清单中的项目不存在时掉落如何检查

Question

I have this linq query but its falling over when no gender has been set against the user it says squencce

List<StandardLookUpList> _AnalsisCodes = GetAnayalsisCodesForExportCode();

var codesForThisItem = _AnalsisCodes.Where(w => w.ItemCode == item.ItemCode);
if (codesForThisItem.Count()  > 0 )
{
     if (codesForThisItem.First(x => x.code == Constants.Sport) != null)
      sport = codesForThisItem.First(x => x.code == Constants.Sport);

       if(codesForThisItem.First(x => x.code == Constants.Gender) !=null)
       gender = codesForThisItem.First(x => x.code == Constants.Gender);
}     

I thought that this line was enough to fix this?.

if (codesForThisItem.First(x => x.code == Constants.Sport)  

But its actually the code for this item is failing I cant use count to strap that as it may have the other codes held against it what is my best way of trapping if its not in the list replace with null string instead.

Answer 1

you can use .FirstOrDefault() instead and then check whether the result is null before proceeding. The problem with what you've written is that .First() always expects there to be a matching result:

List<StandardLookUpList> _AnalsisCodes = GetAnayalsisCodesForExportCode();
var codesForThisItem = _AnalsisCodes.Where(w => w.ItemCode == item.ItemCode);

if (codesForThisItem.Any())
{
     var sportResult = codesForThisItem.FirstOrDefault(x => x.code == Constants.Sport);
     if (sportResult != null) sport = sportResult;

     var genderResult = codesForThisItem.FirstOrDefault(x => x.code == Constants.Gender); 
     if (genderResult != null) gender = genderResult;
}

In fact, if it's always OK for sport and gender themslves to potentially be null (I don't know what they're set to before this code runs or what rules you have about them), you could just do:

List<StandardLookUpList> _AnalsisCodes = GetAnayalsisCodesForExportCode();
var codesForThisItem = _AnalsisCodes.Where(w => w.ItemCode == item.ItemCode);

if (codesForThisItem.Any())
{
     sport = codesForThisItem.FirstOrDefault(x => x.code == Constants.Sport);
     gender = codesForThisItem.FirstOrDefault(x => x.code == Constants.Gender); 
}

Answer 2

Use FirstOrDefault instead of First . First will throw an exception (Sequence contains no matching element) when your predicate does not match any elements.

List<StandardLookUpList > _AnalsisCodes = GetAnayalsisCodesForExportCode();

var codesForThisItem = _AnalsisCodes.Where(w => w.ItemCode == item.ItemCode);
if (codesForThisItem.Any())
{
    if (codesForThisItem.FirstOrDefault(x => x.code == Constants.Sport) != null)
    {
        sport = codesForThisItem.First(x => x.code == Constants.Sport);
    }

    if (codesForThisItem.FirstOrDefault(x => x.code == Constants.Gender) != null)
    {
        gender = codesForThisItem.First(x => x.code == Constants.Gender);
    }
}