Check that FFmpeg exists on launch

YoutubeDownloader.Core/Downloading/FFmpeg.cs

@@ -0,0 +1,32 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+
+namespace YoutubeDownloader.Core.Downloading;
+
+public static class FFmpeg
+{
+    public static string? TryGetCliFilePath()
+    {
+        static IEnumerable<string> GetProbeDirectoryPaths()
+        {
+            yield return AppContext.BaseDirectory;
+            yield return Directory.GetCurrentDirectory();
+
+            foreach (
+                var path in Environment.GetEnvironmentVariable("PATH")?.Split(Path.PathSeparator)
+                    ?? Enumerable.Empty<string>()
+            )
+            {
+                yield return path;
+            }
+        }
+
+        return GetProbeDirectoryPaths()
+            .Select(dirPath =>
+                Path.Combine(dirPath, OperatingSystem.IsWindows() ? "ffmpeg.exe" : "ffmpeg")
+            )
+            .FirstOrDefault(File.Exists);
+    }
+}

YoutubeDownloader.Core/Downloading/VideoDownloader.cs

@@ -67,6 +67,7 @@ await _youtube.Videos.DownloadAsync(
             downloadOption.StreamInfos,
             trackInfos,
             new ConversionRequestBuilder(filePath)
+                .SetFFmpegPath(FFmpeg.TryGetCliFilePath() ?? "ffmpeg")
                 .SetContainer(downloadOption.Container)
                 .SetPreset(ConversionPreset.Medium)
                 .Build(),

YoutubeDownloader/Services/SettingsService.cs

@@ -17,7 +17,7 @@ namespace YoutubeDownloader.Services;
 [INotifyPropertyChanged]
 public partial class SettingsService()
     : SettingsBase(
-        Path.Combine(AppDomain.CurrentDomain.BaseDirectory, "Settings.dat"),
+        Path.Combine(AppContext.BaseDirectory, "Settings.dat"),
         SerializerContext.Default
     )
 {

YoutubeDownloader/ViewModels/MainViewModel.cs

@@ -3,6 +3,8 @@
 using System.Threading.Tasks;
 using Avalonia;
 using CommunityToolkit.Mvvm.Input;
+using YoutubeDownloader.Core;
+using YoutubeDownloader.Core.Downloading;
 using YoutubeDownloader.Framework;
 using YoutubeDownloader.Services;
 using YoutubeDownloader.Utils;
@@ -71,6 +73,29 @@ private async Task ShowDevelopmentBuildMessageAsync()
             ProcessEx.StartShellExecute(Program.ProjectReleasesUrl);
     }
 
+    private async Task ShowFFmpegMessageAsync()
+    {
+        if (!string.IsNullOrWhiteSpace(FFmpeg.TryGetCliFilePath()))
+            return;
+
+        var dialog = viewModelManager.CreateMessageBoxViewModel(
+            "FFmpeg is missing",
+            $"""
+            FFmpeg is required for {Program.Name} to work. Please download it and make it available in the application directory or on the system PATH.
+
+            Click DOWNLOAD to go to the FFmpeg download page. You can also install FFmpeg using a package manager instead.
+            """,
+            "DOWNLOAD",
+            "CLOSE"
+        );
+
+        if (await dialogManager.ShowDialogAsync(dialog) == true)
+            ProcessEx.StartShellExecute("https://ffmpeg.org/download.html");
+
+        if (Application.Current?.ApplicationLifetime?.TryShutdown(3) != true)
+            Environment.Exit(3);
+    }
+
     private async Task CheckForUpdatesAsync()
     {
         try
@@ -106,6 +131,7 @@ private async Task InitializeAsync()
     {
         await ShowUkraineSupportMessageAsync();
         await ShowDevelopmentBuildMessageAsync();
+        await ShowFFmpegMessageAsync();
         await CheckForUpdatesAsync();
     }