Implement async enumerators and enhance related documentation and tests

README.md

@@ -103,7 +103,7 @@ Relative ordering matches the lower-level writers above (XLS fastest, then XLSB,
 | `struct` (`ExcelParser<T>`) | 15.10 ms | 1.59 MB |
 | `ref struct` + span binding (`RefParser.ParseNamed<T>`) | 12.91 ms | 17.17 KB |
 
-Parsing into a `ref struct` with a `ReadOnlySpan<byte>` text column removes essentially all per-row allocation — ~99.6% less than the `class` baseline — and is ~15% faster, since there's no per-row model allocation and no per-row `string` allocation for the text column. It is not AOT/trim-safe (reflection-based, same tradeoff as `ExcelParser<T>`) and is sync-only: a `ref struct` element type cannot appear in `IAsyncEnumerable<T>`, so this has no async counterpart, permanently.
+Parsing into a `ref struct` with a `ReadOnlySpan<byte>` text column removes essentially all per-row allocation — ~99.6% less than the `class` baseline — and is ~15% faster, since there's no per-row model allocation and no per-row `string` allocation for the text column. It is not AOT/trim-safe (reflection-based, same tradeoff as `ExcelParser<T>`). It can be consumed with `foreach` or `await foreach` but not through `IEnumerable<T>`/`IAsyncEnumerable<T>`/LINQ — a `ref struct` element can't be boxed through those interfaces.
 
 ### Cold start
 
@@ -189,12 +189,24 @@ CSV is exposed as a single, unnamed sheet (`SheetCount == 1`, `SheetName == ""`)
 
 ## Read asynchronously
 
-`Row` and `Cell` are `ref struct` types, so async reading uses a manual loop instead of `await foreach`.
-For XLSX files, the async reader buffers one row at a time and uses the same row parser as the sync reader, so sync and async reads stay behaviorally aligned while awaits happen only when more bytes are needed.
+Every reader supports `await foreach`. For XLSX files, the async reader buffers one row at a time and uses the same row parser as the sync reader, so sync and async reads stay behaviorally aligned while awaits happen only when more bytes are needed.
 
 ```csharp
 using ExcelReader.Core.Reader;
 
+await using var reader = await Excel.FromFileAsync("report.xlsx", cancellationToken);
+
+await foreach (var row in reader)
+{
+    Console.WriteLine(row[0].GetString());
+}
+```
+
+`await foreach` binds to the reader's `GetAsyncEnumerator()` by pattern — the sheet is opened synchronously and only each row advance is awaited. Because `Row` and `Cell` are `ref struct` types, the current row cannot be held across an `await` inside the loop body: read its cells (or copy the values out) before awaiting anything else.
+
+When you need the sheet *opened* asynchronously too (e.g. the first read touches the network), or you need to `await` while a row is in scope, drive the enumerator manually via `GetAsyncEnumeratorAsync`, which awaits the open and threads the cancellation token:
+
+```csharp
 await using var reader = await Excel.FromFileAsync("report.xlsx", cancellationToken);
 await using var rows = await reader.GetAsyncEnumeratorAsync(cancellationToken);
 
@@ -205,6 +217,8 @@ while (await rows.MoveNextAsync())
 }
 ```
 
+`await foreach` does not accept `.WithCancellation(ct)`: `Row` being a `ref struct` rules out `IAsyncEnumerable<Row>`, so the loop binds to the pattern rather than the interface. Pass the token at open time (as above), or use the manual `GetAsyncEnumeratorAsync(ct)` loop.
+
 ## Parse typed rows
 
