There are a few ways to do this, but no library per-se that will work with C# for any/all .mp4 video files. That is nothing that is 100% reliable.

Both are command line options (in a sense). Basically, what you’d be doing is running a process from your ASP.NET application using System.Diagnostics.Process.

On of them is to use ffmpeg. With ffmpeg if you just give it a file (as a command line argument if will return various metadata about the file. This information can be parsed to extract the dimensions.

The other is to use MediaInfo. It’s a great tool for this. But again, you’ll have to use the Command line version (CLI version) and pretty much give it the file name as a command line argument. It has an otion to produce and xml response so you can easily parse this and other information if can provide.

ffmpeg can transcode your video as well. Though I don’t see the point of transcoding from mp4. flv?

Alturos.VideoInfo is a wrapper for ffprobe that makes it rather easy to integrate into a C# project.

It can be installed from NuGet like this:

PM> Install-Package Alturos.VideoInfo

To use it, you need ffprobe.exe. If it’s not in your project already, you can run this code to download it:

var ffmpegDownloader = new FileDownloader();
var ffmpegUrl = ffmpegDownloader.GetFfmpegPackageUrl();
var ffmpegDownloadResult = await ffmpegDownloader.DownloadAsync(ffmpegUrl, @"ffprobeffprobe.exe");
var ffprobePath = ffmpegDownloadResult.FfprobePath;

The dimensions of the video can then easily be retrieved by getting the video stream:

var videoAnalyzer = new VideoAnalyzer(ffprobePath);
var analyzeResult = videoAnalyzer.GetVideoInfo(/* your video file path here */);

if (analyzeResult.Successful)
{
    var videoStream = analyzeResult.VideoInfo.Streams.Single(o => o.CodecType == "video");

    var width = videoStream.Width;
    var height = videoStream.Height;
}