Is it possible to overload the null-coalescing operator for a class in C#?
Say for example I want to return a default value if an instance is null and return the instance if it's not. The code would look like something like this:
return instance ?? new MyClass("Default");
But what if I would like to use the null-coalescing operator to also check if the MyClass.MyValue is set?
Good question! It's not listed one way or another in the list of overloadable and non-overloadable operators and nothing's mentioned on the operator's page.
So I tried the following:
public class TestClass
{
public static TestClass operator ??(TestClass test1, TestClass test2)
{
return test1;
}
}
and I get the error "Overloadable binary operator expected". So I'd say the answer is, as of .NET 3.5, a no.
According to the ECMA-334 standard, it is not possible to overload the ?? operator.
Similarly, you cannot overload the following operators:
=
&&
||
?:
?.
checked
unchecked
new
typeof
as
is
Simple answer: No
C# design principles do not allow operator overloading that change semantics of the language. Therefore complex operators such as compound assignment, ternary operator and ... can not be overloaded.
This is rumored to be part of the next version of C#. From http://damieng.com/blog/2013/12/09/probable-c-6-0-features-illustrated
7. Monadic null checking
Removes the need to check for nulls before accessing properties or methods. Known as the Safe Navigation Operator in Groovy).
Before
if (points != null) {
var next = points.FirstOrDefault();
if (next != null && next.X != null) return next.X;
}
return -1;
After
var bestValue = points?.FirstOrDefault()?.X ?? -1;
I was trying to accomplish this with a struct I wrote that was very similar Nullable<T>. With Nullable<T> you can do something like
Nullable<Guid> id1 = null;
Guid id2 = id1 ?? Guid.NewGuid();
It has no problem implicitly converting id1 from Nullable<Guid> to a Guid despite the fact that Nullable<T> only defines an explicit conversion to type T. Doing the same thing with my own type, it gives an error
Operator '??' cannot be applied to operands of type 'MyType' and
'Guid'
So I think there's some magic built into the compiler to make a special exception for Nullable<T>. So as an alternative...
tl;dr
We can't override the ?? operator, but if you want the coalesce operator to evaluate an underlying value rather than the class (or struct in my case) itself, you could just use a method resulting in very few extra keystrokes required. With my case above it looks something like this:
public struct MyType<T>
{
private bool _hasValue;
internal T _value;
public MyType(T value)
{
this._value = value;
this._hasValue = true;
}
public T Or(T altValue)
{
if (this._hasValue)
return this._value;
else
return altValue;
}
}
Usage:
MyType<Guid> id1 = null;
Guid id2 = id1.Or(Guid.Empty);
This works well since it's a struct and id1 itself can't actually be null. For a class, an extension method could handle if the instance is null as long as the value you're trying to check is exposed:
public class MyClass
{
public MyClass(string myValue)
{
MyValue = myValue;
}
public string MyValue { get; set; }
}
public static class MyClassExtensions
{
public static string Or(this MyClass myClass, string altVal)
{
if (myClass != null && myClass.MyValue != null)
return myClass.MyValue;
else
return altVal;
}
}
Usage:
MyClass mc1 = new MyClass(null);
string requiredVal = mc1.Or("default"); //Instead of mc1 ?? "default";
If anyone is here looking for a solution, the closest example would be to do this
return instance.MyValue != null ? instance : new MyClass("Default");
Related
I am wondering if someone can point me to the official documentation for how the == operator resolves implicit conversions.
I have this struct:
public readonly struct Name
{
private readonly string _value;
public Name(string name)
{
if (name == null)
throw new ArgumentNullException(nameof(name));
if (string.IsNullOrWhiteSpace(name))
throw new ArgumentException("Invalid name.");
_value = name;
}
public static implicit operator string(Name name) => name._value;
public override string ToString()
{
return _value;
}
}
Then I run the following code:
var a = new Name("a");
var b = new Name("a");
var eq = a == b;
Because I have defined the implicit conversion operator to string on the struct, this equality check returns true.
It has somehow resolved that Name can be converted to string, and uses the string comparison since == doesn't work on user defined structs without specifically overloading it.