 `ExcelParser<T>` maps worksheet columns to the public settable properties of `T`. Columns match on the property name, or on `[ExcelColumn("header")]` aliases — repeat the attribute to accept several headers. The first row is the header by default.
@@ -330,11 +344,21 @@ foreach (ChangeRowRef item in RefParser.ParseNamed<ChangeRowRef>(reader))
 }
 ```
 
+The sequence also supports `await foreach`, so a `ref struct` model can be parsed asynchronously — the rows are streamed via `MoveNextAsync` while the model stays a zero-copy `ref struct`:
+
+```csharp
+await using var reader = await Excel.FromFileAsync("changes.xlsx");
+
+await foreach (ChangeRowRef item in RefParser.ParseNamed<ChangeRowRef>(reader))
+{
+    Console.WriteLine($"{Encoding.UTF8.GetString(item.File)}: +{item.LinesAdded}");
+}
+```
+
 A few differences from `ExcelParser<T>`:
 
-- **Span fields alias the reader's row buffer** — valid only until the next row. Copy them out (e.g. `Encoding.UTF8.GetString(span)`) if you need to keep the value past the loop body.
-- **Sync only, permanently.** `IAsyncEnumerable<T>` cannot have a `ref struct` element type, so there is no async counterpart — not a missing feature, a language limitation.
-- **`foreach` only.** The returned sequence cannot be consumed through `IEnumerable<T>`/LINQ — a `ref struct` element can't be boxed through that interface — so iterate it directly.
+- **Span fields alias the reader's row buffer** — valid only until the next row. Copy them out (e.g. `Encoding.UTF8.GetString(span)`) if you need to keep the value past the loop body. Under `await foreach`, the same rule means the model can't be held across an `await` in the loop body.
+- **`foreach` / `await foreach` only.** Consumption is pattern-based — the sequence cannot be surfaced through `IEnumerable<T>`, `IAsyncEnumerable<T>`, or LINQ, because a `ref struct` element can't be boxed through those interfaces (`IAsyncEnumerable<T>` in particular forbids a `ref struct` element type — CS9267). Iterate it directly.
 - **Not AOT/trim-safe**, same tradeoff as `ExcelParser<T>` (both reflect over `T`'s properties and compile setters at runtime).
 - A regular `struct`/`class` model works with `ParseNamed` too — only a genuine `ref struct` model gets the extra zero-copy span-property binding.
 

src/ExcelReader.Core/Parser/Internal/CsvEnumerable.cs

@@ -54,6 +54,8 @@ IAsyncEnumerator<T> IAsyncEnumerable<T>.GetAsyncEnumerator(CancellationToken can
             return GetAsyncEnumerator(cancellationToken);
         }
 
+        [SuppressMessage("Performance", "HLQ006:GetAsyncEnumerator should return a value type",
+            Justification = "Async enumerator requires a class to host the async state machine.")]
         public AsyncEnumerator GetAsyncEnumerator(CancellationToken cancellationToken = default)
         {
             TypeMapInfo<T> info = TypeMapper<T>.GetCsvInfo();

src/ExcelReader.Core/Parser/Internal/NamedRefRowEnumerable.cs

@@ -39,11 +39,33 @@ internal NamedRefRowEnumerable(
             _context = new ExcelRowContext(reader.IsDate1904, formatProvider ?? CultureInfo.InvariantCulture);
         }
 
-        // Zero-allocation struct enumerator — the supported way to consume this sequence.
+        // The supported way to consume this sequence via foreach.
+        [System.Diagnostics.CodeAnalysis.SuppressMessage("Performance", "HLQ006:GetEnumerator should return a value type",
+            Justification = "The enumerator is a class so the same type can also expose MoveNextAsync for the async path.")]
         public NamedRefRowEnumerator<TModel, TEnumerator> GetEnumerator()
         {
-            return new NamedRefRowEnumerator<TModel, TEnumerator>(
-                _reader.GetEnumerator(), _context, _typeInfo, _comparer, _normalization, _headerRow);
+            return new(_reader.GetEnumerator(), _context, _typeInfo, _comparer, _normalization, _headerRow);
+        }
+
+        // The 'await foreach' entry point: C#'s pattern-based async binding picks up this parameterless
+        // GetAsyncEnumerator() (the enumerator it returns has MoveNextAsync/Current/DisposeAsync). Opens
+        // the sheet synchronously — the reader's GetAsyncEnumerator() is a sync open (no I/O await), the
+        // async work is per-row via MoveNextAsync. A ref-struct TModel can't be surfaced through
+        // IAsyncEnumerable<TModel> (CS9267), so this stays a pattern match, never the interface.
+        [System.Diagnostics.CodeAnalysis.SuppressMessage("Performance", "HLQ006:GetAsyncEnumerator should return a value type",
+            Justification = "The enumerator is a class so the same type can also expose MoveNextAsync for the async path.")]
