Type int? vs type int
35
I've this comparison which equals false as expected
bool eq = typeof(int?).Equals(typeof(int));
now I have this code
List<object> items = new List<object>() { (int?)123 };
int result = items.OfType<int>().FirstOrDefault();
but this returns 123 - anyway that value is of type int?
How can this be?
c# casting
asked Mar 27 at 8:27
Dr. SnailDr. Snail
748728
|
show 2 more comments
35
I've this comparison which equals false as expected
bool eq = typeof(int?).Equals(typeof(int));
now I have this code
List<object> items = new List<object>() { (int?)123 };
int result = items.OfType<int>().FirstOrDefault();
but this returns 123 - anyway that value is of type int?
How can this be?
c# casting
asked Mar 27 at 8:27
Dr. SnailDr. Snail
748728
int?boxed asint, and basically every Nullable type, Edit : Marc Gravell have the full answer
– styx
Mar 27 at 8:30
Related post about nullable type: stackoverflow.com/questions/4028830/…. This is called as "type lifting".
– Tetsuya Yamamoto
Mar 27 at 8:32
Before reading this topic I wouldn't even guess that evenList<int?>already holds justinttypes. Proof
– Sinatr
Mar 27 at 8:46
7
@Sinatr no, that is incorrect;List<int?>holdsint?. The important distinction in this example is the use ofList<object>. What you're seeing in that "proof" is something very different;GetType()on anyT?either returns theT, or throws a NRE. It never returnsT?- better example: dotnetfiddle.net/3Gy3Fa - and as for why: becauseGetType()is non-virtual, it cannot be overridden, and thus callingGetType()is a boxing operation (even if used via "constrained call"). And when you box aT?, you either get aTas anobject, or anull.
– Marc Gravell♦
Mar 27 at 8:49
@TetsuyaYamamoto That is not a relevant link. How is the boxing/unboxing behavior of Nullable<T> possible? is.
– GSerg
Mar 27 at 11:17
|
show 2 more comments
35
35
35
3
I've this comparison which equals false as expected
bool eq = typeof(int?).Equals(typeof(int));
now I have this code
List<object> items = new List<object>() { (int?)123 };
int result = items.OfType<int>().FirstOrDefault();
but this returns 123 - anyway that value is of type int?
How can this be?
c# casting
asked Mar 27 at 8:27
Dr. SnailDr. Snail
748728
I've this comparison which equals false as expected
bool eq = typeof(int?).Equals(typeof(int));
now I have this code
List<object> items = new List<object>() { (int?)123 };
int result = items.OfType<int>().FirstOrDefault();
but this returns 123 - anyway that value is of type int?
How can this be?
c# casting
c# casting
asked Mar 27 at 8:27
Dr. SnailDr. Snail
748728
asked Mar 27 at 8:27
Dr. SnailDr. Snail
748728
asked Mar 27 at 8:27
Dr. SnailDr. Snail
748728
asked Mar 27 at 8:27
Dr. SnailDr. Snail
748728
asked Mar 27 at 8:27
Dr. SnailDr. Snail
748728
748728
int?boxed asint, and basically every Nullable type, Edit : Marc Gravell have the full answer
– styx
Mar 27 at 8:30
Related post about nullable type: stackoverflow.com/questions/4028830/…. This is called as "type lifting".
– Tetsuya Yamamoto
Mar 27 at 8:32
Before reading this topic I wouldn't even guess that evenList<int?>already holds justinttypes. Proof
– Sinatr
Mar 27 at 8:46
7
@Sinatr no, that is incorrect;List<int?>holdsint?. The important distinction in this example is the use ofList<object>. What you're seeing in that "proof" is something very different;GetType()on anyT?either returns theT, or throws a NRE. It never returnsT?- better example: dotnetfiddle.net/3Gy3Fa - and as for why: becauseGetType()is non-virtual, it cannot be overridden, and thus callingGetType()is a boxing operation (even if used via "constrained call"). And when you box aT?, you either get aTas anobject, or anull.
– Marc Gravell♦
Mar 27 at 8:49
@TetsuyaYamamoto That is not a relevant link. How is the boxing/unboxing behavior of Nullable<T> possible? is.
