Uses the Windows Update Agent API (WUA API) COM Object to check if there are definition updates available for Windows Defender

defender_update_check.cs

// Add a reference to "WUAPI 2.0 Type Library" in Visual Studio
// References:
//     - https://github.com/xonv/nagios-net-client/blob/0920114874ecc85fc7ab3a4426e547c9dc63a44a/NscaWinUpdateModule/WindowsUpdate.cs
//     - https://docs.microsoft.com/en-us/windows/win32/wua_sdk/portal-client

using System;
using WUApiLib;

namespace WinUpdateTest
{
    class Program
    {
        static void Main()
        {
            var uSess = new UpdateSession();
            var uSearcher = uSess.CreateUpdateSearcher();
            var searchResult = uSearcher.Search("IsInstalled=0 and Type='Software'");

            if (searchResult.Updates.Count > 0)
            {
                foreach (IUpdate x in searchResult.Updates)
                {
                    Console.WriteLine(x.Title);
                    foreach (ICategory cat in x.Categories)
                    {
                        switch (cat.Type)
                        {
                            case "UpdateClassification":
                                if (cat.Name == "Definition Updates")
                                    Console.WriteLine("Windows Defender updates are available!");
                                break;
                        }
                    }
                }
            }
        }
    }
}