Custom CSV Formatter in ASP.NET Web API

Introduction

Well, we are quite dependent on XML and JSON responses. This tip illustrates how to create a custom CSV formatter in ASP.NET Web API.

Background

The web APIs mostly write XML or JSON responses to their output streams. Here is an alternative approach to write CSV response to web API's output stream.

Using the Code

Begin with a custom Class with the name CsvMediaTypeFormatter and inherit this class from MediaTypeFormatter. After that, override the basic method of MediaTypeFormatter to implement the required functionality.

public class CsvMediaTypeFormatter : MediaTypeFormatter
    {
        public CsvMediaTypeFormatter()
        {
            this.SupportedMediaTypes.Add(new MediaTypeHeaderValue("text/csv"));
            
            //SupportedEncodings.Add(new UTF8Encoding(encoderShouldEmitUTF8Identifier: false));
            //SupportedEncodings.Add(Encoding.GetEncoding("iso-8859-1"));
        }

        public CsvMediaTypeFormatter(MediaTypeMapping mediaTypeMapping) : this()
        {
            MediaTypeMappings.Add(mediaTypeMapping);
        }
        
        public CsvMediaTypeFormatter(IEnumerable<MediaTypeMapping> mediaTypeMappings) : this()
        {
            foreach (var mediaTypeMapping in mediaTypeMappings)
            {
                MediaTypeMappings.Add(mediaTypeMapping);
            }
        }
        
        public override bool CanWriteType(Type type)
        {
            if (type == null)
                throw new ArgumentNullException("type");

            return isTypeOfIEnumerable(type);
        }

        private bool isTypeOfIEnumerable(Type type)
        {
            foreach (Type interfaceType in type.GetInterfaces())
            {
                if (interfaceType == typeof(IEnumerable))
                    return true;
            }

            return false;
        }

        public override bool CanReadType(Type type)
        {
            return false;
        }

        public override void SetDefaultContentHeaders
        (Type type, HttpContentHeaders headers, MediaTypeHeaderValue mediaType)
        {
            base.SetDefaultContentHeaders(type, headers, mediaType);
            headers.Add("Content-Disposition", "attachment; 
            filename="+ "test_"+ 
            DateTime.Now.ToString("yyyyMMddhhmmssfff")+ ".csv");
        }
        
        public override Task WriteToStreamAsync(Type type, object value, 
        Stream stream, HttpContent content, TransportContext transportContext)
        {           
            writeStream(type, value, stream, content);
            var tcs = new TaskCompletionSource<int>();
            tcs.SetResult(0);
            return tcs.Task;
        }

        private void writeStream(Type type, object value, Stream stream, HttpContent contentHeaders)
        {            
            //NOTE: We have check the type inside CanWriteType method
            //If request comes this far, the type is IEnumerable. We are safe.
           
            Type itemType = type.GetGenericArguments()[0];

            StringWriter _stringWriter = new StringWriter();

            _stringWriter.WriteLine(
                string.Join<string>(
                    ",", itemType.GetProperties().Select(x => x.Name)
                )
            );

            foreach (var obj in (IEnumerable<object>)value)
            {

                var vals = obj.GetType().GetProperties().Select(
                    pi => new {
                        Value = pi.GetValue(obj, null)
                    }
                );

                string _valueLine = string.Empty;

                foreach (var val in vals)
                {
                    if (val.Value != null)
                    {
                        var _val = val.Value.ToString();

                        //Check if the value contains a comma and place it in quotes if so
                        if (_val.Contains(","))
                            _val = string.Concat("\"", _val, "\"");

                        //Replace any \r or \n special characters from a new line with a space
                        if (_val.Contains("\r"))
                            _val = _val.Replace("\r", " ");
                        if (_val.Contains("\n"))
                            _val = _val.Replace("\n", " ");

                        _valueLine = string.Concat(_valueLine, _val, ",");
                    }
                    else
                    {
                        _valueLine = string.Concat(string.Empty, ",");
                    }
                }

                _stringWriter.WriteLine(_valueLine.TrimEnd(','));
            }

            var streamWriter = new StreamWriter(stream);            
            streamWriter.Write(_stringWriter.ToString());
            streamWriter.Flush();
            streamWriter.Close();
        }        
    }

After creating CsvMediaTypeFormatter class, add the following code in Global.asax.cs file.

GlobalConfiguration.Configuration.Formatters.Add(
                                     new CsvMediaTypeFormatter(
                                     new QueryStringMapping
                                     ("format", "csv", "text/csv")   ) );