– GSerg
Mar 27 at 11:17
|
show 2 more comments
int?boxed asint, and basically every Nullable type, Edit : Marc Gravell have the full answer
– styx
Mar 27 at 8:30
Related post about nullable type: stackoverflow.com/questions/4028830/…. This is called as "type lifting".
– Tetsuya Yamamoto
Mar 27 at 8:32
Before reading this topic I wouldn't even guess that evenList<int?>already holds justinttypes. Proof
– Sinatr
Mar 27 at 8:46
7
@Sinatr no, that is incorrect;List<int?>holdsint?. The important distinction in this example is the use ofList<object>. What you're seeing in that "proof" is something very different;GetType()on anyT?either returns theT, or throws a NRE. It never returnsT?- better example: dotnetfiddle.net/3Gy3Fa - and as for why: becauseGetType()is non-virtual, it cannot be overridden, and thus callingGetType()is a boxing operation (even if used via "constrained call"). And when you box aT?, you either get aTas anobject, or anull.
– Marc Gravell♦
Mar 27 at 8:49
@TetsuyaYamamoto That is not a relevant link. How is the boxing/unboxing behavior of Nullable<T> possible? is.
– GSerg
Mar 27 at 11:17
int? boxed as int , and basically every Nullable type, Edit : Marc Gravell have the full answer– styx
Mar 27 at 8:30
int? boxed as int , and basically every Nullable type, Edit : Marc Gravell have the full answer– styx
Mar 27 at 8:30
Related post about nullable type: stackoverflow.com/questions/4028830/…. This is called as "type lifting".
– Tetsuya Yamamoto
Mar 27 at 8:32
Related post about nullable type: stackoverflow.com/questions/4028830/…. This is called as "type lifting".
– Tetsuya Yamamoto
Mar 27 at 8:32
Before reading this topic I wouldn't even guess that even
List<int?> already holds just int types. Proof– Sinatr
Mar 27 at 8:46
Before reading this topic I wouldn't even guess that even
List<int?> already holds just int types. Proof– Sinatr
Mar 27 at 8:46
7
7
@Sinatr no, that is incorrect;
List<int?> holds int?. The important distinction in this example is the use of List<object>. What you're seeing in that "proof" is something very different; GetType() on any T? either returns the T, or throws a NRE. It never returns T? - better example: dotnetfiddle.net/3Gy3Fa - and as for why: because GetType() is non-virtual, it cannot be overridden, and thus calling GetType() is a boxing operation (even if used via "constrained call"). And when you box a T?, you either get a T as an object, or a null.– Marc Gravell♦
Mar 27 at 8:49
@Sinatr no, that is incorrect;
List<int?> holds int?. The important distinction in this example is the use of List<object>. What you're seeing in that "proof" is something very different; GetType() on any T? either returns the T, or throws a NRE. It never returns T? - better example: dotnetfiddle.net/3Gy3Fa - and as for why: because GetType() is non-virtual, it cannot be overridden, and thus calling GetType() is a boxing operation (even if used via "constrained call"). And when you box a T?, you either get a T as an object, or a null.– Marc Gravell♦
Mar 27 at 8:49
@TetsuyaYamamoto That is not a relevant link. How is the boxing/unboxing behavior of Nullable<T> possible? is.
– GSerg
Mar 27 at 11:17
@TetsuyaYamamoto That is not a relevant link. How is the boxing/unboxing behavior of Nullable<T> possible? is.
– GSerg
Mar 27 at 11:17
|
show 2 more comments
2 Answers
2
active
oldest
votes
47
Nullable types have special "boxing" rules; "boxing" is when a value-type is treated as object, as per your code. Unlike regular value-types, a nullable value-type is boxed either as null (regular null, no type), or as the non-nullable type (the T in T?). So: an int? is boxed as an int, not an int?. Then when you use OfType<int>() on it, you get all the values that are int, which is: the single value you passed in, since it is of type int.
answered Mar 27 at 8:28
Marc Gravell♦Marc Gravell
793k19821602565
phew ok thank you for that explanaion. Is that C# basic knowledge?
