lambda function used in kinect

Question

i was reading this book about kinect ,

and i have face a problem in this code :

void kinectSensor_SkeletonFrameReady(object sender, SkeletonFrameReadyEventArgs e)
{
    SkeletonFrame frame = e.OpenSkeletonFrame();
    if (frame == null)
        return;
    Skeleton[] skeletons ;
    skeletons = frame.GetSkeletons();
    if (skeletons.All(s => s.TrackingState == SkeletonTrackingState.NotTracked))
        return;
}

in this section :

if (skeletons.All(s => s.TrackingState == SkeletonTrackingState.NotTracked))

i want to know who provide the value of the parameter s in the lambda exp above ?

and also what skeletons.All mean and what it's return ?

Answer 1

The All method accepts a Func<Skeleton, bool> and a Func<Skeleton, bool> is a delegate that encapsulates a method that accepts a Skeleton parameter and returns a bool.

You could define such a method yourself:

private bool YourMethod(Skeleton s)
{
    return s.TrackingState == SkeletonTrackingState.NotTracked
}

...and pass this one to the All method:

if (skeletons.All(YourMethod))

YourMethod will be called for each Skeleton in skeletons and the All method will return true if YourMethod returns true for all those Skeleton objects.

s => s.TrackingState == SkeletonTrackingState.NotTracked is an anonymous version of YourMethod: https://learn.microsoft.com/en-us/dotnet/csharp/programming-guide/statements-expressions-operators/anonymous-methods