如果文件类型没有关联程序，如何使用 Process.Start 打开文件？

Question

When I want to open an .ico file using Process.Start it throws an error System.ComponentModel.Win32Exception and this is because there is no default program to open that file. I need to show the window to select the default program instead of throwing exception. How can I do that?

private void btnOpenFile_Click(object sender, EventArgs e)
{
   Process.Start(txtSavedAs.Text);
}

Answer 1

What you want to do is pinvoke the AssocQueryString API. Documentation here .

I use that API to get a command string associated with a shell verb. So, for example, if I use the .txt extension, it would return:

C:\Windows\system32\NOTEPAD.EXE %1

Now we know that shell knows what program to execute and how to pass in the command-line argument for that specific extension.

So, if there is a "Command" associated with that extension, it is safe to assume shell will know how to execute that type of file; Hence we should be able to use ShellExecute normally.

If there is no "Command" associated with that file extension, we will show the "openas" dialog allowing the user to pick the application they want to open the file.

Here is a class I put together to do that work:

AppAssociation.cs

using System.Diagnostics;
using System.IO;
using System.Runtime.InteropServices;
using System.Text;

public static class AppAssociation
{
    private static class Win32Native
    {
        public const int ASSOCF_NONE = 0;
        public const int ASSOCSTR_COMMAND = 1;

        [DllImport("shlwapi.dll", CharSet = CharSet.Unicode,
            EntryPoint = "AssocQueryStringW")]
        public static extern uint AssocQueryString(int flags, int str,
            string pszAssoc, string pszExtra, StringBuilder pszOut, ref uint pcchOut);
    }

    public static Process StartProcessForFile(FileInfo file)
    {
        var command = GetCommandForFileExtention(file.Extension);
        return Process.Start(new ProcessStartInfo()
        {
            WindowStyle = ProcessWindowStyle.Normal,
            FileName = file.FullName,
            Verb = string.IsNullOrEmpty(command) ? "openas" : null,
            UseShellExecute = true,
            ErrorDialog = true
        });
    }

    private static string GetCommandForFileExtention(string ext)
    {
        // query length of the buffer we need
        uint length = 0;
        if (Win32Native.AssocQueryString(Win32Native.ASSOCF_NONE,
                Win32Native.ASSOCSTR_COMMAND, ext, null, null, ref length) == 1)
        {
            // build the buffer
            var sb = new StringBuilder((int)length);
            // ask for the actual command string with the right-sized buffer
            if (Win32Native.AssocQueryString(Win32Native.ASSOCF_NONE,
                    Win32Native.ASSOCSTR_COMMAND, ext, null, sb, ref length) == 0)
            {
                return sb.ToString();
            }
        }
        return null;
    }
}

You would call this like so:

AppAssociation.StartProcessForFile(new FileInfo(@"c:\MyFiles\TheFile.txt"));