– Dr. Snail
Mar 27 at 8:30
12
@Dr.Snail "basic" is relative / subjective, and I'd wager that a good percentage of developers never have a need to know that nuance; it is useful context if you're dealing with boxing, though... and technically it isn't really C# knowledge, but rather: .NET knowledge (it would apply to all languages)
– Marc Gravell♦
Mar 27 at 8:31
5
@Sinatr you can't - the list never containsint?- it only containsintbecause of the boxing rules on nullable types
– Marc Gravell♦
Mar 27 at 8:34
1
@KyleJohnson you added a nullable int to the list. Naively, if nullable ints and ints are different things, you'd expect asking for all the ints in the list to return nothing. There are languages that do it that way, but C# has chosen to do it differently.
– mbrig
Mar 27 at 21:24
1
(and yes, my first sentence there is technically wrong. The nullable int never got added to the list. But if you don't know what's happening here, that's what it looks like is happening)
– mbrig
Mar 27 at 21:26
|
show 2 more comments
6
A nullable value type is boxed by the following rules:
- If
HasValuereturnsfalse, the null reference is produced. - If
HasValuereturnstrue, a value of the underlying value typeTis
boxed, not the instance of nullable.
In your example second rule has been followed as you have value, e.g.
var i = (object)(int?)123;
answered Mar 27 at 9:11
JohnnyJohnny
3,6201021
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2 Answers
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2 Answers
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47
Nullable types have special "boxing" rules; "boxing" is when a value-type is treated as object, as per your code. Unlike regular value-types, a nullable value-type is boxed either as null (regular null, no type), or as the non-nullable type (the T in T?). So: an int? is boxed as an int, not an int?. Then when you use OfType<int>() on it, you get all the values that are int, which is: the single value you passed in, since it is of type int.
answered Mar 27 at 8:28
Marc Gravell♦Marc Gravell
793k19821602565
phew ok thank you for that explanaion. Is that C# basic knowledge?
– Dr. Snail
Mar 27 at 8:30
12
@Dr.Snail "basic" is relative / subjective, and I'd wager that a good percentage of developers never have a need to know that nuance; it is useful context if you're dealing with boxing, though... and technically it isn't really C# knowledge, but rather: .NET knowledge (it would apply to all languages)
– Marc Gravell♦
Mar 27 at 8:31
5
@Sinatr you can't - the list never containsint?- it only containsintbecause of the boxing rules on nullable types
– Marc Gravell♦
Mar 27 at 8:34
1
@KyleJohnson you added a nullable int to the list. Naively, if nullable ints and ints are different things, you'd expect asking for all the ints in the list to return nothing. There are languages that do it that way, but C# has chosen to do it differently.
– mbrig
Mar 27 at 21:24
1
(and yes, my first sentence there is technically wrong. The nullable int never got added to the list. But if you don't know what's happening here, that's what it looks like is happening)
– mbrig
Mar 27 at 21:26
|
show 2 more comments
47
Nullable types have special "boxing" rules; "boxing" is when a value-type is treated as object, as per your code. Unlike regular value-types, a nullable value-type is boxed either as null (regular null, no type), or as the non-nullable type (the T in T?). So: an int? is boxed as an int, not an int?. Then when you use OfType<int>() on it, you get all the values that are int, which is: the single value you passed in, since it is of type int.
answered Mar 27 at 8:28
Marc Gravell♦Marc Gravell
793k19821602565
phew ok thank you for that explanaion. Is that C# basic knowledge?
– Dr. Snail
Mar 27 at 8:30
12
@Dr.Snail "basic" is relative / subjective, and I'd wager that a good percentage of developers never have a need to know that nuance; it is useful context if you're dealing with boxing, though... and technically it isn't really C# knowledge, but rather: .NET knowledge (it would apply to all languages)
– Marc Gravell♦
Mar 27 at 8:31
5
@Sinatr you can't - the list never containsint?- it only containsintbecause of the boxing rules on nullable types
– Marc Gravell♦
Mar 27 at 8:34
1
@KyleJohnson you added a nullable int to the list. Naively, if nullable ints and ints are different things, you'd expect asking for all the ints in the list to return nothing. There are languages that do it that way, but C# has chosen to do it differently.
