Question

I’m trying to update a single record in a table, but when I run .Firstordefault(), I get the error: “Object reference not set to an instance of an object.”, and if use it with .First(), I get “Sequence contains no elements”.

Using it another place, its working fine, but this time its causing errors.

Here’s the code:

public class AllownceDetails
{
  public int ta_id{get;set;}
  public int tvrid{get;set;}
  public DateTime ofDate{get;set;}
  public string status{get;set;}
  public string userid {get;set;}
}
//Update Method
public void Update(AllownceDetails Allowncedtl)
    {

        var ta = (from a in ce.tbl_tvrallownce
                  where a.tvrid == Allowncedtl.tvrid 
                   //error: Sequence contains no elements

                  select a).SingleOrDefault();

        ta.status = Allowncedtl.status; 
                   //error:Object reference not set to an instance of an object
        ce.SaveChanges();


    }

Answer 1

The Single method throws this exception when there are no elements in list(query) or there are multiple elements.

The SingleOrDefault method throws an exception when there are multiple elements in the list. Returns null when there are no elements.

The FirstOrDefault method return the first item in the list or null. There are no exceptions.

Use it and the check the reference for null

if (ta != null )
   {
      ta.status = Allowncedtl.status; 
      ce.SaveChanges()
   }

The source will always be an object, so no worries about it in your case.