defaultifempty

src/UniTask.NetCore/Linq/DefaultIfEmpty.cs

@@ -73,11 +73,16 @@ namespace Cysharp.Threading.Tasks.Linq
                 cancellationToken.ThrowIfCancellationRequested();
                 completionSource.Reset();
 
-                if (iteratingState == IteratingState.Empty)
+                if (iteratingState == IteratingState.Completed)
                 {
                     return CompletedTasks.False;
                 }
 
+                if (enumerator == null)
+                {
+                    enumerator = source.GetAsyncEnumerator(cancellationToken);
+                }
+
                 awaiter = enumerator.MoveNextAsync().GetAwaiter();
 
                 if (awaiter.IsCompleted)

src/UniTask.NetCoreTests/Linq/Concat.cs

@@ -112,5 +112,33 @@ namespace NetCoreTests.Linq
                 await Assert.ThrowsAsync<UniTaskTestException>(async () => await zs);
             }
         }
+
+        [Fact]
+        public async Task DefaultIfEmpty()
+        {
+            {
+                var xs = await Enumerable.Range(1, 0).ToUniTaskAsyncEnumerable().DefaultIfEmpty(99).ToArrayAsync();
+                var ys = Enumerable.Range(1, 0).DefaultIfEmpty(99).ToArray();
+                xs.Should().BeEquivalentTo(ys);
+            }
+            {
+                var xs = await Enumerable.Range(1, 1).ToUniTaskAsyncEnumerable().DefaultIfEmpty(99).ToArrayAsync();
+                var ys = Enumerable.Range(1, 1).DefaultIfEmpty(99).ToArray();
+                xs.Should().BeEquivalentTo(ys);
+            }
+            {
+                var xs = await Enumerable.Range(1, 10).ToUniTaskAsyncEnumerable().DefaultIfEmpty(99).ToArrayAsync();
+                var ys = Enumerable.Range(1, 10).DefaultIfEmpty(99).ToArray();
+                xs.Should().BeEquivalentTo(ys);
+            }
+            // Throw
+            {
+                foreach (var item in UniTaskTestException.Throws())
+                {
+                    var xs = item.DefaultIfEmpty().ToArrayAsync();
+                    await Assert.ThrowsAsync<UniTaskTestException>(async () => await xs);
+                }
+            }
+        }
     }
 }

src/UniTask.NetCoreTests/Linq/_Exception.cs

@@ -23,14 +23,14 @@ namespace NetCoreTests.Linq
         }
 
 
-        public static IEnumerable<IUniTaskAsyncEnumerable<int>> Throws()
+        public static IEnumerable<IUniTaskAsyncEnumerable<int>> Throws(int count = 3)
         {
             yield return ThrowImmediate();
             yield return ThrowAfter();
             yield return ThrowInMoveNext();
-            yield return UniTaskAsyncEnumerable.Range(1, 3).Concat(ThrowImmediate());
+            yield return UniTaskAsyncEnumerable.Range(1, count).Concat(ThrowImmediate());
-            yield return UniTaskAsyncEnumerable.Range(1, 3).Concat(ThrowAfter());
+            yield return UniTaskAsyncEnumerable.Range(1, count).Concat(ThrowAfter());
-            yield return UniTaskAsyncEnumerable.Range(1, 3).Concat(ThrowInMoveNext());
+            yield return UniTaskAsyncEnumerable.Range(1, count).Concat(ThrowInMoveNext());
         }
     }