+        public NamedRefRowEnumerator<TModel, TEnumerator> GetAsyncEnumerator()
+        {
+            return new(_reader.GetAsyncEnumerator(), _context, _typeInfo, _comparer, _normalization, _headerRow);
+        }
+
+        // Manual-use alternative that opens the sheet asynchronously (awaits the reader's async open).
+        // Not reachable by 'await foreach' — its shape (returning a ValueTask of the enumerator) doesn't
+        // match the pattern. Await it, then drive the returned enumerator with MoveNextAsync in a loop.
+        public async ValueTask<NamedRefRowEnumerator<TModel, TEnumerator>> GetAsyncEnumeratorAsync(CancellationToken ct = default)
+        {
+            var enumerator = await _reader.GetAsyncEnumeratorAsync(ct).ConfigureAwait(false);
+            return new(enumerator, _context, _typeInfo, _comparer, _normalization, _headerRow);
         }
 
         IEnumerator<TModel> IEnumerable<TModel>.GetEnumerator()

src/ExcelReader.Core/Parser/Internal/NamedRefRowEnumerator.cs

@@ -16,11 +16,17 @@ namespace ExcelReader.Core.Parser.Internal
     // call, immediately writing into a caller-supplied `ref T model`. Every existing caller stores
     // that result in a class FIELD (SyncRowEnumerator<T,TRows>.CurrentValue) — illegal for a
     // ref-struct-constrained T (CS8345: a ref struct field is only legal inside another ref struct).
-    // So this type splits the same two responsibilities instead: MoveNext() only advances/classifies
-    // (skip pre-header rows, build the column map once at the header row), and Current's getter parses
-    // the row into a fresh LOCAL model on every access — never stored as a field, safe to call
-    // repeatedly (idempotent) for the same row.
-    public struct NamedRefRowEnumerator<TModel, TEnumerator> : IDisposable
+    // So this type splits the same two responsibilities instead: MoveNext()/MoveNextAsync() only
+    // advance/classify (skip pre-header rows, build the column map once at the header row), and Current's
+    // getter parses the row into a fresh LOCAL model on every access — never stored as a field, safe to
+    // call repeatedly (idempotent) for the same row.
+    //
+    // A reference type (not a struct): MoveNextAsync's awaiting slow path mutates enumeration state
+    // (_rowNumber, and the one-time column map) across an await, which a struct would silently lose to the
+    // async state machine's by-value `this` copy. A class shares the one instance, exactly like
+    // AsyncRowEnumerator<T,TReader,TRows>. The lazy-Current design above is still required regardless:
+    // a ref-struct TModel can never be stored in a field (CS8345), class or struct.
+    public sealed class NamedRefRowEnumerator<TModel, TEnumerator> : IDisposable, IAsyncDisposable
         where TModel : allows ref struct
         where TEnumerator : class, IExcelRowEnumerator
     {
@@ -55,7 +61,7 @@ internal NamedRefRowEnumerator(
         }
 
         // Recomputed on every access (see class remarks) — never cached in a field.
-        public readonly TModel Current
+        public TModel Current
         {
             get
             {
@@ -85,6 +91,63 @@ public bool MoveNext()
             return false;
         }
 
+        // Async twin of MoveNext, mirroring AsyncRowEnumerator<T,TReader,TRows>.MoveNextAsync: a non-async
+        // fast path that stays synchronous whenever the underlying row-enumerator resolves synchronously
+        // (the common case — no second state machine on top of _rows' own), only falling to an awaiting
+        // continuation on a genuine buffer miss. Every state mutation (ClassifyRow's ref _rowNumber,
+        // BuildColumnMap) runs on the shared class instance, so it survives the await (see class remarks).
+        [SuppressMessage("SharpSource", "SS034:Use await to get the result of a Task",
+            Justification = "The .Result access is guarded by IsCompletedSuccessfully immediately above it — never blocks.")]
+        [SuppressMessage("VisualStudio.Threading", "VSTHRD103:Result synchronously blocks",
+            Justification = "The .Result access is guarded by IsCompletedSuccessfully immediately above it — never blocks.")]
+        public ValueTask<bool> MoveNextAsync()
+        {
+            while (true)
+            {
+                ValueTask<bool> moveTask = _rows.MoveNextAsync();
+                if (!moveTask.IsCompletedSuccessfully)
+                {
+                    return AwaitThenContinueAsync(moveTask);
+                }
+                if (!moveTask.Result)
+                {
+                    return new ValueTask<bool>(false);
+                }
