Fixed issue 627

src/MiniExcel/OpenXml/ExcelOpenXmlSheetWriter.DefaultOpenXml.cs

@@ -183,7 +183,10 @@ private Tuple<string, string, string> GetCellValue(int rowIndex, int cellIndex,
             return Tuple.Create(RegularCellStyleIndex, StringDataType, string.Empty);
 
         if (value is string str)
-            return Tuple.Create(RegularCellStyleIndex, StringDataType, ExcelOpenXmlUtils.EncodeXML(str));
+        {
+            var styleIndex = columnInfo?.ExcelFormatId is { } fmt and not -1 ? fmt.ToString() : RegularCellStyleIndex; 
+            return Tuple.Create(styleIndex, StringDataType, ExcelOpenXmlUtils.EncodeXML(str));
+        }
 
         var type = GetValueType(value, columnInfo);
 

tests/MiniExcelTests/MiniExcelIssueAsyncTests.cs

@@ -224,7 +224,7 @@
 
     private class Issue243Dto
     {
         public string Name { get; set; }
         public int Age { get; set; }
         public DateTime InDate { get; set; }
     }
@@ -358,11 +358,11 @@
         var sheets = new DataSet();
 
         var users = JsonConvert.DeserializeObject<DataTable>(JsonConvert.SerializeObject(new[] { new { Name = "Jack", Age = 25 }, new { Name = "Mike", Age = 44 } }));
         users.TableName = "users";
         sheets.Tables.Add(users);
 
         var department = JsonConvert.DeserializeObject<DataTable>(JsonConvert.SerializeObject(new[] { new { ID = "01", Name = "HR" }, new { ID = "02", Name = "IT" } }));
         department.TableName = "department";
         sheets.Tables.Add(department);
 
         var rowsWritten = await MiniExcel.SaveAsAsync(path, sheets);
@@ -1888,4 +1888,27 @@
         Assert.All(rows, x => Assert.Single(x));
         Assert.Equal("Name", rows[0].A);
     }
+
+    [Fact]
+    public async Task TestIssue627()
+    {
+        var data = new[] { new { LongNumber = "1550432695793487872" } };
+
+        var config = new OpenXmlConfiguration
+        {
+            DynamicColumns = [
+                new DynamicExcelColumn("LongNumber") { Format = "@" } 
+            ]
+        };
+
+        await using var ms = new MemoryStream();
+        await ms.SaveAsAsync(data, configuration: config);
+        ms.Seek(0, SeekOrigin.Begin);
+
+        using var package = new ExcelPackage(ms);
+        var cell = package.Workbook.Worksheets[0].Cells["A2"];
+
+        Assert.Equal("1550432695793487872", cell.GetValue<string>());
+        Assert.Equal("@", cell.Style.Numberformat.Format);
+    }
 }