#2325 - Throws NotSupportedException if opening workbook saved in Strict OpenXML format

src/EPPlus/ExcelWorkbook.cs

@@ -1127,14 +1127,33 @@ public bool Date1904
             }
         }
 
+        private const string StrictSpreadsheetMainNamespace = "http://purl.oclc.org/ooxml/spreadsheetml/main";
+
+        // Call this when loading the workbook XML
+        private void ValidateWorkbookNamespace()
+        {
+            string defaultNamespace = _workbookXml.DocumentElement.NamespaceURI;
+            if (defaultNamespace == StrictSpreadsheetMainNamespace)
+            {
+                throw new NotSupportedException(
+                    "The workbook is saved in the \"Strict Open XML Spreadsheet\" format, " +
+                    "which is not supported by EPPlus. " +
+                    "Please re-save the workbook as a regular \"Excel Workbook (.xlsx)\" " +
+                    "in the Save As dialog and try again.");
+            }
+        }
+
 
         /// <summary>
         /// Create or read the XML for the workbook.
         /// </summary>
         private void CreateWorkbookXml(XmlNamespaceManager namespaceManager)
         {
             if (_package.ZipPackage.PartExists(WorkbookUri))
+            {
                 _workbookXml = _package.GetXmlFromUri(WorkbookUri);
+                ValidateWorkbookNamespace();
+            }  
             else
             {
                 // create a new workbook part and add to the package

src/EPPlusTest/EPPlus.Test.csproj

@@ -250,6 +250,9 @@
 		<None Update="Workbooks\SignedWorkbook1.xlsm">
 			<CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
 		</None>
+		<None Update="Workbooks\StrictOpenXml.xlsx">
+		  <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+		</None>
 		<None Update="Workbooks\SubtotalFilters.xlsx">
 			<CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
 		</None>

src/EPPlusTest/Issues/WorksheetIssues.cs

@@ -1130,5 +1130,16 @@ public void InsertAndShift_ShouldNotThrow_WhenArrayIsFull()
             sheet.InsertColumn(5, 1);
         }
 
+        [TestMethod]
+        public void Issue2325()
+        {
+            var ex = Assert.ThrowsExactly<NotSupportedException>(() =>
+            {
+                var package = OpenTemplatePackage("StrictOpenXml.xlsx");
+                var ws = package.Workbook.Worksheets.First();
+            });
+            Assert.IsTrue(ex.Message.Contains("Strict Open XML"));
+        }
+
     }
 }