Fixing Resharper's "Implicitly captured closure: this"

Question

As far as I understand, in the resharper suggestion " Implicitly captured closure: this", this refers to the instance of HomeController class. The question is now why did the following solutions worked to fix the problem and removed the Resharper suggestion.

public class HomeController : Controller
{
    private ExcelDocument _excelDoc = new ExcelDocument();
    private IDatabase _database = new CoreDatabase();

    [HttpPost]
    public JsonResult Insert(HttpPostedFileBase file, int programId)
    {
        _excelDoc.ReadData(firstDataRowIdx: 1, headerRowIdx: 0, excelFile: file);

        _excelDoc.CompareToDatabaseCodes(_database.
             .GetCodesWhere(c => c.ProgramID == programId && c.Active)// In this lambda Resharper says: 'Implicitly captured closure: this'
             .Select(rc => rc.Code)
             .ToList());

        _excelDoc.InsertTireIds(_database
             .GetTiersWhere(t => t.ProgramID == programId && _excelDoc.DistinctTiers.Contains(t.Tier)));

        //More code...
    }
}

I've found two ways to remove the Resharper suggestion without really understanding the reason behind those fixes.

1. Assign programId parameter to local variable like this:

   [HttpPost]
   public JsonResult Insert(HttpPostedFileBase file, int programId)
   {
       _excelDoc.ReadData(firstDataRowIdx: 1, headerRowIdx: 0, excelFile: file);
   
       // Fix 
       int selectedProgramId = programId;
   
       _excelDoc.CompareToDatabaseCodes(_database.
            .GetCodesWhere(c => c.ProgramID == selectedProgramId && c.Active)// In this lambda Resharper says: 'Implicitly captured closure: this'
            .Select(rc => rc.Code)
            .ToList());
   
       _excelDoc.InsertTireIds(_database
            .GetTiersWhere(t => t.ProgramID == selectedProgramId && _excelDoc.DistinctTiers.Contains(t.Tier)));
       //More code...
   }
   

2. Assign _excelDoc.DistinctTiers (in second lambda) to a local variable:

   [HttpPost]
   public JsonResult Insert(HttpPostedFileBase file, int programId)
   {
       _excelDoc.ReadData(firstDataRowIdx: 1, headerRowIdx: 0, excelFile: file); 
   
       _excelDoc.CompareToDatabaseCodes(_database.
            .GetCodesWhere(c => c.ProgramID == programId && c.Active)// In this lambda Resharper says: 'Implicitly captured closure: this'
            .Select(rc => rc.Code)
            .ToList());
   
       // Fix 
       List<int> distinctTiers = _excelDoc.DistinctTiers;
   
       _excelDoc.InsertTireIds(_database
            .GetTiersWhere(t => t.ProgramID == programId && distinctTiers.Contains(t.Tier)));
   
       //More code...
   }
   

Please help me understand why my fixes worked.

Let me know if more info is required to explain the problem better.

Thanks!!!

Answer 1

What it means is that you're including a field from the class in a lamda expression. Because Resharper doesn't know what that lamda is going to be used for, it can't be certain that it isn't going to survive beyond the scope of the method call. If, for instance, that lamda is being assigned to some long-lived field of another class, it will refer to the current value of this._excelDoc - which might in the meantime have been changed, or set to null etc...

Resharper is simply telling you that the lamda's lifecycle shouldn't go beyond the scope of the method, or it will continue to operation on the _excelDoc as referred to by this instance.