Application.Current.Resources.TryLookup throws hresult_error despite it being a Try* method

[asklar]

In the cppwinrt generated header we have this in windows.foundation.collections.0.h, and we go down the first branch in the code, which will throw if the item is not found (in this case, XAML's ResourceDictionary returns E_FAIL):

        auto TryLookup(param_type<K> const& key) const
        {
            if constexpr (std::is_base_of_v<Windows::Foundation::IUnknown, V>)
            {
                V result{ nullptr };
                impl::check_hresult_allow_bounds(WINRT_IMPL_SHIM(Windows::Foundation::Collections::IMap<K, V>)->Lookup(get_abi(key), put_abi(result)));
                return result;
            }
            else
            {
                std::optional<V> result;
                V value{ empty_value<V>() };

                if (0 == impl::check_hresult_allow_bounds(WINRT_IMPL_SHIM(Windows::Foundation::Collections::IMap<K, V>)->Lookup(get_abi(key), put_abi(value))))
                {
                    result = std::move(value);
                }

                return result;
            }
        }

[oldnewthing]

The Try means that it returns null if the key is missing. The contractual way to report a missing key is to return E_BOUNDS, but ResourceDictionary went its own way. Will accommodate the nonstandard behavior. See upcoming PR.

The Try does not mean "ignore all errors". Try will throw if the key could not be looked up at all, because returning null in those cases would be misleading. The key isn't missing. We couldn't even get to the point where we could check whether the key is present or not, e.g., because you are on the wrong thread, or because you don't have access, or the map is otherwise unavailable.