Firstordefault: Default Value Other Than Null
As I Understand It, in Linq the Method Firstordefault() Can Return a Default Value of Something Other Than Null. What I Haven't Worked out Is What Kind of...
As I understand it, in Linq the method FirstOrDefault() can return a Default value of something other than null. What I haven't worked out is what kind of things other than null can be returned by this (and similar) method when there are no items in the query result. Is there any particular way that this can be set up so that if there is no value for a particular query some predefined value is returned as the default value?
14 Answers
As I understand it, in Linq the method FirstOrDefault() can return a Default value of something other than null.
No. Or rather, it always returns the default value for the element type... which is either a null reference, the null value of a nullable value type, or the natural "all zeroes" value for a non-nullable value type.
Is there any particular way that this can be set up so that if there is no value for a particular query some predefined value is returned as the default value?
For reference types, you can just use:
var result = query.FirstOrDefault() ?? otherDefaultValue;
Of course this will also give you the "other default value" if the first value is present, but is a null reference...
You can use DefaultIfEmpty followed by First:
T customDefault = ...;
IEnumerable<T> mySequence = ...;
mySequence.DefaultIfEmpty(customDefault).First();
General case, not just for value types:
static class ExtensionsThatWillAppearOnEverything
{
public static T IfDefaultGiveMe<T>(this T value, T alternate)
{
if (value.Equals(default(T))) return alternate;
return value;
}
}
var result = query.FirstOrDefault().IfDefaultGiveMe(otherDefaultValue);
Again, this can't really tell if there was anything in your sequence, or if the first value was the default.
If you care about this, you could do something like
static class ExtensionsThatWillAppearOnIEnumerables
{
public static T FirstOr<T>(this IEnumerable<T> source, T alternate)
{
foreach(T t in source)
return t;
return alternate;
}
}
and use as
var result = query.FirstOr(otherDefaultValue);
although as Mr. Steak points out this could be done just as well by .DefaultIfEmpty(...).First().
From the documentation for FirstOrDefault
[Returns] default(TSource) if source is empty;
From the documentation for default(T):
the default keyword, which will return null for reference types and zero for numeric value types. For structs, it will return each member of the struct initialized to zero or null depending on whether they are value or reference types. For nullable value types, default returns a System.Nullable, which is initialized like any struct.
Therefore, the default value can be null or 0 depending on whether the type is a reference or value type, but you cannot control the default behaviour.
Copied over from comment by @sloth
Instead of YourCollection.FirstOrDefault(), you could use YourCollection.DefaultIfEmpty(YourDefault).First() for example.
Example:
var viewModel = new CustomerDetailsViewModel
{
MainResidenceAddressSection = (MainResidenceAddressSection)addresses.DefaultIfEmpty(new MainResidenceAddressSection()).FirstOrDefault( o => o is MainResidenceAddressSection),
RiskAddressSection = addresses.DefaultIfEmpty(new RiskAddressSection()).FirstOrDefault(o => !(o is MainResidenceAddressSection)),
};
Actually, I use two approaches to avoid NullReferenceException when I'm working with collections:
public class Foo
{
public string Bar{get; set;}
}
void Main()
{
var list = new List<Foo>();
//before C# 6.0
string barCSharp5 = list.DefaultIfEmpty(new Foo()).FirstOrDefault().Bar;
//C# 6.0 or later
var barCSharp6 = list.FirstOrDefault()?.Bar;
}
Must Read
For C# 6.0 or later:
Use ?. or ?[ to test if is null before perform a member access Null-conditional Operators documentation
Example:
var barCSharp6 = list.FirstOrDefault()?.Bar;
Marcus Vance is a cybersecurity auditor and technology writer dedicated to educating the public about online safety, data privacy regulations, enterprise security, and emerging cyber threats.
