Open file with associated application

This is an old thread but just in case anyone comes across it like I did. pi.FileName needs to be set to the file name (and possibly full path to file ) of the executable you want to use to open your file. The below code works for me to open a video file with VLC.

var path = files[currentIndex].fileName;
var pi = new ProcessStartInfo(path)
{
    Arguments = Path.GetFileName(path),
    UseShellExecute = true,
    WorkingDirectory = Path.GetDirectoryName(path),
    FileName = "C:\\Program Files (x86)\\VideoLAN\\VLC\\vlc.exe",
    Verb = "OPEN"
};
Process.Start(pi)

Tigran's answer works but will use windows' default application to open your file, so using ProcessStartInfo may be useful if you want to open the file with an application that is not the default.

In .Net Core (as of v2.2) it should be:

new Process
{
    StartInfo = new ProcessStartInfo(@"file path")
    {
        UseShellExecute = true
    }
}.Start();

Related github issue can be found here

Just write

System.Diagnostics.Process.Start(@"file path");

example

System.Diagnostics.Process.Start(@"C:\foo.jpg");
System.Diagnostics.Process.Start(@"C:\foo.doc");
System.Diagnostics.Process.Start(@"C:\foo.dxf");
...

And shell will run associated program reading it from the registry, like usual double click does.