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C# okay with comparing value types to null
I came a cross something I find strange in the C# (4.0) compiler just now.
int x = 0;
if (x == null) // Only gives a warning - 'expression is always false'
x = 1;
int y = (int)null; // Compile error
int z = (int)(int?)null; // Compiles, but runtime error 'Nullable object must have a value.'
If you cannot assign null to an int, why does the compiler allow you to compare them (It gives a warning only)?
Interestingly, the compiler does not allow the following:
struct myStruct
{
};
myStruct s = new myStruct();
if (s == null) // does NOT compile
;
Why does the struct example not compile, but the int example does?
When the comparison is made, the compiler tries to make it so both operands of the comparison have compatible types if possible.
It had an int value and a constant null value (with no particular type). The only compatible type between the two values is int? so they are coerced to int? and compared as int? == int?. Some int value as an int? is definitely non-null and null is definitely null. The compiler realizes that and since a non-null value is not equal to a definite null value, the warning is given.
actually compilation allow comparing 'int?' to 'int' not 'int' to null which make sense
e.g.
int? nullableData = 5;
int data = 10;
data = (int)nullableData;// this make sense
nullableData = data;// this make sense
// you can assign null to int
nullableData = null;
// same as above statment.
nullableData = (int?)null;
data = (int)(int?)null;
// actually you are converting from 'int?' to 'int'
// which can be detected only at runtime if allowed or not
and that's what you are trying to do in int z = (int)(int?)null;
Related
Can someone please explain me the logic reason why I must cast null to int?
When the left argument type can have their both types ?
Insted of doing
int? k = (DateTime.Now.Ticks%5 > 3 ? 1 : null);
I must do
int? k = (DateTime.Now.Ticks%5 > 3 ? 1 : (int?) null);
although int? k = null is perfectly valid.
An opposite example :
I didn't have to do it in:
string k = (DateTime.Now.Ticks%5 > 3 ? "lala" : null);
int? k = (DateTime.Now.Ticks%5 > 3 ? 1 : (int?) null);
In this case what we have is 1 is int and null is actually null
Now the confusion is the ternary operator is confused as what is the return type int or well null and since its int it won't accept null
So you would need to cast it to a nullable int
Now in the other case you have a string and null is perfectly acceptable for a string
Further explanation can be found at Type inference - Eric
The second and third operands of the ?: operator control the type of
the conditional expression. Let X and Y be the types of the second and
third operands. Then,
If X and Y are the same type, then this is the type of the conditional expression.
Otherwise, if an implicit conversion exists from X to Y, but not from Y to X, then Y is the type of the conditional expression.
Otherwise, if an implicit conversion exists from Y to X, but not from X to Y, then X is the type of the conditional expression.
Otherwise, no expression type can be determined, and a compile-time error occurs.
Because the compiler doesn't use the type of the variable in the left hand side to determine the type of the expression on the right hand side. First it determines what type the expression is, then it determines if it's possible to put it in the variable.
There is no type close enough that is common between an int and a null value. You have to either make the int value nullable or the null value "intable" for the compiler to find a common ground for the values.
When you have a string and a null value the compiler can simply use one of the types, because a string is already nullable.
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Conditional operator assignment with Nullable<value> types?
Why does the conditional operator “?:” not work here when my function is returning a nullable integer “int?”? “return null” works but with “?:” I have to cast “null” to “(int?)” first.
public int? IsLongName(string name) {
int length = name.Length;
// this works without problems
if (name.Length > 10) {
return null;
} else {
return name.Length;
}
// this reports:
// Type of conditional expression cannot be determined because
// there is no implicit conversion between '<null>' and 'int'
return name.Length > 10 ? null : name.Length;
}
Try changing your last line to this:
return name.Length > 10 ? null : (int?)name.Length;
The compiler can't understand what's the return type of the ?: operator. It has conflicting values - null and int. By casting the int to nullable, the compiler can understand the return type is nullable int, and null would be accepted as well.
Both a null value and an int value can be implicitly converted to an int? data type, but a literal of null on its own isn't known by the compiler to be anything other than an object if you don't tell it. There is no common data type that both object and int can be implicitly converted to, which is what the compiler is complaining about.
As Yorye says, you can cast the int to int? to let the compiler do the conversion; or, you can cast the null to int?, which then allows the compiled to use the implicit conversion from int to int?.
Both conditions of the ?: operator must be implicitly compatible. int can never be null, so there is a compile-time error (likewise, null can never be int). You have to cast, there's no way around it using the ternary.
I don't think you would have the same problem with an if statement, because the compiler would only check that the method returns a compatible value to the return type from any given path, not that the return value from any given exit point is implicitly compatible with another block's return value.
The ?: operator only considers the types of its two possible return values. It is not aware of the type of the variable that will receive its result (indeed, in more complex expressions, no explicit variable may exist).