– mbrig
Mar 27 at 21:24
1
(and yes, my first sentence there is technically wrong. The nullable int never got added to the list. But if you don't know what's happening here, that's what it looks like is happening)
– mbrig
Mar 27 at 21:26
|
show 2 more comments
47
47
47
Nullable types have special "boxing" rules; "boxing" is when a value-type is treated as object, as per your code. Unlike regular value-types, a nullable value-type is boxed either as null (regular null, no type), or as the non-nullable type (the T in T?). So: an int? is boxed as an int, not an int?. Then when you use OfType<int>() on it, you get all the values that are int, which is: the single value you passed in, since it is of type int.
answered Mar 27 at 8:28
Marc Gravell♦Marc Gravell
793k19821602565
Nullable types have special "boxing" rules; "boxing" is when a value-type is treated as object, as per your code. Unlike regular value-types, a nullable value-type is boxed either as null (regular null, no type), or as the non-nullable type (the T in T?). So: an int? is boxed as an int, not an int?. Then when you use OfType<int>() on it, you get all the values that are int, which is: the single value you passed in, since it is of type int.
answered Mar 27 at 8:28
Marc Gravell♦Marc Gravell
793k19821602565
answered Mar 27 at 8:28
Marc Gravell♦Marc Gravell
793k19821602565
answered Mar 27 at 8:28
Marc Gravell♦Marc Gravell
793k19821602565
answered Mar 27 at 8:28
Marc Gravell♦Marc Gravell
793k19821602565
793k19821602565
phew ok thank you for that explanaion. Is that C# basic knowledge?
– Dr. Snail
Mar 27 at 8:30
12
@Dr.Snail "basic" is relative / subjective, and I'd wager that a good percentage of developers never have a need to know that nuance; it is useful context if you're dealing with boxing, though... and technically it isn't really C# knowledge, but rather: .NET knowledge (it would apply to all languages)
– Marc Gravell♦
Mar 27 at 8:31
5
@Sinatr you can't - the list never containsint?- it only containsintbecause of the boxing rules on nullable types
– Marc Gravell♦
Mar 27 at 8:34
1
@KyleJohnson you added a nullable int to the list. Naively, if nullable ints and ints are different things, you'd expect asking for all the ints in the list to return nothing. There are languages that do it that way, but C# has chosen to do it differently.
– mbrig
Mar 27 at 21:24
1
(and yes, my first sentence there is technically wrong. The nullable int never got added to the list. But if you don't know what's happening here, that's what it looks like is happening)
– mbrig
Mar 27 at 21:26
|
show 2 more comments
phew ok thank you for that explanaion. Is that C# basic knowledge?
– Dr. Snail
Mar 27 at 8:30
12
@Dr.Snail "basic" is relative / subjective, and I'd wager that a good percentage of developers never have a need to know that nuance; it is useful context if you're dealing with boxing, though... and technically it isn't really C# knowledge, but rather: .NET knowledge (it would apply to all languages)
– Marc Gravell♦
Mar 27 at 8:31
5
@Sinatr you can't - the list never containsint?- it only containsintbecause of the boxing rules on nullable types
– Marc Gravell♦
Mar 27 at 8:34
1
@KyleJohnson you added a nullable int to the list. Naively, if nullable ints and ints are different things, you'd expect asking for all the ints in the list to return nothing. There are languages that do it that way, but C# has chosen to do it differently.
– mbrig
Mar 27 at 21:24
1
(and yes, my first sentence there is technically wrong. The nullable int never got added to the list. But if you don't know what's happening here, that's what it looks like is happening)
– mbrig
Mar 27 at 21:26
phew ok thank you for that explanaion. Is that C# basic knowledge?
– Dr. Snail
Mar 27 at 8:30
phew ok thank you for that explanaion. Is that C# basic knowledge?
– Dr. Snail
Mar 27 at 8:30
12
12
@Dr.Snail "basic" is relative / subjective, and I'd wager that a good percentage of developers never have a need to know that nuance; it is useful context if you're dealing with boxing, though... and technically it isn't really C# knowledge, but rather: .NET knowledge (it would apply to all languages)
– Marc Gravell♦
Mar 27 at 8:31
@Dr.Snail "basic" is relative / subjective, and I'd wager that a good percentage of developers never have a need to know that nuance; it is useful context if you're dealing with boxing, though... and technically it isn't really C# knowledge, but rather: .NET knowledge (it would apply to all languages)
– Marc Gravell♦
Mar 27 at 8:31
5
5
@Sinatr you can't - the list never contains
int? - it only contains int because of the boxing rules on nullable types– Marc Gravell♦
Mar 27 at 8:34
@Sinatr you can't - the list never contains
int? - it only contains int because of the boxing rules on nullable types– Marc Gravell♦
Mar 27 at 8:34
1
1
@KyleJohnson you added a nullable int to the list. Naively, if nullable ints and ints are different things, you'd expect asking for all the ints in the list to return nothing. There are languages that do it that way, but C# has chosen to do it differently.
