What do two question marks together mean in C

Question

Ran across this line of code   FormsAuth   formsAuth    new FormsAuthenticationWrapper      What do the two question marks mean  is it some kind of ternary operator  It s hard to look up in Google

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It s the null coalescing operator  and quite like the ternary  immediate-if  operator  See also    Operator - MSDN   FormsAuth   formsAuth    new FormsAuthenticationWrapper      expands to   FormsAuth   formsAuth    null   formsAuth   new FormsAuthenticationWrapper      which further expands to   if formsAuth    null      FormsAuth   formsAuth  else     FormsAuth   new FormsAuthenticationWrapper      In English  it means  If whatever is to the left is not null  use that  otherwise use what s to the right    Note that you can use any number of these in sequence  The following statement will assign the first non-null Answer  to Answer  if all Answers are null then the Answer is null    string Answer   Answer1    Answer2    Answer3    Answer4      Also it s worth mentioning while the expansion above is conceptually equivalent  the result of each expression is only evaluated once  This is important if for example an expression is a method call with side effects   Credit to  Joey for pointing this out

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Others have described the Null Coalescing Operator quite well  In cases where a single test for null is required  the shortened syntax     can add readability  Legacy null test  if  myvariable    null        myvariable   new MyConstructor       Using the Null Coalescing Operator this can be written  myvariable   myvariable    new MyConstructor     which can also be written with the shortened syntax  myvariable     new MyConstructor     Some find it more readable and succinct

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If you re familiar with Ruby  its     seems akin to C  s    to me   Here s some Ruby   irb main  001 0 gt  str1   nil   gt  nil irb main  002 0 gt  str1      new value    gt   new value  irb main  003 0 gt  str2    old value    gt   old value  irb main  004 0 gt  str2      another new value    gt   old value  irb main  005 0 gt  str1   gt   new value  irb main  006 0 gt  str2   gt   old value    And in C    string str1   null  str1   str1     new value   string str2    old value   str2   str2     another new value

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Nothing dangerous about this  In fact  it is beautiful   You can add default value if that is desirable  for example   CODE  int x   x1    x2    x3    x4    0

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Just because no-one else has said the magic words yet  it s the null coalescing operator  It s defined in section 7 12 of the C  3 0 language specification   It s very handy  particularly because of the way it works when it s used multiple times in an expression  An expression of the form   a    b    c    d   will give the result of expression a if it s non-null  otherwise try b  otherwise try c  otherwise try d  It short-circuits at every point   Also  if the type of d is non-nullable  the type of the whole expression is non-nullable too

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It s short hand for the ternary operator     FormsAuth    formsAuth    null    formsAuth   new FormsAuthenticationWrapper      Or for those who don t do ternary   if  formsAuth    null      FormsAuth   formsAuth    else     FormsAuth   new FormsAuthenticationWrapper

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Some of the examples here of getting values using coalescing are inefficient   What you really want is   return  formsAuthWrapper    formsAuthWrapper    new FormsAuthenticationWrapper      or  return  formsAuthWrapper      formsAuthWrapper   new FormsAuthenticationWrapper       This prevents the object from being recreated every time  Instead of the private variable remaining null and a new object getting created on every request  this ensures the private variable is assigned if the new object is created

User · Answer

Just because no-one else has said the magic words yet  it s the null coalescing operator  It s defined in section 7 12 of the C  3 0 language specification   It s very handy  particularly because of the way it works when it s used multiple times in an expression  An expression of the form   a    b    c    d   will give the result of expression a if it s non-null  otherwise try b  otherwise try c  otherwise try d  It short-circuits at every point   Also  if the type of d is non-nullable  the type of the whole expression is non-nullable too

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Some of the examples here of getting values using coalescing are inefficient   What you really want is   return  formsAuthWrapper    formsAuthWrapper    new FormsAuthenticationWrapper      or  return  formsAuthWrapper      formsAuthWrapper   new FormsAuthenticationWrapper       This prevents the object from being recreated every time  Instead of the private variable remaining null and a new object getting created on every request  this ensures the private variable is assigned if the new object is created

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Thanks everybody  here is the most succinct explanation I found on the MSDN site      y   x  unless x is null  in which case y   -1  int y   x    -1

