dotnet/wpf · error · InvalidOperationException

SR.MediaElement_CannotSetSourceOnMediaElementDrivenByClock

Error message

SR.MediaElement_CannotSetSourceOnMediaElementDrivenByClock

What it means

AVElementHelper.MemberOnInvalidateSource throws this InvalidOperationException when the Source property of a MediaElement is changed while the element is driven by a MediaClock. A clock-driven MediaElement has its timeline owned by the clock, so setting Source directly is not allowed.

Solutions

1. Stop using the clock (e.g. remove the MediaClock/ClockController) before changing Source, or create a new MediaElement.
2. Control playback exclusively through ClockController when using clocks, and set the source at clock creation time.
3. Guard Source updates with a check that the element is not clock-driven.

Example fix

// before
media.Clock = clock;
media.Source = new Uri("other.wmv"); // throws
// after
media.Clock = null; // detach clock first
media.Source = new Uri("other.wmv");

if (media.Clock != null) throw new InvalidOperationException("Detach the MediaClock before changing Source");

bool IsClockDriven(MediaElement m) => m.Clock != null;

try { media.Source = newUri; } catch (InvalidOperationException) { media.Clock = null; media.Source = newUri; }

When it happens

Trigger: Assigning mediaElement.Source (or a binding/style updating Source) on a MediaElement whose Clock was set via ClockController or MediaClock assignment.

Common situations: Mixing clock-based playback (Storyboard/MediaClock with ClockController.Seek/Play) with source updates; data-binding Source on a clock-controlled element; style triggers that change Source during clock playback.

Understand the failure class

Background: "Invalid state transition" errors: "status must be X, actually Y", "already rejected/charging/uninstalled", "cannot ... while running" — what they mean when a library rejects your call — this error's family across 31 libraries.

Related errors

• Animation_Invalid_DefaultValue
• Cannot remove signature from read-only file.
• Image_EncoderNoColorContext
• Image_EncoderNoGlobalMetadata
• Image_EncoderNoGlobalThumbnail

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/6ae76361b93582b5. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Controls/AVElementHelper.cs:721

        /// <summary>
        /// Raised when source is changed
        /// </summary>
        internal static void OnSourceChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            if (e.IsASubPropertyChange)
            {
                return;
            }

            AVElementHelper aveh = AVElementHelper.GetHelper(d);
            aveh.MemberOnInvalidateSource(e); // call non-static
        }

        private void MemberOnInvalidateSource(DependencyPropertyChangedEventArgs e)
        {
            if (_clock._value != null)
            {
                throw new InvalidOperationException(SR.MediaElement_CannotSetSourceOnMediaElementDrivenByClock);
            }

            _source._value = (Uri)e.NewValue;
            _source._wasSet = _source._isSet = true;

            HandleStateChange();
        }

        /// <summary>
        /// Raised when there is a error with media playback
        /// </summary>
        private
        void
        OnMediaFailed(
            object sender,
            ExceptionEventArgs args
            )
        {

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