+                switch (ProjectionRules.ClassifyRow(ref _rowNumber, _headerRow, _bindings is not null))
+                {
+                    case ProjectionStep.Yield:
+                        return new ValueTask<bool>(true);
+                    case ProjectionStep.BuildMap:
+                        BuildColumnMap(_rows.Current);
+                        break;
+                    case ProjectionStep.Stop:
+                        return new ValueTask<bool>(false);
+                        // Skip: loop again, still synchronous.
+                }
+            }
+        }
+
+        private async ValueTask<bool> AwaitThenContinueAsync(ValueTask<bool> pendingMoveNext)
+        {
+            if (!await pendingMoveNext.ConfigureAwait(false))
+            {
+                return false;
+            }
+            ProjectionStep step = ProjectionRules.ClassifyRow(ref _rowNumber, _headerRow, _bindings is not null);
+            switch (step)
+            {
+                case ProjectionStep.Yield:
+                    return true;
+                case ProjectionStep.BuildMap:
+                    BuildColumnMap(_rows.Current);
+                    break; // map built at the header row — resume the fast path for the next row.
+                case ProjectionStep.Stop:
+                    return false;
+                // Skip: resume the fast path.
+            }
+            return await MoveNextAsync().ConfigureAwait(false);
+        }
+
         private void BuildColumnMap(Row row)
         {
             int propertyCount = _typeInfo.PropertyCount;
@@ -143,7 +206,7 @@ private void BuildColumnMap(Row row)
             _seen = requireValueCount > 0 ? new bool[bindings.Length] : [];
         }
 
-        private readonly void ParseCurrentRow(Row row, ref TModel model)
+        private void ParseCurrentRow(Row row, ref TModel model)
         {
             NamedColumnBinding<TModel>[] bindings = _bindings!;
             bool track = _requireValueCount > 0;
@@ -187,21 +250,31 @@ private readonly void ParseCurrentRow(Row row, ref TModel model)
             }
         }
 
-        private readonly void ValidateRowValues(NamedColumnBinding<TModel>[] bindings)
+        private void ValidateRowValues(NamedColumnBinding<TModel>[] bindings)
         {
             for (int i = 0; i < bindings.Length; i++)
             {
-                if (bindings[i].RequireValue && !_seen[i])
+                ref readonly var binding = ref bindings[i];
+                if (binding.RequireValue && !_seen[i])
                 {
-                    throw ProjectionRules.MissingRequiredValue(bindings[i].Name, _rowNumber);
+                    throw ProjectionRules.MissingRequiredValue(binding.Name, _rowNumber);
                 }
             }
         }
 
-        public readonly void Dispose()
+        [SuppressMessage("IDisposableAnalyzers.Correctness", "IDISP007:Don't dispose injected",
+            Justification = "_rows is created for this enumerator alone by NamedRefRowEnumerable.Get(Async)Enumerator() — owned here, not injected.")]
+        public void Dispose()
         {
             _rows.Dispose();
         }
+
+        [SuppressMessage("IDisposableAnalyzers.Correctness", "IDISP007:Don't dispose injected",
+            Justification = "_rows is created for this enumerator alone by NamedRefRowEnumerable.Get(Async)Enumerator() — owned here, not injected.")]
+        public ValueTask DisposeAsync()
+        {
+            return _rows.DisposeAsync();
+        }
     }
 }
 #endif

src/ExcelReader.Core/Parser/Internal/RowEnumeratorBase.cs

@@ -13,13 +13,13 @@
     // (a base class can never be less accessible than its derived type).
     [SuppressMessage("Design", "CA1034:Nested types should not be visible",
         Justification = "Base class of the public nested Enumerator types; not itself meant for direct external use.")]
     public abstract class SyncRowEnumerator<T, TRows> : IEnumerator<T>
         where TRows : class, IExcelRowEnumerator
     {
         [SuppressMessage("Performance", "HLQ011:ReadOnlyEnumeratorField",
             Justification = "TRows is constrained to `class` here, so it is always a reference type — no copy-on-mutate risk from a readonly field.")]
         protected readonly TRows Rows;
         protected T CurrentValue = default!;
 
         protected SyncRowEnumerator(TRows rows)
         {
@@ -47,6 +47,8 @@
 
         private protected abstract ProjectionStep Project();
 
+        [SuppressMessage("IDisposableAnalyzers.Correctness", "IDISP007:Don't dispose injected",
+            Justification = "Rows is created for this enumerator alone by the enclosing enumerable's GetEnumerator (reader.GetEnumerator()) — owned here, not injected.")]