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Thanks everybody  here is the most succinct explanation I found on the MSDN site      y   x  unless x is null  in which case y   -1  int y   x    -1

User · Answer

As correctly pointed in numerous answers that is the  null coalescing operator        speaking of which you might also want to check out its cousin the  Null-conditional Operator      or     that is an operator that many times it is used in conjunction with     Null-conditional Operator     Used to test for null before performing a member access      or index      operation  These operators help you write less code to handle null checks  especially for descending into data structures    For example       if  customers  or  Order  property or  Price  property  is null     dollarAmount will be 0     otherwise dollarAmount will be equal to  customers Order Price   int dollarAmount   customers  Order  Price    0     the old way without    and    of doing this is  int dollarAmount   customers    null                      amp  amp  customers Order  null                     amp  amp  customers Order Price  null                        customers Order Price   0     which is more verbose and cumbersome

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Note  I have read whole this thread and many others but I can t find as thorough answer as this is  By which I completely understood the  quot why to use    and when to use    and how to use     quot  Source  Windows communication foundation unleashed By Craig McMurtry ISBN 0-672-32948-4 Nullable Value Types There are two common circumstances in which one would like to know whether a value has been assigned to an instance of a value type  The first is when the instance represents a value in a database  In such a case  one would like to be able to examine the instance to ascertain whether a value is indeed present in the database  The other circumstance  which is more pertinent to the subject matter of this book  is when the instance represents a data item received from some remote source  Again  one would like to determine from the instance whether a value for that data item was received  The  NET Framework 2 0 incorporates a generic type definition that provides for cases like these in which one wants to assign null to an instance of a value type  and test whether the value of the instance is null  That generic type definition is System Nullable lt T gt   which constrains the generic type arguments that may be substituted for T to value types  Instances of types constructed from System Nullable lt T gt  can be assigned a value of null  indeed  their values are null by default  Thus  types constructed from System Nullable lt T gt  may be referred to as nullable value types  System Nullable lt T gt  has a property  Value  by which the value assigned to an instance of a type constructed from it can be obtained if the value of the instance is not null  Therefore  one can write  System Nullable lt int gt  myNullableInteger   null  myNullableInteger   1  if  myNullableInteger    null    Console WriteLine myNullableInteger Value      The C  programming language provides an abbreviated syntax for declaring types constructed from System Nullable lt T gt   That syntax allows one to abbreviate  System Nullable lt int gt  myNullableInteger   to int  myNullableInteger   The compiler will prevent one from attempting to assign the value of a nullable value type to an ordinary value type in this way  int  myNullableInteger   null  int myInteger   myNullableInteger   It prevents one from doing so because the nullable value type could have the value null  which it actually would have in this case  and that value cannot be assigned to an ordinary value type  Although the compiler would permit this code  int  myNullableInteger   null  int myInteger   myNullableInteger Value   The second statement would cause an exception to be thrown because any attempt to access the System Nullable lt T gt  Value property is an invalid operation if the type constructed from System Nullable lt T gt  has not been assigned a valid value of T  which has not happened in this case  Conclusion  One proper way to assign the value of a nullable value type to an ordinary value type is to use the System Nullable lt T gt  HasValue property to ascertain whether a valid value of T has been assigned to the nullable value type  int  myNullableInteger   null  if  myNullableInteger HasValue    int myInteger   myNullableInteger Value     Another option is to use this syntax  int  myNullableInteger   null  int myInteger   myNullableInteger    -1   By which the ordinary integer myInteger is assigned the value of the nullable integer  quot myNullableInteger quot  if the latter has been assigned a valid integer value  otherwise  myInteger is assigned the value of -1

User · Answer

It s short hand for the ternary operator     FormsAuth    formsAuth    null    formsAuth   new FormsAuthenticationWrapper      Or for those who don t do ternary   if  formsAuth    null      FormsAuth   formsAuth    else     FormsAuth   new FormsAuthenticationWrapper

User · Answer

Thanks everybody  here is the most succinct explanation I found on the MSDN site      y   x  unless x is null  in which case y   -1  int y   x    -1

User · Answer

coalescing operator  it s equivalent to  FormsAuth   formsAUth    null   new FormsAuthenticationWrapper     formsAuth

