Use CoCreateInstance for .NET Core compatibility

src/Windows/Avalonia.Win32/Interop/UnmanagedMethods.cs

@@ -842,8 +842,9 @@ namespace Avalonia.Win32.Interop
             return SetClassLong64(hWnd, nIndex, dwNewLong);
         }
 
-        [ComImport, ClassInterface(ClassInterfaceType.None), TypeLibType(TypeLibTypeFlags.FCanCreate), Guid("DC1C5A9C-E88A-4DDE-A5A1-60F82A20AEF7")]
-        internal class FileOpenDialogRCW { }
+        [DllImport("ole32.dll", PreserveSig = true)]
+        internal static extern int CoCreateInstance(ref Guid clsid,
+            IntPtr ignore1, int ignore2, ref Guid iid, [MarshalAs(UnmanagedType.IUnknown), Out] out object pUnkOuter);
 
         
         [DllImport("shell32.dll", CharSet = CharSet.Unicode, SetLastError = true)]

src/Windows/Avalonia.Win32/SystemDialogImpl.cs

@@ -157,7 +157,11 @@ namespace Avalonia.Win32
                 string result = string.Empty;
 
                 var hWnd = parent?.Handle?.Handle ?? IntPtr.Zero;
-                var frm = (IFileDialog)(new UnmanagedMethods.FileOpenDialogRCW());
+                var clsid = Guid.Parse("DC1C5A9C-E88A-4DDE-A5A1-60F82A20AEF7");
+                var iid  = Guid.Parse("42F85136-DB7E-439C-85F1-E4075D135FC8");
+
+                UnmanagedMethods.CoCreateInstance(ref clsid, IntPtr.Zero, 1, ref iid, out var unk);
+                var frm = (IFileDialog)unk;
                 uint options;
                 frm.GetOptions(out options);
                 options |= (uint)(UnmanagedMethods.FOS.FOS_PICKFOLDERS | UnmanagedMethods.FOS.FOS_FORCEFILESYSTEM | UnmanagedMethods.FOS.FOS_NOVALIDATE | UnmanagedMethods.FOS.FOS_NOTESTFILECREATE | UnmanagedMethods.FOS.FOS_DONTADDTORECENT);