If one of those return values is null, it has no type information - it can only check the other return value's type, and check whether a conversion exists.
In your case, we have null and a return value of type int. There is no conversion available. There would be a conversion to int?, but that is neither of the possible return types that ?: is considering.
int n == 0;
if (n == null)
{
Console.WriteLine("......");
}
Is it true that the result of expression (n == null) is always false since
a value of type int is never equal to null of type int? (see warning below)
Warning CS0472 The result of the expression is always 'false' since a value of type 'int' is never equal to 'null' of type 'int?'
If you want your integer variable to allow null values, declare it to be a nullable type:
int? n = 0;
Note the ? after int, which means that type can have the value null. Nullable types were introduced with v2.0 of the .NET Framework.
In C# using an uninitialized variable is not allowed.
So
int i;
Console.Writeline(i);
Results in a compilation error.
You can initialize int with new such as:
int anInt = new int();
This will result in the Default value for int which is 0. In cases where you do wish to have a generic int one can make the int nullable with the syntax
int? nullableInt = null;
Because int is a value type rather than a reference type. The C# Language Specification doesn't allow an int to contain null. Try compiling this statement:
int x = null ;
and see what you get.
You get the compiler warning because it's a pointless test and the compiler knows it.
"Value types" in .NET (like int, double, and bool) cannot, by definition, be null - they always have an actual value assigned. Check out this good intro to value types vs. reference types.
The usage of NULL applies to Pointers and References in general. A value 0 assigned to an integer is not null. However if you can assign a pointer to the integer and assign it to NULL, the statement is valid.
To sum up =>
/*Use the keyword 'null' while assigning it to pointers and references. Use 0 for integers.*/
Very simply put, an int is a very basic item. It's small and simple so that it can be handled quickly. It's handled as the value directly, not along the object/pointer model. As such, there's no legal "NULL" value for it to have. It simply contains what it contains. 0 means a 0. Unlike a pointer, where it being 0 would be NULL. An object storing a 0 would have a non-zero pointer still.
If you get the chance, take the time to do some old-school C or assembly work, it'll become much clearer.
public static int? n { get; set; } = null;
OR
public static Nullable<int> n { get; set; }
or
public static int? n = null;
or
public static int? n
or just
public static int? n { get; set; }
static void Main(string[] args)
{
Console.WriteLine(n == null);
//you also can check using
Console.WriteLine(n.HasValue);
Console.ReadKey();
}
The null keyword is a literal that represents a null reference, one that does not refer to any object.
In programming, nullable types are a feature of the type system of some programming languages which allow the value to be set to the special value NULL instead of the usual possible values of the data type.
https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/null
https://en.wikipedia.org/wiki/Null
No, because int is a value type. int is a Value type like Date, double, etc. So there is no way to assigned a null value.
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Casting: (NewType) vs. Object as NewType
Why is the C# “as” operator so popular?
Hey,
I know this may be a silly question but this doubt came to me today.
What is the difference between doing
String text = (String) variable;
and
String text = variable as String;
?
A cast can do three things:
Perform a user-defined conversion
Perform an unboxing conversion
Perform a reference conversion
An as operation is almost always a reference conversion, the only exception being unboxing to a nullable type:
object x = "hello";
int? y = x as int?; // y is null afterwards
Then there's the behaviour with conversions which fail at execution time. So the differences are:
Casts performing reference conversions or unboxing will throw InvalidCastException on failure; as will result in the null value of the target type instead
Casts can perform user-defined conversions; as can't
Casts can unbox to non-nullable value types; as can only be used for unboxing if the target type is a nullable value type
as will return null if variable isn't actually of that type (String in this case). The cast will throw an exception.
Here is the link to Eric Lippert's blog on casting in C#. I'd summarize it, but it's pretty short and he'll explain it much better than me.
http://blogs.msdn.com/ericlippert/archive/2009/10/08/what-s-the-difference-between-as-and-cast-operators.aspx
And here is his post on the cast operator:
http://blogs.msdn.com/ericlippert/archive/2009/03/19/representation-and-identity.aspx
There are a lot of different ways to cast in C#.
This will try to cast the reference to a String reference. If the cast fails, it throws an exception:
string text = (String) variable;
This will try to cast the reference to a String reference. If the cast fails, it will return a null reference to be assigned to the variable:
string text = varible as String;
This will cast a string reference to an object reference, which is a safe casting as String inherits from Object:
object text = (object)"1337";
Casting to a parent class can also be done implicitly:
object text = "1337";
This will box a value inside an object, then unbox it to a plain value again:
int value = 42;
object boxed = (object)value;
int valueAgain = (int)boxed;
The boxing can also be done implicitly:
int value = 42;
object boxed = value;
int valueAgain = (int)boxed;
This will make a widening conversion from byte to int:
byte a = 42;
int b = (int)a;
The same works as an implicit conversion:
byte a = 42;
int b = a;
This will make a narrowing conversion from int to byte, throwing away the overflow:
int a = 512;
byte b = (byte)a; // b now contains 0
I've noticed that some .NET structs can be compared to null.