User · Answer

Others have described the Null Coalescing Operator quite well  In cases where a single test for null is required  the shortened syntax     can add readability  Legacy null test  if  myvariable    null        myvariable   new MyConstructor       Using the Null Coalescing Operator this can be written  myvariable   myvariable    new MyConstructor     which can also be written with the shortened syntax  myvariable     new MyConstructor     Some find it more readable and succinct

User · Answer

If you re familiar with Ruby  its     seems akin to C  s    to me   Here s some Ruby   irb main  001 0 gt  str1   nil   gt  nil irb main  002 0 gt  str1      new value    gt   new value  irb main  003 0 gt  str2    old value    gt   old value  irb main  004 0 gt  str2      another new value    gt   old value  irb main  005 0 gt  str1   gt   new value  irb main  006 0 gt  str2   gt   old value    And in C    string str1   null  str1   str1     new value   string str2    old value   str2   str2     another new value

User · Answer

As correctly pointed in numerous answers that is the  null coalescing operator        speaking of which you might also want to check out its cousin the  Null-conditional Operator      or     that is an operator that many times it is used in conjunction with     Null-conditional Operator     Used to test for null before performing a member access      or index      operation  These operators help you write less code to handle null checks  especially for descending into data structures    For example       if  customers  or  Order  property or  Price  property  is null     dollarAmount will be 0     otherwise dollarAmount will be equal to  customers Order Price   int dollarAmount   customers  Order  Price    0     the old way without    and    of doing this is  int dollarAmount   customers    null                      amp  amp  customers Order  null                     amp  amp  customers Order Price  null                        customers Order Price   0     which is more verbose and cumbersome

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is there to provide a value for a nullable type when the value is null  So  if formsAuth  is null  it will return new FormsAuthenticationWrapper

User · Answer

For your amusement only  knowing you are all C  guys  -    I think it originated in Smalltalk  where it has been around for many years  It is defined there as   in Object     anArgument       self   in UndefinedObject  aka nil s class      anArgument       anArgument   There are both evaluating     and non-evaluating versions      of this   It is often found in getter-methods for lazy-initialized private  instance  variables  which are left nil until really needed

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The two question marks      indicate that its a Coalescing operator   Coalescing operator returns the first NON-NULL value from a chain  You can see this youtube video which demonstrates the whole thing practically   But let me add more to what the video says   If you see the English meaning of coalescing it says    consolidate together     For example below is a simple coalescing code which chains four strings   So if str1 is null it will try str2  if str2 is null it will try str3 and so on until it finds a string with a non-null value   string final   str1    str2    str3    str4    In simple words Coalescing operator returns the first NON-NULL value from a chain

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It s the null coalescing operator   http   msdn microsoft com en-us library ms173224 aspx  Yes  nearly impossible to search for unless you know what it s called   -   EDIT  And this is a cool feature from another question   You can chain them   Hidden Features of C

User · Answer

Just because no-one else has said the magic words yet  it s the null coalescing operator  It s defined in section 7 12 of the C  3 0 language specification   It s very handy  particularly because of the way it works when it s used multiple times in an expression  An expression of the form   a    b    c    d   will give the result of expression a if it s non-null  otherwise try b  otherwise try c  otherwise try d  It short-circuits at every point   Also  if the type of d is non-nullable  the type of the whole expression is non-nullable too

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coalescing operator  it s equivalent to  FormsAuth   formsAUth    null   new FormsAuthenticationWrapper     formsAuth

User · Answer

For your amusement only  knowing you are all C  guys  -    I think it originated in Smalltalk  where it has been around for many years  It is defined there as   in Object     anArgument       self   in UndefinedObject  aka nil s class      anArgument       anArgument   There are both evaluating     and non-evaluating versions      of this   It is often found in getter-methods for lazy-initialized private  instance  variables  which are left nil until really needed

User · Answer

It s a null coalescing operator that works similarly to a ternary operator       a    b    gt  a   null   a   b    Another interesting point for this is   A nullable type can contain a value  or it can be undefined   So if you try to assign a nullable value type to a non-nullable value type  you will get a compile-time error   int  x   null     x is nullable value type int z   0     z is non-nullable value type z   x     compile error will be there    So to do that using    operator   z   x    1     with    operator there are no issues

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coalescing operator  it s equivalent to  FormsAuth   formsAUth    null   new FormsAuthenticationWrapper     formsAuth

