public ObservableCollection<WindowInfo> Windows { get; } = new ObservableCollection<WindowInfo>();
public ObservableCollection<ScreenInfo> Screens { get; } = new ObservableCollection<ScreenInfo>();

// Omitting declarations for the selected screen & selected window code...

public async Task Refresh()
{
    this.SelectedScreen = null;
    this.SelectedWindow = null;

    this.Windows.Clear();
    this.Screens.Clear();

    var securityCheck = await WinRtGraphicsCaptureHelper.RequestPermissionsAsync();
    if (!securityCheck)
    {
        MessageBox.Show("Could not get capture permissions.");
        return;
    }

    var screens = await WinRtGraphicsCaptureHelper.GetAvailableScreensAsync();
    foreach(var item in screens)
    {
        this.Screens.Add(item);
    }

    var windows = await WinRtGraphicsCaptureHelper.GetAvailableWindowsAsync();
    foreach (var item in windows)
    {
        this.Windows.Add(item);
    }
}

private DisplayCaptureInstance? capture;

public void ToggleCapture()
{
    if(this.capture is not null)
    {
        this.capture.Stop();
        return;
    }

    var frameHandler = new NdiFrameHandler()
    {
        NdiSenderName = this.NdiSenderName
    };

    if (this.windowCapture && this.selectedWindow is not null)
    {
        this.capture = WinRtGraphicsCaptureHelper.CreateCaptureForWindow(this.selectedWindow, frameHandler);
    }
    else if(this.screenCapture && this.selectedScreen is not null)
    {
        this.capture = WinRtGraphicsCaptureHelper.CreateCaptureForScreen(this.selectedScreen, frameHandler);
    }
    else
    {
        this.capture = null;
        MessageBox.Show("No source selected. Not started.");
    }

    if(this.capture != null)
    {
        this.capture.CaptureSessionStopped += this.OnCaptureSessionStopped;
    }
}

private void OnCaptureSessionStopped()
{
    if(this.capture != null)
    {
        this.capture.CaptureSessionStopped -= this.OnCaptureSessionStopped;
    }
    this.capture = null;
}

using NewTek;
using NewTek.NDI;
using SharpDX;
using System;
using System.Linq;
using System.Runtime.InteropServices;
using Tractus.WinRtCaptureWrapper;

namespace WpfRtTestbed;

public class NdiFrameHandler : FrameHandler
{
    IntPtr sendInstancePtr = IntPtr.Zero;

    public string NdiSenderName { get; set; }

    protected override void OnDisposing()
    {
        if (this.sendInstancePtr != IntPtr.Zero)
        {
            NDIlib.send_destroy(this.sendInstancePtr);
            this.sendInstancePtr = IntPtr.Zero;
        }
    }

    public override void SendFrame(DataRectangle rect, int width, int height)
    {
        int xres = width; // NdiWidth;
        int yres = height; //NdiHeight;

        int frNum = 30;
        int frDen = 1;

        if (sendInstancePtr == IntPtr.Zero)
        {
            var ptr = new NDIlib.send_create_t
            {
                clock_audio = false,
                clock_video = true,
                p_groups = IntPtr.Zero,
                p_ndi_name = UTF.StringToUtf8(this.NdiSenderName)
            };
            sendInstancePtr = NDIlib.send_create(ref ptr);
        }

        var stride = rect.Pitch; // (xres * 32/*BGRA bpp*/ + 7) / 8;
        var requiredFrameSize = yres * stride;
        var aspectRatio = xres / (float)yres;

        // We are going to create a progressive frame at 60Hz.
        NDIlib.video_frame_v2_t videoFrame = new NDIlib.video_frame_v2_t()
        {
            // Resolution
            xres = xres,
            yres = yres,
            // Use BGRA video
            FourCC = NDIlib.FourCC_type_e.FourCC_type_BGRA,
            // The frame-eate
            frame_rate_N = frNum,
            frame_rate_D = frDen,
            // The aspect ratio
            picture_aspect_ratio = aspectRatio,
            // This is a progressive frame
            frame_format_type = NDIlib.frame_format_type_e.frame_format_type_progressive,
            // Timecode.
            timecode = NDIlib.send_timecode_synthesize,
            // The video memory used for this frame
            p_data = rect.DataPointer,
            // The line to line stride of this image
            line_stride_in_bytes = stride,
            // no metadata
            p_metadata = IntPtr.Zero,
            // only valid on received frames
            timestamp = 0,
        };

        NDIlib.send_send_video_v2(this.sendInstancePtr, ref videoFrame);
        //// add it to the output queue
        //AddFrame(videoFrame);    }
    }
}