DefaultAppHelper.cs

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

// https://www.pinvoke.net/default.aspx/shlwapi.assocquerystring
// https://docs.microsoft.com/en-us/windows/win32/api/shlwapi/nf-shlwapi-assocquerystringa
// https://docs.microsoft.com/en-us/windows/win32/api/shellapi/nf-shellapi-findexecutablea
// https://stackoverflow.com/a/9540278

internal static class DefaultAppHelper
{
    [DllImport("shell32.dll", EntryPoint = "FindExecutable")]
    private static extern long FindExecutable(string lpFile, string lpDirectory, StringBuilder lpResult);

    [DllImport("Shlwapi.dll", CharSet = CharSet.Unicode)]
    private static extern uint AssocQueryString(
        AssocF flags,
        AssocStr str,
        string pszAssoc,
        string pszExtra,
        [Out] StringBuilder pszOut,
        ref uint pcchOut);

	public static bool TryFindExecutable(string path, out string executablePath)
	{
		var executable = new StringBuilder(1024);
		var result = FindExecutable(path, string.Empty, executable);
		if (result > 32)
		{
			executablePath = executable.ToString();
			return true;
		}

		var extension = new FileInfo(path).Extension;
		executablePath = AssocQueryString(AssocStr.Executable, extension);
		return !string.IsNullOrEmpty(executablePath);
	}

	private static string AssocQueryString(AssocStr association, string extension)
	{
		uint length = 0;
		uint ret = AssocQueryString(AssocF.None, association, extension, null!, null!, ref length);
		if (ret != 1) // expected S_FALSE
		{
			return string.Empty;
		}

		var sb = new StringBuilder((int)length); // (length-1) will probably work too as null termination is added
		ret = AssocQueryString(AssocF.None, association, extension, null!, sb, ref length);
		if (ret != 0) // expected S_OK
		{
			return string.Empty;
		}

		return sb.ToString();
	}

	[Flags]
	private enum AssocF
	{
		None = 0,
		Init_NoRemapCLSID = 0x1,
		Init_ByExeName = 0x2,
		Open_ByExeName = 0x2,
		Init_DefaultToStar = 0x4,
		Init_DefaultToFolder = 0x8,
		NoUserSettings = 0x10,
		NoTruncate = 0x20,
		Verify = 0x40,
		RemapRunDll = 0x80,
		NoFixUps = 0x100,
		IgnoreBaseClass = 0x200,
		Init_IgnoreUnknown = 0x400,
		Init_Fixed_ProgId = 0x800,
		Is_Protocol = 0x1000,
		Init_For_File = 0x2000
	}

	private enum AssocStr
	{
		Command = 1,
		Executable,
		FriendlyDocName,
		FriendlyAppName,
		NoOpen,
		ShellNewValue,
		DDECommand,
		DDEIfExec,
		DDEApplication,
		DDETopic,
		InfoTip,
		QuickTip,
		TileInfo,
		ContentType,
		DefaultIcon,
		ShellExtension,
		DropTarget,
		DelegateExecute,
		Supported_Uri_Protocols,
		ProgID,
		AppID,
		AppPublisher,
		AppIconReference,
		Max
	}
}