Can't set nullable int to LINQ query's return value?

Question

I want to retrieve an Id from a table based on certain criteria. If the criteria isn't met, I just want to return the default value (in this case, I'm assuming null). Here's my code:

int? circuitTypeId = cimsContext.CircuitTypes.FirstOrDefault(ct => ct.Name == circuitType).Id;

I am assuming that if there isn't a match between ct.Name and circuitType that the value returned is null . However, when running my program, I get an error on this line saying "Null reference exception". I know that the record isn't there, but shouldn't I be able to assign the null returned by the query to my nullable int variable?

Answer 1

Your assumption is wrong, as per your query:-

int? circuitTypeId = cimsContext.CircuitTypes
                                .FirstOrDefault(ct => ct.Name == circuitType).Id;

This will return int , when match is found but it will throw a Null Reference Exception when no name matches with that of circuitType . you should do this:-

var circuitTypeObj = cimsContext.CircuitTypes
                                .FirstOrDefault(ct => ct.Name == circuitType);
int? circuitTypeId = circuitTypeObj == null ? (int?)null : circuitTypeObj.Id;

Answer 2

尝试这个 ：

int? circuitTypeId = cimsContext.CircuitTypes.Where(ct => ct.Name == circuitType).Select(p => p.Id).FirstOrDefault();

Answer 3

You should first check for null before assigning the circuitTypeId variable:

int? circuitTypeId;

var circuitType = cimsContext.CircuitTypes.FirstOrDefault(ct => ct.Name == circuitType);
if(circuitType != null)
{
    circuitTypeId = circuitType.Id;
}

Answer 4

The problem is .Id is probably trying to reference a null CirctuitType.

var circuitType = cimsContext.CircuitTypes.FirstOrDefault(ct => ct.Name == circuitType);
int? circuitTypeId = circuitType.Id; // if circuitType is null, a null reference exception is thrown

Instead, you'll need to test the reference for null:

var circuitType = cimsContext.CircuitTypes.FirstOrDefault(ct => ct.Name == circuitType);
int? circuitTypeId = (circuitType == null ? (int?)null : circuitType.Id);

Answer 5

In C# version 6 you can simply add a ? before id to prevent the properties of null objects from being referenced.

int? circuitTypeId = cimsContext.CircuitTypes.FirstOrDefault(ct => ct.Name == circuitType)?.Id;