Lazy-load SharedStringsTable with streaming parser, dirty tracking, and allocation reduction

benchmarks/NPOI.Benchmarks/LargeExcelFileBenchmark.cs

@@ -14,6 +14,11 @@ public class LargeExcelFileBenchmark
     private static MemoryStream _memoryStream;
     private string _filePath;
 
+    // 36 MB .xlsx sourced from https://github.com/mini-software/MiniExcel/tree/master/benchmarks/MiniExcel.Benchmarks
+    // 1,000,000 rows × 10 cols, all cells are shared strings → uniqueCount=1,000,000
+    // Uncompressed sharedStrings.xml is ~31 MB, making SST the dominant parse cost.
+    private string _largeFileWithSstPath;
+
     [GlobalSetup]
     public void GlobalSetup()
     {
@@ -37,6 +42,8 @@ public void GlobalSetup()
 
         _loadedWorkBook = new XSSFWorkbook(copyPath);
         _memoryStream = new MemoryStream();
+
+        _largeFileWithSstPath = Path.Combine("data", "Test1000000x10_SharingStrings.xlsx");
     }
 
     [Benchmark]
@@ -46,6 +53,35 @@ public void XSSFWorkbookLoad()
         workbook.Dispose();
     }
 
+    /// <summary>
+    /// Opens a 36 MB workbook whose sharedStrings.xml decompresses to ~31 MB
+    /// (1,000,000 unique strings) and immediately disposes without reading any cells.
+    /// With lazy SST loading the shared strings table is never parsed, so this
+    /// benchmark represents the minimum overhead of opening the workbook.
+    /// Compare with <see cref="XSSFWorkbookLargeSstLoadStrings"/> which forces
+    /// SST parsing to be able to read cell values.
+    /// </summary>
+    [Benchmark]
+    public void XSSFWorkbookLargeSstOpenDispose()
+    {
+        using var workbook = new XSSFWorkbook(_largeFileWithSstPath, true);
+    }
+
+    /// <summary>
+    /// Opens the same 36 MB workbook and explicitly forces the SST to load by
+    /// accessing <see cref="NPOI.XSSF.Model.SharedStringsTable.Count"/>.
+    /// This is the baseline that shows how expensive eager DOM-based SST parsing
+    /// would be; with lazy loading + streaming parser the cost is deferred and
+    /// reduced in allocation compared to the old DOM path.
+    /// </summary>
+    [Benchmark]
+    public void XSSFWorkbookLargeSstLoadStrings()
+    {
+        using var workbook = new XSSFWorkbook(_largeFileWithSstPath, true);
+        // Force SST parse
+        _ = workbook.GetSharedStringSource().Count;
+    }
+
     [Benchmark]
     public void XSSFReaderLoad()
     {

benchmarks/NPOI.Benchmarks/NPOI.Benchmarks.csproj

@@ -19,6 +19,9 @@
       <None Update="data\test-performance.xlsx">
         <CopyToOutputDirectory>Always</CopyToOutputDirectory>
       </None>
+      <None Update="data\Test1000000x10_SharingStrings.xlsx">
+        <CopyToOutputDirectory>Always</CopyToOutputDirectory>
+      </None>
     </ItemGroup>
 
 </Project>