public void MapAsync_MapSecondAsyncFuncIsNull_ExpectArgumentNullException( TaggedUnion <RefType, StructType> source) { var ex = Assert.ThrowsAsync <ArgumentNullException>( async() => _ = await source.MapAsync <decimal, object>(_ => Task.FromResult(21.5m), null !)); Assert.AreEqual("mapSecondAsync", ex !.ParamName); }