revit encountered a system.nullreferenceexception object reference not set to ob

[wahedfazeli]

i have this code for import from excel data to revit this is my code:

{

UIApplication uiapp = commandData.Application;
            UIDocument uidoc = uiapp.ActiveUIDocument;
            //Application app = uiapp.Application;
            Document doc = uidoc.Document;
            string fileman = Path.Combine(Path.GetTempPath(), "MyExcelbad.xlsx");

            using (Transaction t = new Transaction(doc, "param"))
            {
                t.Start();
                using (ExcelPackage package = new ExcelPackage(new FileInfo(fileman)))
                {
                    ExcelWorksheet sheet = package.Workbook.Worksheets["Wall"];

foreach (WallType elem in new FilteredElementCollector(doc).OfClass(typeof(WallType)).WhereElementIsElementType())
                    {
                        //stacked don't have cost
                        if (elem.Kind == WallKind.Stacked)
                        {
                            continue;
                        }
                        for (int value = 2; value <= sheet.Dimension.Rows; value++)
                        {
                            if (sheet.Cells[value, 1].Value.Equals(elem.Name))
                            {
                                if (sheet.Cells[value, 1].Value.Equals("Exterior - Brick on Mtl. Stud"))
                                {
                                    continue;

                                }
                                if (sheet.Cells[value, 1].Value.Equals("Exterior - Block on Mtl. Stud"))
                                {
                                    continue;

                                }
                                sheet.Cells[value, 8].Value = elem.Name;
                                if (null != elem.get_Parameter(BuiltInParameter.ALL_MODEL_COST))
                                {
                                    Parameter cost = elem.get_Parameter(BuiltInParameter.ALL_MODEL_COST);
                                    cost.Set(sheet.Cells[value, 4].Value.ToString());
                                    elem.get_Parameter(BuiltInParameter.KEYNOTE_PARAM).Set(10);
                                }
                            }
                        }
                    }
                    package.Save();
                }
                    t.Commit();
            }
            return Result.Succeeded;
       }

but i have this error

i dont know why this error happened for me

this is line 64

:

[jeremytammik]

Step through your code line by line and you will see what is causing the problem.

[wahedfazeli]

Thanks dear jeremy
I found my problem the cells of excel were null this create problem for me
Thank you

[Anonymous]

To fully understand why a NullReferenceException is thrown, it is important to know the difference between value types and reference types.

So, if you're dealing with value types, NullReferenceExceptions can not occur. Though you need to keep alert when dealing with reference types!

Only reference types, as the name is suggesting, can hold references or point literally to nothing (or 'null'). Whereas value types always contain a value.

Reference types (these ones must be checked):

• dynamic
  object
  string

Value types (you can simply ignore these ones):

• Numeric types
  Integral types
  Floating-point types
  decimal
  bool
  User defined structs