For example:
TimeSpan y = new TimeSpan();
if (y == null)
return;
will compile just fine (the same with the Guid struct).
Now I know that stucts are value type and that the code above should not compile, unless there's an overload of operator == which takes an object. But, as far as I could tell there isn't.
I've looked at the class with Reflector, and also at the docs on MSDN.
The two of them do implement the following interfaces:
IComparable, IComparable<T>, IEquatable<T>
but, trying to implment the same Interfaces did not seem to help:
struct XX : IComparable, IComparable<XX>, IEquatable<XX> {
public int CompareTo(Object obj) {
return 0;
}
public int CompareTo (XX other){
return 0;
}
public bool Equals (XX other){
return false;
}
public override bool Equals(object value){
return false;
}
public static int Compare(XX t1, XX t2){
return 0;
}
}
I'm using: .NET 2.0 Visual Studio 2005.
Does anyone has any idea what's the reason for this ?
I am just trying to get a better understanding. This isn't an issue as I know I shouldn't compare structs to null anyway.
It's the == operator.
The TimeSpan class has an overload of the equality operator:
public static bool operator ==(DateTime d1, DateTime d2)
{
return (t1._ticks == t2._ticks);
}
This in itself doesn't make it possible to compare with null, but...
With the arrival of nullable types, each struct is implicitly convertible to its nullable type, so when you see something like
TimeSpan y = new TimeSpan();
if (y == null)
return;
You don't see that this is happening:
TimeSpan y = new TimeSpan();
if ((Nullable<TimeSpan>)y == (Nullable<TimeSpan>)null)
return;
Null gets the implicit conversion (implicit assignment?), but not all System.Object objects do:
TimeSpan y = new TimeSpan();
object o = null;
if (y == o) //compiler error
return;
Okay, but the equality operator doesn't take nullable arguments, does it?
Well, msdn is of help here, stating:
The predefined unary and binary
operators and any user-defined
operators that exist for value types
may also be used by nullable types.
These operators produce a null value
if [any of] the operands are null; otherwise,
the operator uses the contained value
to calculate the result.
So you effectively get a nullable implementation for each operator for free, with a fixed defined behaviour. The "contained value" mentioned above is the actual value the non-nullable operator would return.
This problem was effectively introduced when nullable types were included. There's an implicit conversion from TimeSpan to TimeSpan?, and there's a comparison between TimeSpan? and the null value of that type.
The compiler issues a warning for some types which makes it clearer what it's trying to do:
int x = 10;
if (x == null)
{
Console.WriteLine();
}
Gives this warning:
Test.cs(9,13): warning CS0472: The result of the expression is always 'false'
since a value of type 'int' is never equal to 'null' of type 'int?'
I believe Marc Gravell and I worked out the circumstances under which the warning is given once... it's a shame it's not consistent.
This case is covered for generics in section 7.9.6 of the C# language specification.
The x == null construct is permitted even though T could represent a value type, and the result is simply defined to be false when T is a value type.
I dug through the spec for a bit and couldn't find a more general rule. Jon's answer indicates it's a nullable promotion issue.
This rule (or a similar variation) does seem to be being applied here. If you look at the reflected output closely you'll notice the comparison isn't there. The C# compiler is apparently optimizing this comparison away and replacing it with false.
For instance, if you type the following
var x = new TimeSpan();
var y = x == null;
Console.WriteLine(x);
Then decompile it you'll see the following
var x = new TimeSpan();
var y = false;
Console.WriteLine(x);
See also: C# 3 (.NET 3.5) version of csc fails to report CS0162 for unrechable code (struct/null)
Starting with the C# 3 compiler that means it sometimes doesn't even warn you about this ;-p
Because Guid / TimeSpan etc provide ==, they fall into this trap where it doesn't warn you.
I FOUND IT :)
The following gives a warning:
int i = 0;
if (i == null)
// ^^ Warning: The result of the expression is always 'false' since a value of
// type 'int' is never equal to 'null' of type 'int?'
The compiler is just failing to emit the correct warning that the null you typed was converted to type TimeSpan? for the comparison.
Edit: The related section in the spec is §13.7.1 stating that null can be implicitly converted to any nullable type, and (the very difficult to read) section §13.7.2 stating a value type T can be implicitly converted to T?.
What I originally wrote:
Whatever's happening is something in the C# spec because like JaredPar says it compiles to simply false.
Note that this doesn't compile:
TimeSpan ts = new TimeSpan();
object o = null;
if (ts == o) // error, Operator '==' cannot be applied to operands of type 'System.TimeSpan' and 'object'
...