The example Recursively search for files and replace text in them in C# lets you search a directory hierarchy for files matching a pattern. It can also replace a target string with a new string in those files.

This version just adds the ability to open files. For example, you can search for a file and then double-click on it to open it in the system's default application for that file. That lets you search for files containing a string and then open some of them to examine them.

The following code shows how the programs opens the selected files when you double-click on the list of files.

// Open the double-clicked file with the system's default application.
private void lstFiles_DoubleClick(object sender, EventArgs e)
{
    foreach (string file in lstFiles.SelectedItems)
    {
        System.Diagnostics.Process.Start(file);
    }
}

Usually you will probably double-click a single file but the code loops through the ListBox's SelectedItems collection to open all of the selected files (in case you shift-double-click or something).

For each selected file, the program calls Process.Start to "run" the file. That makes the system open it using the appropriate application.