter(response.Body);

        var recordType = typeof(T);

        var props = recordType.GetProperties();

        // a data structure to hold the nested property info
        List<FieldDefinition> fields = new List<FieldDefinition>();

        // write the headers, and build the nested property data structure.
        var idx = 0;
        foreach (var prop in props)
        {
            if (idx++ > 0)
            {
                await writer.WriteAsync(',');
            }
            var propType = prop.PropertyType;
            if (IsPrimitive(propType))
            {
                // technically, I think a property name could contain a quote
                // but, probably not in the context of an EF query.
                await writer.WriteAsync(prop.Name);
            }
            else
            {
                var second = prop.PropertyType.GetProperties();
                foreach (var prop2 in second)
                {
                    propType = prop2.PropertyType;
                    if (IsPrimitive(propType))
                    {
                        if (idx++ > 0)
                        {
                            await writer.WriteAsync(',');
                        }
                        await writer.WriteAsync(prop2.Name);
                    }
                }
                fields.Add(new FieldDefinition(prop, second));
            }
        }
        await writer.WriteLineAsync();
        // headers are now written.

        foreach (var row in _data)
        {
            idx = 0;
            foreach (var f in fields)
            {
                if (idx++ > 0)
                {
                    await writer.WriteAsync(',');
                }

                var prop = f.First;
                var propType = prop.PropertyType;
                var val = prop.GetValue(row, null);

                if (IsPrimitive(propType))
                {
                    await WriteValue(writer, val);                    
                }
                else
                {
                    if (f.Second != null)
                    {
                        foreach (var prop2 in f.Second)
                        {
                            propType = prop2.PropertyType;

                            if (IsPrimitive(propType))
                            {
                                var val2 = prop.GetValue(val, null);
                                await WriteValue(writer, val);
                            }
                        }
                    }
                }
            }

            await writer.WriteLineAsync();
        }
        await writer.FlushAsync();
    }
}