I have tried to find documentation for how the == operator resolves which types it can compare, but can someone point me in the right direction?
You may view the following documentation regarding equality operators from the official MSDN site.
https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/language-specification/expressions#reference-type-equality-operators
I have a class which contains a nullable strings, I want to make a check to see whether they stay null or somebody has set them.
simliar to strings, the class contains integers which are nullable, where i can perform this check by doing an equality comparison
with the .HasValue() method - it seems like strings dont have this?
So how do check whether it goes from null to notNull?
public class Test
{
public string? a
public string? b
public int? c
}
var oldQ = new Test(c=123)
var newQ = new Test(c=546)
bool isStilValid = newQ.c.HasValue() == oldQ.c.HasValue() //(this is not possible?)&& newQ.b.HasValue() == oldQ.b.HasValue()
why is this not possible?
HasValue property belongs to Nullable<T> struct, where T is also restricted to be a value type only. So, HasValue is exist only for value types.
Nullable reference types are implemented using type annotations, you can't use the same approach with nullable value types. To check a reference type for nullability you could use comparison with null or IsNullOrEmpty method (for strings only). So, you can rewrite your code a little bit
var oldQ = new Test() { c = 123 };
var newQ = new Test() { c = 456 };
bool isStilValid = string.IsNullOrEmpty(newQ.b) == string.IsNullOrEmpty(oldQ.b);
Or just use a regular comparison with null
bool isStilValid = (newQ.b != null) == (oldQ.b != null);
Only struct in C# have HasValue method, but you can simple create your own string extension as below and that will solve your problem.
public static class StringExtension {
public static bool HasValue(this string value)
{
return !string.IsNullOrEmpty(value);
}
}
I hope this is helpful for someone.
The equivalent comparing to null would be:
bool isStillValid = (newQ.c != null) == (oldQ.c != null) && (newQ.b != null) == (oldQ.b != null);
That's the equivalent to your original code, but I'm not sure the original code is correct...
isStillValid will be true if ALL the items being tested for null are actually null. Is that really what you intended?
That is, if newQ.c is null and oldQ.c is null and newQ.b is null and oldQ.b is null then isStillValid will be true.
The Nullable<T> type requires a type T that is a non-nullable value type for example int or double.
string typed variables are already null, so the nullable string typed variable doesn't make sense.
You need to use string.IsNullOrEmpty or simply null
Given a class with an implicit conversion operator to a Guid:
class MyId
{
private readonly Guid innerGuid;
public MyId(Guid innerGuid)
{
this.innerGuid = innerGuid;
}
public static implicit operator Guid(MyId id)
{
return id.innerGuid;
}
}
When assigning to a Nullable<Guid>:
static void Main(string[] args)
{
MyId someId = null;
Guid? optionalId = someId;
Console.WriteLine("optionalId = " + (optionalId.HasValue ? optionalId.Value.ToString() : "NULL"));
}
I would expect the null reference to simply propagate across from someId to optionalId. ie. get the console output:
optionalId = NULL
However the compiler seems to take precedence on the Nullable's inner Guid type, and it attempts to execute the implicit conversion, which throws a NRE as the id parameter is obviously null.
Is this a bug or by design?
It can be fixed with a null coalescing operator and explicit null value:
static void Main(string[] args)
{
MyId someId = null;
Guid? optionalId = someId ?? (Guid?) null;
Console.WriteLine("optionalId = " + (optionalId.HasValue ? optionalId.Value.ToString() : "NULL"));
}
But that seems very weird. Resharper even fades it out, implying its unnecessary and states:
'??' right operand is always null
because you've a variable of type MyId, not of Guid? and you're assigning that variable to a Guid? and the MyId type has an IMPLICIT conversion operator. If you made it EXPLICIT then the compiler would probably complain about no conversion. What exactly would you expect to happen by assigning a null Foo class instance to a Guid? variable? Do you really expect that to be meaningful? –
Basically I want the following generic function:
public string StringOrNull<T> (T value)
{
if (value != null)
{
return value.ToString();
}
return null;
}
I know I could use a constraint such as where T: class, but T can be a primitive type, Nullable<>, or a class. Is there a generic way to do this?
