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Type int? vs type int

Nullable<int> vs. int? - Is there any difference?How is the boxing/unboxing behavior of Nullable<T> possible?Cast int to enum in C#Create Generic method constraining T to an EnumRandom number generator only generating one random numberUsing LINQ to remove elements from a List<T>Get int value from enum in C#Type Checking: typeof, GetType, or is?Distinct() with lambda?How do I generate a random int number?Call one constructor from anotherTry-catch speeding up my code?

24

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

share|improve this question

asked 15 hours ago

Dr. SnailDr. Snail

693628

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  int? boxed as int , and basically every Nullable type, Edit : Marc Gravell have the full answer
  
  – styx
  15 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Related post about nullable type: stackoverflow.com/questions/4028830/…. This is called as "type lifting".
  
  – Tetsuya Yamamoto
  15 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Before reading this topic I wouldn't even guess that even List<int?> already holds just int types. Proof
  
  – Sinatr
  15 hours ago
  

  
  
  
  


• 
  
  
  

  
  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♦
  15 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @TetsuyaYamamoto That is not a relevant link. How is the boxing/unboxing behavior of Nullable<T> possible? is.
  
  – GSerg
  12 hours ago
  

  
  
  
  

 | 
show 2 more comments

24

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

share|improve this question

asked 15 hours ago

Dr. SnailDr. Snail

693628

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  int? boxed as int , and basically every Nullable type, Edit : Marc Gravell have the full answer
  
  – styx
  15 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Related post about nullable type: stackoverflow.com/questions/4028830/…. This is called as "type lifting".
  
  – Tetsuya Yamamoto
  15 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Before reading this topic I wouldn't even guess that even List<int?> already holds just int types. Proof
  
  – Sinatr
  15 hours ago
  

  
  
  
  


• 
  
  
  

  
  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♦
  15 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @TetsuyaYamamoto That is not a relevant link. How is the boxing/unboxing behavior of Nullable<T> possible? is.
  
  – GSerg
  12 hours ago
  

  
  
  
  

 | 
show 2 more comments

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

share|improve this question

asked 15 hours ago

Dr. SnailDr. Snail

693628

bool eq = typeof(int?).Equals(typeof(int));

List<object> items = new List<object>() (int?)123 ;
int result = items.OfType<int>().FirstOrDefault();

share|improve this question

asked 15 hours ago

Dr. SnailDr. Snail

693628

share|improve this question

asked 15 hours ago

Dr. SnailDr. Snail

693628

asked 15 hours ago

Dr. SnailDr. Snail

693628

asked 15 hours ago

Dr. SnailDr. Snail

693628

2 Answers
2

active

oldest

votes

39

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.

share|improve this answer

answered 15 hours ago

Marc Gravell♦Marc Gravell

792k19721582561

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  phew ok thank you for that explanaion. Is that C# basic knowledge?
  
  – Dr. Snail
  15 hours ago
  

  
  
  
  


• 
  
  
  

  
  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♦
  15 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  How would the query to obtain only int? from above items list will look like?
  
  – Sinatr
  15 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  4
  

  
  
  
  
  
  

  
  
  @Sinatr you can't - the list never contains int? - it only contains int because of the boxing rules on nullable types
  
  – Marc Gravell♦
  15 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Maybe I'm missing something but it returning 123 makes perfect sense to be. This long explanation does not. You added an int to a list and said give me all the integers in the list. The ? makes it so you can pass a null in the place of an int.
  
  – Kyle Johnson
  4 hours ago
  
  

  
  
  
  

 | 
show 2 more comments

4

A nullable value type is boxed by the following rules:

• If HasValue returns false, the null reference is produced.


• If HasValue returns true, a value of the underlying value type T is
  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;

share|improve this answer

edited 10 hours ago

answered 14 hours ago

JohnnyJohnny

3,4251021

add a comment | 

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39

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.

share|improve this answer

answered 15 hours ago

Marc Gravell♦Marc Gravell

792k19721582561

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  phew ok thank you for that explanaion. Is that C# basic knowledge?
  
  – Dr. Snail
  15 hours ago
  

  
  
  
  


• 
  
  
  

  
  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♦
  15 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  How would the query to obtain only int? from above items list will look like?
  
  – Sinatr
  15 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  4
  

  
  
  
  
  
  

  
  
  @Sinatr you can't - the list never contains int? - it only contains int because of the boxing rules on nullable types
  
  – Marc Gravell♦
  15 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Maybe I'm missing something but it returning 123 makes perfect sense to be. This long explanation does not. You added an int to a list and said give me all the integers in the list. The ? makes it so you can pass a null in the place of an int.
  
  – Kyle Johnson
  4 hours ago
  
  

  
  
  
  

 | 
show 2 more comments

39

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.

share|improve this answer

answered 15 hours ago

Marc Gravell♦Marc Gravell

792k19721582561

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  phew ok thank you for that explanaion. Is that C# basic knowledge?
  
  – Dr. Snail
  15 hours ago
  

  
  
  
  


• 
  
  
  

  
  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♦
  15 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  How would the query to obtain only int? from above items list will look like?
  
  – Sinatr
  15 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  4
  

  
  
  
  
  
  

  
  
  @Sinatr you can't - the list never contains int? - it only contains int because of the boxing rules on nullable types
  
  – Marc Gravell♦
  15 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Maybe I'm missing something but it returning 123 makes perfect sense to be. This long explanation does not. You added an int to a list and said give me all the integers in the list. The ? makes it so you can pass a null in the place of an int.
  
  – Kyle Johnson
  4 hours ago
  
  

  
  
  
  

 | 
show 2 more comments

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.

share|improve this answer

answered 15 hours ago

Marc Gravell♦Marc Gravell

792k19721582561

share|improve this answer

answered 15 hours ago

Marc Gravell♦Marc Gravell

792k19721582561

answered 15 hours ago

Marc Gravell♦Marc Gravell

792k19721582561

answered 15 hours ago

Marc Gravell♦Marc Gravell

792k19721582561

4

A nullable value type is boxed by the following rules:

• If HasValue returns false, the null reference is produced.


• If HasValue returns true, a value of the underlying value type T is
  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;

share|improve this answer

edited 10 hours ago

answered 14 hours ago

JohnnyJohnny

3,4251021

add a comment | 

4

A nullable value type is boxed by the following rules:

• If HasValue returns false, the null reference is produced.


• If HasValue returns true, a value of the underlying value type T is
  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;

share|improve this answer

edited 10 hours ago

answered 14 hours ago

JohnnyJohnny

3,4251021

add a comment | 

A nullable value type is boxed by the following rules:

• If HasValue returns false, the null reference is produced.


• If HasValue returns true, a value of the underlying value type T is
  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;

share|improve this answer

edited 10 hours ago

answered 14 hours ago

JohnnyJohnny

3,4251021

share|improve this answer

edited 10 hours ago

answered 14 hours ago

JohnnyJohnny

3,4251021

answered 14 hours ago

JohnnyJohnny

3,4251021

answered 14 hours ago

JohnnyJohnny

3,4251021