Fix WellKnownFolder.Download on Windows (#17107)

src/Avalonia.Base/Platform/Storage/FileIO/BclStorageProvider.cs

@@ -109,7 +109,7 @@ internal abstract class BclStorageProvider : IStorageProvider
         if (OperatingSystemEx.IsWindows())
         {
             return Environment.OSVersion.Version.Major < 6 ? null :
-                Marshal.PtrToStringUni(SHGetKnownFolderPath(s_folderDownloads, 0, IntPtr.Zero));
+                TryGetWindowsKnownFolder(s_folderDownloads);
         }
 
         if (OperatingSystemEx.IsLinux())
@@ -148,7 +148,31 @@ internal abstract class BclStorageProvider : IStorageProvider
         }
     }
 
+    private static unsafe string? TryGetWindowsKnownFolder(Guid guid)
+    {
+        nint path = IntPtr.Zero;
+        string? result = null;
+
+        try
+        {
+            var hr = SHGetKnownFolderPath(guid, 0, 0, &path);
+            if (hr == 0)
+            {
+                result = Marshal.PtrToStringUni(path);
+            }
+        }
+        finally
+        {
+            if (path != IntPtr.Zero)
+            {
+                Marshal.FreeCoTaskMem(path);
+            }
+        }
+
+        return result;
+    }
+
     private static readonly Guid s_folderDownloads = new Guid("374DE290-123F-4565-9164-39C4925E467B");
     [DllImport("shell32.dll")]
-    private static extern IntPtr SHGetKnownFolderPath([MarshalAs(UnmanagedType.LPStruct)] Guid id, int flags, IntPtr token);
+    private static unsafe extern int SHGetKnownFolderPath([MarshalAs(UnmanagedType.LPStruct)] Guid id, int flags, nint token, nint* ppszPath);
 }