Edit
Turns out I jumped the gun. This actually works just fine as this sample shows:
class Program
{
static void Main(string[] args)
{
int i = 7;
Nullable<int> n_i = 7;
Nullable<int> n_i_asNull = null;
String foo = "foo";
String bar = null;
Console.WriteLine(StringOrNull(i));
Console.WriteLine(StringOrNull(n_i));
Console.WriteLine(StringOrNull(n_i_asNull));
Console.WriteLine(StringOrNull(foo));
Console.WriteLine(StringOrNull(bar));
}
static private string StringOrNull<T>(T value)
{
if (value != null)
{
return value.ToString();
}
return null;
}
}
default Keyword in Generic Code
In generic classes and methods, one issue that arises is how to assign a default value to a parameterized type T when you do not know the following in advance:
Whether T will be a reference type or a value type.
If T is a value type, whether it will be a numeric value or a struct.
Here's a fun one:
public static class ExtensionFunctions{
public static string ToStringOrNull( this object target ) {
return target != null ? target.ToString() : null;
}
}
The cool part? This will work:
( (string) null ).ToStringOrNull();
So will this:
5.ToStringOrNull();
Extension functions are pretty awesome... they even work on null objects!
If you pass a primitive type, it will automatically be boxed, so you don't need to worry about the null comparison. Since boxing occurs automatically, you can even explicitly compare an int to null without an error, but the result will always be false (and you'll probably get a compiler warning telling you so).
You can use default keyword to return the default of T:
public string StringOrNull<T> (T value)
{
.....
return default(T).ToString();
}
Why generic?
public string StringOrNull (object value)
{
if (value != null){
return value.ToString();
}
return null;
}
This question already has answers here:
Closed 12 years ago.
Possible Duplicate:
Type result with Ternary operator in C#
I ran into this scenario, and there doesn't seem to be a natural way to return a nullable int. The code below gives compilation error because the ternary operator doesn't like null.
public int? userId
{
get
{
int rv;
return int.TryParse(userIdString, out rv) ? rv : null;
}
}
So you (or just me) really have to go all the way and spell it all out:
public int? userId
{
get
{
int id;
if(int.TryParse(userIdString, out id)){
return id;
}
return null;
}
}
EDIT: Is there a more natural way of instantiating a nullable, to make the ternary operator work?
public int? userId
{
get
{
int rv;
return int.TryParse(userIdString, out rv) ? (int?)rv : null;
}
}
public int? userId
{
get
{
int rv;
return int.TryParse(userIdString, out rv) ? (int?)rv : null;
}
}
EDIT: I hadn't read the question carefully enough; the problem isn't that the conditional operator doesn't like nulls - it's that it needs to know what the overall type of the expression should be... and that type has to be the type of either the left-hand side, or the right-hand side. null itself is a type-less expression which can be converted to many types; int is a perfectly valid type, but it's one of the types which null can't be converted to. You can either make the right-hand side explicitly of type int? and get the implicit conversion of int to int? from the left-hand side, or you can perform a cast on the left-hand side, and get the implicit conversion of null to int?.
My answer is like James's, but casting the null instead:
public int? userId
{
get
{
int rv;
return int.TryParse(userIdString, out rv) ? rv : (int?) null;
}
}
This is to emphasize that it's not a null reference; it's a null value of type int?. At that point, the conversion of the int rv is obvious.
There are two other alternatives along the same lines to consider though:
return int.TryParse(userIdString, out rv) ? rv : new int?();
return int.TryParse(userIdString, out rv) ? rv : default(int?);
Personally I think the "casted null" is the nicest form, but you can make up your own mind.
Another alternative would be to have a generic static method:
public static class Null
{
public static T? For<T>() where T : struct
{
return default(T?);
}
}
and write:
return int.TryParse(userIdString, out rv) ? rv : Null.For<int>();
I don't think I really like that, but I offer it for your inspection :)
You're returning a (non-nullable) int or null in the same expression. You'll need to explicitly return a int? in your ternary expression for that to work.