First method of Linq giving error

Question

In my webmethod of my WCF rest service i am trying to find record using Linq's First method like below

  [WebInvoke(UriTemplate = "UpdateProductObject", Method = "PUT")]
    public Result UpdateProductObject(ProductObjectToSave prodSave)
    {

        IUnitOfWork unitOfWork = new UnitOfWork((IObjectContext)_objectSetFactory);
        var versions = prodSave.VersionDetails;

        foreach (var versionDetail in versions)
        {
            var detail = versionDetail;

            var dbVersionentity = _productVersionEntityRepository.First(x => x.Id == detail.Id);

            if (detail.Id  < 0)
            {

                dbVersionentity.Id = GetNextTableId("vProductVersion");

            }

            dbVersionentity.Name = detail.Name;
            dbVersionentity.Code = detail.Name;

            if (detail.Id > 0){

            _productVersionEntityRepository.Update(dbVersionentity);
            }
            else
            {
                _productVersionEntityRepository.Insert(dbVersionentity);
            }


        }


        try
        {
            unitOfWork.Commit();
        }
        catch (Exception e)
        {

            return new Result() { Error = e.Message };
        }

        return new Result() { Error = "Record updated successfully" };

    }

The "_productVersionEntityRepository" is defined as below in my service.

 private readonly Repository<ProductVersionEntity> _productVersionEntityRepository;

When there are no records it throws exception "Sequence contains no elements" . I have done some finding that we can use FirstOrDefault method. But somehow i am not getting that option to use FirstOrDefault. I am really new to this so may be i have missed some links where solution could have been described. Please help me or suggest me other way to do some error handling if First method fails

Answer 1

That's the way that First() works, it will throw an exception if the element can't be found. You can use FirstorDefault() instead and check if the element is null .

Edit: I realised that you're using a custom repository. If you have access to the source code, I advise you to add a new method called .FirstOrDefault() that will take as parameter a predicate and returns null if no entities are found.

Edit 2: Add this method to your repository:

T FirstOrDefault(Expression<Func<T, bool>> where, params Expression<Func<T, object>>[] includeProperties) 
{ 
    IQueryable<T> query = AsQueryable(); 
    query = PerformInclusions(includeProperties, query); 

    return query.FirstOrDefault(where); 
}

Then you can do something like this in your code:

foreach (var versionDetail in versions)
{
    bool isNew = false;
    var detail = versionDetail;

    var dbVersionentity = _productVersionEntityRepository.FirstOrDefault(x => x.Id == detail.Id);

    // not found in database
    if(dbVersionentity == null)
    {
        isNew = true;

        // create entity here
        dbVersionentity = new .....; 

        // you don't need to do this if id is auto-generated, 
        // i.e. Identity column in SQL Server
        dbVersionentity.Id = GetNextTableId("vProductVersion");
    }

    dbVersionentity.Name = detail.Name;
    dbVersionentity.Code = detail.Name;


    if (isNew)
    {
        _productVersionEntityRepository.Insert(dbVersionentity);
    }
    else
    {
        _productVersionEntityRepository.Update(dbVersionentity);
    }
}

Answer 2

尝试

var dbVersionentity = _productVersionEntityRepository.Where(x => x.Id == detail.Id).FirstOrDefault();