public void Distinct_Null() { Assert.Throws <ArgumentNullException>(() => AsyncEnumerableEx.Distinct <int, int>(Return42, default(Func <int, int>))); Assert.Throws <ArgumentNullException>(() => AsyncEnumerableEx.Distinct(default(IAsyncEnumerable <int>), x => x)); Assert.Throws <ArgumentNullException>(() => AsyncEnumerableEx.Distinct(Return42, default(Func <int, int>))); Assert.Throws <ArgumentNullException>(() => AsyncEnumerableEx.Distinct(default(IAsyncEnumerable <int>), x => x, EqualityComparer <int> .Default)); Assert.Throws <ArgumentNullException>(() => AsyncEnumerableEx.Distinct(Return42, default(Func <int, int>), EqualityComparer <int> .Default)); }