User · Answer

The    operator is called the null-coalescing operator  It returns the left-hand operand if the operand is not null  otherwise it returns the right hand operand   int  variable1   null  int variable2    variable1    100    Set variable2 to the value of variable1  if variable1 is NOT null  otherwise  if variable1    null  set variable2 to 100

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It s short hand for the ternary operator     FormsAuth    formsAuth    null    formsAuth   new FormsAuthenticationWrapper      Or for those who don t do ternary   if  formsAuth    null      FormsAuth   formsAuth    else     FormsAuth   new FormsAuthenticationWrapper

User · Answer

Note  I have read whole this thread and many others but I can t find as thorough answer as this is  By which I completely understood the  quot why to use    and when to use    and how to use     quot  Source  Windows communication foundation unleashed By Craig McMurtry ISBN 0-672-32948-4 Nullable Value Types There are two common circumstances in which one would like to know whether a value has been assigned to an instance of a value type  The first is when the instance represents a value in a database  In such a case  one would like to be able to examine the instance to ascertain whether a value is indeed present in the database  The other circumstance  which is more pertinent to the subject matter of this book  is when the instance represents a data item received from some remote source  Again  one would like to determine from the instance whether a value for that data item was received  The  NET Framework 2 0 incorporates a generic type definition that provides for cases like these in which one wants to assign null to an instance of a value type  and test whether the value of the instance is null  That generic type definition is System Nullable lt T gt   which constrains the generic type arguments that may be substituted for T to value types  Instances of types constructed from System Nullable lt T gt  can be assigned a value of null  indeed  their values are null by default  Thus  types constructed from System Nullable lt T gt  may be referred to as nullable value types  System Nullable lt T gt  has a property  Value  by which the value assigned to an instance of a type constructed from it can be obtained if the value of the instance is not null  Therefore  one can write  System Nullable lt int gt  myNullableInteger   null  myNullableInteger   1  if  myNullableInteger    null    Console WriteLine myNullableInteger Value      The C  programming language provides an abbreviated syntax for declaring types constructed from System Nullable lt T gt   That syntax allows one to abbreviate  System Nullable lt int gt  myNullableInteger   to int  myNullableInteger   The compiler will prevent one from attempting to assign the value of a nullable value type to an ordinary value type in this way  int  myNullableInteger   null  int myInteger   myNullableInteger   It prevents one from doing so because the nullable value type could have the value null  which it actually would have in this case  and that value cannot be assigned to an ordinary value type  Although the compiler would permit this code  int  myNullableInteger   null  int myInteger   myNullableInteger Value   The second statement would cause an exception to be thrown because any attempt to access the System Nullable lt T gt  Value property is an invalid operation if the type constructed from System Nullable lt T gt  has not been assigned a valid value of T  which has not happened in this case  Conclusion  One proper way to assign the value of a nullable value type to an ordinary value type is to use the System Nullable lt T gt  HasValue property to ascertain whether a valid value of T has been assigned to the nullable value type  int  myNullableInteger   null  if  myNullableInteger HasValue    int myInteger   myNullableInteger Value     Another option is to use this syntax  int  myNullableInteger   null  int myInteger   myNullableInteger    -1   By which the ordinary integer myInteger is assigned the value of the nullable integer  quot myNullableInteger quot  if the latter has been assigned a valid integer value  otherwise  myInteger is assigned the value of -1

User · Answer

coalescing operator  it s equivalent to  FormsAuth   formsAUth    null   new FormsAuthenticationWrapper     formsAuth

User · Answer

is there to provide a value for a nullable type when the value is null  So  if formsAuth  is null  it will return new FormsAuthenticationWrapper

User · Answer

It s the null coalescing operator   http   msdn microsoft com en-us library ms173224 aspx  Yes  nearly impossible to search for unless you know what it s called   -   EDIT  And this is a cool feature from another question   You can chain them   Hidden Features of C

User · Answer

It s short hand for the ternary operator     FormsAuth    formsAuth    null    formsAuth   new FormsAuthenticationWrapper      Or for those who don t do ternary   if  formsAuth    null      FormsAuth   formsAuth    else     FormsAuth   new FormsAuthenticationWrapper

