I have a dilemma. Should I use line 4, 8, 12, or 16?

UseAsOrCast.cs

// Dilemma, should I use this code:
// NOTE: existingJobDetail.Id is defined as a regular (non-nullable) int.

int? parentJobDetailId = existingJobDetail != null ? existingJobDetail.Id as int? : null;

// Or this code:

int? parentJobDetailId = existingJobDetail != null ? (int?)existingJobDetail.Id : null;

// Or (less likely):

int? parentJobDetailId = existingJobDetail != null ? existingJobDetail.Id as Nullable<int>: null;

// Or:

int? parentJobDetailId = existingJobDetail != null ? (Nullable<int>)existingJobDetail.Id : null;

The first pair is identical to the second pair. int? is just an alias for Nullable<int>.

The as casting might work, but straight casting is more appropriate. as is intended for polymorphic casting, such as when accessing an explicit interface implementation or when attempting to upcast a reference type to a more concrete type and the cast isn't guaranteed to succeed. In this case, Nullable<T> is a value type (struct) and so is your base data type (int), and there exists both explicit and implicit conversion operators. Additionally, as probing carries additional overhead although I am not certain whether the compiler will recognize that existingJobDetail.Id as int? will never return null. If it does recognize that, then I would imagine it would treat the two forms the same as far as emitted IL is concerned.

So I would use the form of your second example:

int? parentJobDetailId = existingJobDetail != null ? (int?)existingJobDetail.Id : null;

I'd probably put a sanity check around the statement, like the following since the parentJobDetailId can be null.

if(existingJobDetail != null){ parentJobDetailId.Value = existringJobDetail.Id; }

It's not as terse, but it is more readable and that's a trade off I like to make.

scratch that, if this is the most current c#, I'd do the following.

int? parentJobDetailId = existingJobDetail?.Id ?? null;

Can you not just create a new nullable variable do a SetValue? I don't have a C# compiler handy + have done very little C# for quite a while.