– mbrig
Mar 27 at 21:24
@KyleJohnson you added a nullable int to the list. Naively, if nullable ints and ints are different things, you'd expect asking for all the ints in the list to return nothing. There are languages that do it that way, but C# has chosen to do it differently.
– mbrig
Mar 27 at 21:24
1
1
(and yes, my first sentence there is technically wrong. The nullable int never got added to the list. But if you don't know what's happening here, that's what it looks like is happening)
– mbrig
Mar 27 at 21:26
(and yes, my first sentence there is technically wrong. The nullable int never got added to the list. But if you don't know what's happening here, that's what it looks like is happening)
– mbrig
Mar 27 at 21:26
|
show 2 more comments
6
A nullable value type is boxed by the following rules:
- If
HasValuereturnsfalse, the null reference is produced. - If
HasValuereturnstrue, a value of the underlying value typeTis
boxed, not the instance of nullable.
In your example second rule has been followed as you have value, e.g.
var i = (object)(int?)123;
answered Mar 27 at 9:11
JohnnyJohnny
3,6201021
add a comment |
6
A nullable value type is boxed by the following rules:
- If
HasValuereturnsfalse, the null reference is produced. - If
HasValuereturnstrue, a value of the underlying value typeTis
boxed, not the instance of nullable.
In your example second rule has been followed as you have value, e.g.
var i = (object)(int?)123;
answered Mar 27 at 9:11
JohnnyJohnny
3,6201021
add a comment |
6
6
6
A nullable value type is boxed by the following rules:
- If
HasValuereturnsfalse, the null reference is produced. - If
HasValuereturnstrue, a value of the underlying value typeTis
boxed, not the instance of nullable.
In your example second rule has been followed as you have value, e.g.
var i = (object)(int?)123;
answered Mar 27 at 9:11
JohnnyJohnny
3,6201021
A nullable value type is boxed by the following rules:
- If
HasValuereturnsfalse, the null reference is produced. - If
HasValuereturnstrue, a value of the underlying value typeTis
boxed, not the instance of nullable.
In your example second rule has been followed as you have value, e.g.
var i = (object)(int?)123;
answered Mar 27 at 9:11
JohnnyJohnny
3,6201021
edited Mar 27 at 13:26
answered Mar 27 at 9:11
JohnnyJohnny
3,6201021
answered Mar 27 at 9:11
JohnnyJohnny
3,6201021
answered Mar 27 at 9:11
JohnnyJohnny
3,6201021
3,6201021
add a comment |
add a comment |
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int?boxed asint, and basically every Nullable type, Edit : Marc Gravell have the full answer– styx
Mar 27 at 8:30
Related post about nullable type: stackoverflow.com/questions/4028830/…. This is called as "type lifting".
– Tetsuya Yamamoto
Mar 27 at 8:32
Before reading this topic I wouldn't even guess that even
List<int?>already holds justinttypes. Proof– Sinatr
Mar 27 at 8:46
7
@Sinatr no, that is incorrect;
List<int?>holdsint?. The important distinction in this example is the use ofList<object>. What you're seeing in that "proof" is something very different;GetType()on anyT?either returns theT, or throws a NRE. It never returnsT?- better example: dotnetfiddle.net/3Gy3Fa - and as for why: becauseGetType()is non-virtual, it cannot be overridden, and thus callingGetType()is a boxing operation (even if used via "constrained call"). And when you box aT?, you either get aTas anobject, or anull.– Marc Gravell♦
Mar 27 at 8:49
@TetsuyaYamamoto That is not a relevant link. How is the boxing/unboxing behavior of Nullable<T> possible? is.
– GSerg
Mar 27 at 11:17