         public void Dispose()
         {
             Rows.Dispose();
@@ -66,7 +68,7 @@
     // the row-enumerator's own (e.g. XlsxReader.Enumerator.MoveNextAsync / CsvReader.Enumerator.MoveNextAsync).
     [SuppressMessage("Design", "CA1034:Nested types should not be visible",
         Justification = "Base class of the public nested AsyncEnumerator types; not itself meant for direct external use.")]
     public abstract class AsyncRowEnumerator<T, TReader, TRows> : IAsyncEnumerator<T>
         where TReader : IExcelRowReader<TRows>
         where TRows : class, IExcelRowEnumerator
     {
@@ -74,8 +76,8 @@
         [SuppressMessage("SharpSource", "SS066:Disposable field is not disposed", Justification = "Borrowed, not owned.")]
         private readonly TReader _reader;
         private readonly CancellationToken _ct;
         protected TRows? Rows;
         protected T CurrentValue = default!;
 
         protected AsyncRowEnumerator(TReader reader, CancellationToken ct)
         {

src/ExcelReader.Core/Reader/CsvReader.cs

@@ -97,6 +97,18 @@ IExcelRowEnumerator IExcelRowReader<IExcelRowEnumerator>.GetEnumerator()
             return GetEnumerator();
         }
 
+        [SuppressMessage("Performance", "HLQ006:GetAsyncEnumerator should return a value type",
+            Justification = "Enumerator is a class so the same type can also expose MoveNextAsync for the async path.")]
+        public Enumerator GetAsyncEnumerator()
+        {
+            return GetEnumerator();
+        }
+
+        IExcelRowEnumerator IExcelRowReader<IExcelRowEnumerator>.GetAsyncEnumerator()
+        {
+            return GetAsyncEnumerator();
+        }
+
         [SuppressMessage("IDisposableAnalyzers.Correctness", "IDISP001:Dispose created",
             Justification = "Enumerator ownership transfers to the caller, who disposes it via await using / DisposeAsync.")]
         [SuppressMessage("Reliability", "CA2000:Dispose objects before losing scope",

src/ExcelReader.Core/Reader/IExcelRowReader.cs

@@ -7,6 +7,7 @@ public interface IExcelRowReader<TEnumerator>
     {
         bool IsDate1904 { get; }
         TEnumerator GetEnumerator();
+        TEnumerator GetAsyncEnumerator();
         ValueTask<TEnumerator> GetAsyncEnumeratorAsync(CancellationToken ct = default);
     }
 

src/ExcelReader.Core/Reader/XlsReader.cs

@@ -103,17 +103,37 @@ IExcelRowEnumerator IExcelRowReader<IExcelRowEnumerator>.GetEnumerator()
 
         [SuppressMessage("Performance", "HLQ006:GetAsyncEnumerator should return a value type",
             Justification = "Enumerator is a class so the same type can also expose MoveNextAsync for parity with XlsxReader.")]
-        public Enumerator GetAsyncEnumerator(CancellationToken ct = default)
+        public Enumerator GetAsyncEnumerator()
+        {
+            return GetEnumerator();
+        }
+
+        // ct overload takes no default value: the parameterless GetAsyncEnumerator() above already covers
+        // the no-argument call, so a default here would only shadow it.
+        [SuppressMessage("Performance", "HLQ006:GetAsyncEnumerator should return a value type",
+            Justification = "Enumerator is a class so the same type can also expose MoveNextAsync for parity with XlsxReader.")]
+        public Enumerator GetAsyncEnumerator(CancellationToken ct)
         {
             ct.ThrowIfCancellationRequested();
             return new Enumerator(this, _sheets[_current].Offset, ct);
         }
 
+        IExcelRowEnumerator IExcelRowReader<IExcelRowEnumerator>.GetAsyncEnumerator()
+        {
+            return GetAsyncEnumerator();
+        }
+
+        [SuppressMessage("Reliability", "CA2000:Dispose objects before losing scope",
+            Justification = "Enumerator ownership transfers to the caller, who disposes it via await using / DisposeAsync.")]
         public ValueTask<Enumerator> GetAsyncEnumeratorAsync(CancellationToken ct = default)
         {
             return new ValueTask<Enumerator>(GetAsyncEnumerator(ct));
         }
 
+        [SuppressMessage("Reliability", "CA2000:Dispose objects before losing scope",
+            Justification = "Enumerator ownership transfers to the caller, who disposes it via await using / DisposeAsync.")]
+        [SuppressMessage("Performance", "CA1849:Call async methods when in an async method",
+            Justification = "XlsReader is fully in-memory; opening the enumerator is synchronous, so there is no async open to await here.")]
         ValueTask<IExcelRowEnumerator> IExcelRowReader<IExcelRowEnumerator>.GetAsyncEnumeratorAsync(CancellationToken ct)
         {
             return new ValueTask<IExcelRowEnumerator>(GetAsyncEnumerator(ct));