What is the purpose of a question mark after a type for example int myVariable

Question

Typically the main use of the question mark is for the conditional  x    yes     no    But I have seen another use for it but can t find an explanation of this use of the   operator  for example   public int  myProperty      get     set

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it declares that the type is nullable

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It means that the value type in question is a nullable type     Nullable types are instances of the System Nullable struct  A   nullable type can represent the correct range of values for its   underlying value type  plus an additional null value  For example  a   Nullable lt Int32 gt   pronounced  Nullable of Int32   can be assigned any   value from -2147483648 to 2147483647  or it can be assigned the null   value  A Nullable lt bool gt  can be assigned the values true  false  or   null  The ability to assign null to numeric and Boolean types is   especially useful when you are dealing with databases and other data   types that contain elements that may not be assigned a value  For   example  a Boolean field in a database can store the values true or   false  or it may be undefined   class NullableExample     static void Main             int  num   null            Is the HasValue property true        if  num HasValue                    System Console WriteLine  num       num Value                 else                   System Console WriteLine  num   Null                      y is set to zero       int y   num GetValueOrDefault              num Value throws an InvalidOperationException if num HasValue is false       try                   y   num Value                catch  System InvalidOperationException e                    System Console WriteLine e Message

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Nullable Types      Nullable types are instances of the   System Nullable struct  A nullable   type can represent the normal range of   values for its underlying value type    plus an additional null value  For   example  a  Nullable lt Int32 gt    pronounced    Nullable of Int32   can be assigned   any value from -2147483648 to   2147483647  or it can be assigned the   null value  A  Nullable lt bool gt   can be   assigned the values true or false  or   null  The ability to assign null to   numeric and Boolean types is   particularly useful when dealing with   databases and other data types   containing elements that may not be   assigned a value  For example  a   Boolean field in a database can store   the values true or false  or it may be   undefined

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In   x    yes     no    the   declares an if sentence  Here  x represents the boolean condition  The part before the   is the then sentence and the part after is the else sentence   In  for example   int    the   declares a nullable type  and means that the type before it may have a null value

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practical usage   public string someFunctionThatMayBeCalledWithNullAndReturnsString int  value      if  value    null          return  bad value          return someFunctionThatHandlesIntAndReturnsString value

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int  is shorthand for Nullable lt int gt   The two forms are interchangeable   Nullable lt T gt  is an operator that you can use with a value type T to make it accept null   In case you don t know it  value types are types that accepts values as int  bool  char etc      They can t accept references to values  they would generate a compile-time error if you assign them a null  as opposed to reference types  which can obviously accept it   Why would you need that  Because sometimes your value type variables could receive null references returned by something that didn t work very well  like a missing or undefined variable returned from a database   I suggest you to read the Microsoft Documentation because it covers the subject quite well

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To add on to the answers above  here is a code sample  struct Test       int something    struct NullableTest       int something    class Example       public void Demo                 Test t   new Test            t   null           NullableTest  t2   new NullableTest            t2   null            This would give a compilation error      Error 12  Cannot convert null to  Test  because it is a non-nullable   value type   Notice that there is no compilation error for NullableTest   note the   in the declaration of t2

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It is a shorthand for Nullable lt int gt   Nullable lt T gt  is used to allow a value type to be set to null  Value types usually cannot be null

[c#] What is the purpose of a question mark after a type (for example: int? myVariable)?

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