Why is the default value of the string type null instead of an empty string

Question

It s quite annoying to test all my strings for null before I can safely apply methods like ToUpper    StartWith   etc     If the default value of string were the empty string  I would not have to test  and I would feel it to be more consistent with the other value types like int or double for example  Additionally Nullable lt String gt  would make sense   So why did the designers of C  choose to use null as the default value of strings   Note  This relates to this question  but is more focused on the why instead of what to do with it

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Since string is a reference type and the default value for reference type is null

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A String is an immutable object which means when given a value  the old value doesn t get wiped out of memory  but remains in the old location  and the new value is put in a new location  So if the default value of String a was String Empty  it would waste the String Empty block in memory when it was given its first value   Although it seems minuscule  it could turn into a problem when initializing a large array of strings with default values of String Empty  Of course  you could always use the mutable StringBuilder class if this was going to be a problem

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Why the designers of C  chose to use null as the default value of   strings    Because strings are reference types  reference types are default value is null  Variables of reference types store references to the actual data   Let s use default keyword for this case   string str   default string      str is a string  so it is a reference type  so default value is null   int str    default  int     str is an int  so it is a value type  so default value is zero

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You could also use the following  as of C  6 0  string myString   null  string result   myString  ToUpper      The string result will be null

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Perhaps if you d use    operator when assigning your string variable  it might help you   string str   SomeMethodThatReturnsaString             if SomeMethodThatReturnsaString   returns a null value     is assigned to str

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Why is the default value of the string type null instead of an empty   string    Because string is a reference type and the default value for all reference types is null       It s quite annoying to test all my strings for null before I can   safely apply methods like ToUpper    StartWith   etc      That is consistent with the behaviour of reference types  Before invoking their instance members  one should put a check in place for a null reference      If the default value of string were the empty string  I would not have   to test  and I would feel it to be more consistent with the other   value types like int or double for example    Assigning the default value to a specific reference type other than null would make it inconsistent      Additionally Nullable lt String gt  would make sense    Nullable lt T gt  works with the value types  Of note is the fact that Nullable was not introduced on the original  NET platform so there would have been a lot of broken code had they changed that rule  Courtesy  jcolebrand

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If the default value of string were the empty string  I would not have to test   Wrong  Changing the default value doesn t change the fact that it s a reference type and someone can still explicitly set the reference to be null      Additionally Nullable lt String gt  would make sense    True point  It would make more sense to not allow null for any reference types  instead requiring Nullable lt TheRefType gt  for that feature      So why did the designers of C  choose to use null as the default value of strings    Consistency with other reference types  Now  why allow null in reference types at all  Probably so that it feels like C  even though this is a questionable design decision in a language that also provides Nullable

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The fundamental reason problem is that the designers of the CLS specification  which defines how languages interact with  net  did not define a means by which class members could specify that they must be called directly  rather than via callvirt  without the caller performing a null-reference check  nor did it provide a meany of defining structures which would not be subject to  normal  boxing   Had the CLS specification defined such a means  then it would be possible for  net to consistently follow the lead established by the Common Object Model  COM   under which a null string reference was considered semantically equivalent to an empty string  and for other user-defined immutable class types which are supposed to have value semantics to likewise define default values   Essentially  what would happen would be for each member of String  e g  Length to be written as something like  InvokableOnNull    int String Length   get   if  this  null  return 0  else return  Length       This approach would have offered very nice semantics for things which should behave like values  but because of implementation issues need to be stored on the heap   The biggest difficulty with this approach is that the semantics of conversion between such types and Object could get a little murky   An alternative approach would have been to allow the definition of special structure types which did not inherit from Object but instead had custom boxing and unboxing operations  which would convert to from some other class type    Under such an approach  there would be a class type NullableString which behaves as string does now  and a custom-boxed struct type String  which would hold a single private field Value of type String   Attempting to convert a String to NullableString or Object would return Value if non-null  or String Empty if null   Attempting to cast to String  a non-null reference to a NullableString instance would store the reference in Value  perhaps storing null if the length was zero   casting any other reference would throw an exception   Even though strings have to be stored on the heap  there is conceptually no reason why they shouldn t behave like value types that have a non-null default value   Having them be stored as a  normal  structure which held a reference would have been efficient for code that used them as type  string   but would have added an extra layer of indirection and inefficiency when casting to  object    While I don t foresee  net adding either of the above features at this late date  perhaps designers of future frameworks might consider including them

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Habib is right -- because string is a reference type   But more importantly  you don t have to check for null each time you use it   You probably should throw a ArgumentNullException if someone passes your function a null reference  though   Here s the thing -- the framework would throw a NullReferenceException for you anyway if you tried to call  ToUpper   on a string   Remember that this case still can happen even if you test your arguments for null since any property or method on the objects passed to your function as parameters may evaluate to null   That being said  checking for empty strings or nulls is a common thing to do  so they provide String IsNullOrEmpty   and String IsNullOrWhiteSpace   for just this purpose

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You could write an extension method  for what it s worth    public static string EmptyNull this string str        return str            Now this works safely   string str   null  string upper   str EmptyNull   ToUpper

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Maybe the string keyword confused you  as it looks exactly like any other value type declaration  but it is actually an alias to System String as explained in this question  Also the dark blue color in Visual Studio and the lowercase first letter may mislead into thinking it is a struct

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Empty strings and nulls are fundamentally different  A null is an absence of a value and an empty string is a value that is empty   The programming language making assumptions about the  value  of a variable  in this case an empty string  will be as good as initiazing the string with any other value that will not cause a null reference problem   Also  if you pass the handle to that string variable to other parts of the application  then that code will have no ways of validating whether you have intentionally passed a blank value or you have forgotten to populate the value of that variable   Another occasion where this would be a problem is when the string is a return value from some function  Since string is a reference type and can technically have a value as null and empty both  therefore the function can also technically return a null or empty  there is nothing to stop it from doing so   Now  since there are 2 notions of the  absence of a value   i e an empty string and a null  all the code that consumes this function will have to do 2 checks  One for empty and the other for null    In short  its always good to have only 1 representation for a single state  For a broader discussion on empty and nulls  see the links below   https   softwareengineering stackexchange com questions 32578 sql-empty-string-vs-null-value  NULL vs Empty when dealing with user input

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Since you mentioned ToUpper    and this usage is how I found this thread  I will share this shortcut  string     quot  quot   ToUpper        private string  city      public string City               get                       return  this  city     quot  quot   ToUpper                      set                       this  city   value                   Seems better than          if null    this  city            this  city   this  city ToUpper

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Nullable types did not come in until 2 0   If nullable types had been made in the beginning of the language then string would have been non-nullable and string  would have been nullable  But they could not do this du to backward compatibility   A lot of people talk about ref-type or not ref type  but string is an out of the ordinary class and solutions would have been found to make it possible

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Because a string variable is a reference  not an instance    Initializing it to Empty by default would have been possible but it would have introduced a lot of inconsistencies all over the board

[c#] Why is the default value of the string type null instead of an empty string?

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