User · Answer

The two question marks      indicate that its a Coalescing operator   Coalescing operator returns the first NON-NULL value from a chain  You can see this youtube video which demonstrates the whole thing practically   But let me add more to what the video says   If you see the English meaning of coalescing it says    consolidate together     For example below is a simple coalescing code which chains four strings   So if str1 is null it will try str2  if str2 is null it will try str3 and so on until it finds a string with a non-null value   string final   str1    str2    str3    str4    In simple words Coalescing operator returns the first NON-NULL value from a chain

User · Answer

It s the null coalescing operator   http   msdn microsoft com en-us library ms173224 aspx  Yes  nearly impossible to search for unless you know what it s called   -   EDIT  And this is a cool feature from another question   You can chain them   Hidden Features of C

User · Answer

is there to provide a value for a nullable type when the value is null  So  if formsAuth  is null  it will return new FormsAuthenticationWrapper

User · Answer

Nothing dangerous about this  In fact  it is beautiful   You can add default value if that is desirable  for example   CODE  int x   x1    x2    x3    x4    0

User · Answer

For your amusement only  knowing you are all C  guys  -    I think it originated in Smalltalk  where it has been around for many years  It is defined there as   in Object     anArgument       self   in UndefinedObject  aka nil s class      anArgument       anArgument   There are both evaluating     and non-evaluating versions      of this   It is often found in getter-methods for lazy-initialized private  instance  variables  which are left nil until really needed

User · Answer

Just because no-one else has said the magic words yet  it s the null coalescing operator  It s defined in section 7 12 of the C  3 0 language specification   It s very handy  particularly because of the way it works when it s used multiple times in an expression  An expression of the form   a    b    c    d   will give the result of expression a if it s non-null  otherwise try b  otherwise try c  otherwise try d  It short-circuits at every point   Also  if the type of d is non-nullable  the type of the whole expression is non-nullable too

User · Answer

FormsAuth   formsAuth    new FormsAuthenticationWrapper      is equivalent to   FormsAuth   formsAuth    null   formsAuth   new FormsAuthenticationWrapper      But the cool thing about it is you can chain them  like other people said  The one thin not touched upon is that you can actually use it to throw an exception   A   A    B    throw new Exception  A and B are both NULL

User · Answer

Thanks everybody  here is the most succinct explanation I found on the MSDN site      y   x  unless x is null  in which case y   -1  int y   x    -1

User · Answer

It s the null coalescing operator   http   msdn microsoft com en-us library ms173224 aspx  Yes  nearly impossible to search for unless you know what it s called   -   EDIT  And this is a cool feature from another question   You can chain them   Hidden Features of C

User · Answer

It s a null coalescing operator that works similarly to a ternary operator       a    b    gt  a   null   a   b    Another interesting point for this is   A nullable type can contain a value  or it can be undefined   So if you try to assign a nullable value type to a non-nullable value type  you will get a compile-time error   int  x   null     x is nullable value type int z   0     z is non-nullable value type z   x     compile error will be there    So to do that using    operator   z   x    1     with    operator there are no issues

User · Answer

FormsAuth   formsAuth    new FormsAuthenticationWrapper      is equivalent to   FormsAuth   formsAuth    null   formsAuth   new FormsAuthenticationWrapper      But the cool thing about it is you can chain them  like other people said  The one thin not touched upon is that you can actually use it to throw an exception   A   A    B    throw new Exception  A and B are both NULL

User · Answer

For your amusement only  knowing you are all C  guys  -    I think it originated in Smalltalk  where it has been around for many years  It is defined there as   in Object     anArgument       self   in UndefinedObject  aka nil s class      anArgument       anArgument   There are both evaluating     and non-evaluating versions      of this   It is often found in getter-methods for lazy-initialized private  instance  variables  which are left nil until really needed

User · Answer

is there to provide a value for a nullable type when the value is null  So  if formsAuth  is null  it will return new FormsAuthenticationWrapper

User · Answer

The    operator is called the null-coalescing operator  It returns the left-hand operand if the operand is not null  otherwise it returns the right hand operand   int  variable1   null  int variable2    variable1    100    Set variable2 to the value of variable1  if variable1 is NOT null  otherwise  if variable1    null  set variable2 to 100

[c#] What do two question marks together mean in C#?

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