unoplatform/uno · error · InvalidOperationException

Playlist Items could not be set

Error message

Playlist Items could not be set

What it means

InvalidOperationException thrown in InitializeSource (WebAssembly) when the source is a MediaPlaybackList with Items.Count > 0 but after SetPlaylistItems the _playlistItems list is null or empty — i.e. none of the playlist items produced a usable Uri. SetPlaylistItems maps each item to i.Source.Uri, so items with null Source/Uri yield no playable URI.

Source

Thrown at src/AddIns/Uno.UI.MediaPlayer.WebAssembly/MediaPlayerExtension.cs:310

			return;
		}

		try
		{
			_owner.PlaybackSession.PlaybackState = MediaPlaybackState.Opening;
			InitializePlayer();

			switch (_owner.Source)
			{
				case MediaPlaybackList playlist when playlist.Items.Count > 0:
					SetPlaylistItems(playlist);
					if (_playlistItems is not null && _playlistItems.Any())
					{
						_uri = _playlistItems[0];
					}
					else
					{
						throw new InvalidOperationException("Playlist Items could not be set");
					}
					break;

				case MediaPlaybackItem item:
					_uri = item.Source.Uri;
					break;

				case MediaSource source:
					_uri = source.Uri;
					break;

				default:
					throw new InvalidOperationException("Unsupported media source type");
			}

			ApplyVideoSource();
			Events?.RaiseSourceChanged();
		}

View on GitHub (pinned to 0418340488)

Solutions

  1. Ensure every MediaPlaybackItem in the list has a Source with a non-null Uri before assigning the playlist.
  2. Guard against concurrent modification: snapshot playlist.Items before the count check and the mapping.
  3. Filter out items with null Source/Uri before building the MediaPlaybackList.
  4. If using stream-backed sources, switch to Uri-backed MediaSource.CreateFromUri for WASM compatibility.
  5. Log each item's Source.Uri during construction to identify the offending entry.

Example fix

// before — items may have null Uri
var list = new MediaPlaybackList();
list.Items.Add(new MediaPlaybackItem(MediaSource.CreateFromStream(stream, "")));
player.Source = list;

// after — ensure Uri-backed items
var list = new MediaPlaybackList();
list.Items.Add(new MediaPlaybackItem(MediaSource.CreateFromUri(new Uri(url))));
player.Source = list;
Defensive patterns

Strategy: validation

Validate before calling

// Ensure every playlist item has a Uri before assigning
if (playlist.Items.All(i => i?.Source?.Uri is not null))
    player.Source = playlist;
else
    throw new InvalidOperationException("Playlist contains items without a Uri");

Type guard

bool PlaylistHasPlayableUris(MediaPlaybackList list)
    => list.Items.Count > 0 && list.Items.All(i => i?.Source?.Uri is not null);

Try / catch

try { player.Source = playlist; }
catch (InvalidOperationException ex) when (ex.Message.Contains("Playlist Items could not be set"))
{ Log.Error("Playlist produced no playable URI"); OnMediaFailed(ex); }

Prevention

When it happens

Trigger: MediaPlayer.Source is a MediaPlaybackList whose items exist but each item's Source is null or Source.Uri is null; items were concurrently removed between the Count>0 check and SetPlaylistItems; playlist built with MediaPlaybackItems whose backing MediaSource has no Uri (e.g. stream-backed sources unsupported on WASM).

Common situations: Building a playlist from MediaPlaybackItems constructed without a Uri (e.g. from an IMediaSource/stream); concurrent mutation of the playlist; a deserialization path that created items with null Source. Note: a null Source.Uri would throw NullReferenceException inside SetPlaylistItems' Select rather than reach this guard, so the empty-list case implies items whose Uri resolved to nothing or were cleared.

Related errors

AI-assisted analysis of unoplatform/uno@0418340488 (2026-08-13). Data as JSON: /api/errors/4470c2ee86c791dc. Report an issue: GitHub.