Please let me know if this is in the wrong place, or let me know a better place for it.
This question is not about the syntax, more the idea behind it.
I would like to know what null is essentially, or isn't as the case may be
First off, I just want to clarify exactly what null is. By my understanding, null is
technically nothing. So the statement
string s = null;
is technically incorrect? You are assigning a variable no value but a variable can't not have a value? Am I correct in thinking this?
My idea of null is that it's kind of like thinking, "If I go and get a drink of water, I will need a cup to put the water in". null is the space in which the data will be placed in, but the space doesn't exist yet. Following this idea:
string s = "";
would be more appropriate, no? And along this thought (though a bit less confusing)
int n = 0;
follows the same idea, where 0 is a value, but the value of 0 is nothing?
The original line of code is quite valid. In C#, any reference type variable can have no value, i.e. it refers to no object. null is quite different to an empty string. Consider a variable of some other type, e.g. Form. What would be the equivalent of an empty string then? A null in C# is basically the same as a NULL in a database.
Value types are a but different. Because reference type variables contain a reference, it is possible for them to refer to no object. Value type variables, on the other hand, contain a value and so cannot be null. A struct or enum is a value type and a class or delegate is a reference type. So, if a variable on the stack contains the value zero then, for a reference type that means no object, i.e. null, and for a value type that means the default value for that type, e.g. zero for numbers, false for bool and DateTime.MinValue for DateTime.
C# references are basically tarted-up pointers. Just as in C/C++ a pointer can be null to point to no object, so a C# reference-type variable can be null to refer to no object. In the case of strings, an empty string is an object and is very different to null. An empty balloon is still a balloon and very different to no balloon at all.
Your confusion likely lies in the difference between reference types and value types.
void Foo() {
string s = null;
}
This does not create a string. Instead, s is a reference to a string. However, s is currently referring to (or pointing to, in C terminology) nothing.
s ---> [nothing]
Now when we do this:
s = "Stack Overflow";
we are making s refer to that string. s itself doesn't contain the string.
s ----> "Stack Overflow"
Note that "" itself is a string, and does exist
s ----> ""
Strings are actually a bad example because of string interning.
Value types on the other hand line up with your confusion.
An int for example, must have a value. If you don't assign it one, it will (generally) take the default value of 0.
See more:
Value Types and Reference Types
This is not wrong as much as it is redundant because null is the default value of string
as int num = 0 is redundant because 0 is the default value of int
If you need to initialize your string then you should go for string s = "" or my personal favorite string s = string.Empty
Null means nothing. 0 is a value, but null means that there is no value. Actually in C#:
string s=null;
means that instance of the class string is null and has no value.
Imagine string s is just a variable pointing to a place in memory which was allocated for you.
string s = null; Is allocating a variable but it is not pointng to a place in your memory.
string s = "; micht be the same but it is pointing to a slot in your memory containing a iteral with an empty string.
I hope that made the problem a bit more clear.
Related
Imagine code like this:
var str = (String)null;
Does it differs from:
String str;
Or:
String str = null;
Does the first code cause boxing of null value, or is it rather resolved at compiler time to string?
String is a reference type, so no, there's no boxing.
var str = (String)null;
String str = null;
These two are equivalent. In the first line, the compiler infers the type of str from the right hand side of the expression. In the second line, the cast from null to string is implicit.
String str;
The last one is equivalent to String str = null if it's a field declaration, which means str will be assigned its default value, which is null. If, however, str is a local variable, it'll have to be explicitly assigned before you can use it.
Let's take your question and pick it apart.
Will the code in your question cause boxing?
No, it will not.
This is not because any of the 3 statements operate differently (there are differences though, more below), but boxing is not a concept that occurs when using strings.
Boxing occurs when you take a value type and wrap it up into an object. A string is a reference type, and thus there will never be boxing involved with it.
So boxing is out, what about the rest, are the three statements equal?
These two will do the same:
var str = (String)null;
String str = null;
The third one (second one in the order of your question though) is different in the sense that it only declares the str identifier to be of type String, it does not specifically initialize it to null.
However, if this is a field declaration of a class, this will be the same since all fields are initialized to defaults / zeroes when an object is constructed, and thus it will actually be initialized to null anyway.
If, on the other hand, this is a local variable, you now have an uninitialized variable. Judging from the fact that you write var ..., which is illegal in terms of fields, this is probably more correct for your question.
MSDN says,
Boxing is the process of converting a value type to the type object
String is not a value type and so there will be no boxing/unboxing.
Yes they are equal. Since string is a reference type even if you say string str; it will get default value which is null
These two are equal:
var str = (String)null;
String str = null;
However, this one,
String str;
Depending on the context might or might not be equal to previous expressions. If it's a local variable, then it's not equal. You must explicitly initialise it. If it's a class variable, then it's initialised to null.
Neither cause boxing.
Is it possible to test if a variable is defined as a string if the value inside it is null?
If I write:
string b = null;
bool c = b is string;
Then c will be false because is looks at the content, which is null and not a string.
If I write:
string b = null;
bool c = (b.GetType() == typeof(string));
Then it crashes because s is null and you can't call GetType() on a null value.
So, how can I check b to find out what type it is? Some kind of reflection maybe? Or is there any simpler way?
Edit 1: Clarification of the question!
I was a bit unclear in my question and that was my fault. In the example it looks like I'm trying to test the content of the variable. But I want to test the variable itself without looking at the content. In the code examples given I can see that b is a string, but what if I don't know if b is a string and just want to test the variable s to see if it is a string or not.
So, how can I know what type the variable is defined as? As in this example, but x is an unknown variable that might be defined as a string and it might be null as well (since it might be null this example won't work).
bool c = (x.GetType() == typeof(string));
Edit 2: The working solution!
Thanks to all the answers given I was able to solve it. This is how the working solution became. I first created a help function to test the defined type of a variable that works even if the value is null and it doesn't point to anything.
public static Type GetParameterType<T>(T destination)
{
return typeof(T);
}
Then I can just call this function and test my "suspected string" and find out if it really is a string or not.
// We define s as string just for this examples sake but in my "definition" we wouldn't be sure about whether s is a string or not.
string s = null;
// Now we want to test to see if s is a string
Type t = GetParameterType(s);
b = t == typeof(string); // Returns TRUE because s has the type of a string
b = t is string; // Returns FALSE because the content isn't a string
This is just what I wanted to find out!!! Thank you all for squeezing your brains...
You cannot check the type of null because null has no type. It doesn't reference anything at all, therefore there is nothing that C# can look at to find out the actual type.
(Everyone else seems to be answering the question "How can I tell if a string reference is null or empty - but I think the question is "how can I tell if the underlying type of a null reference is string...)
There might be a way to fiddle it though - you might be able to use a generic method as mentioned here:
.NET : How do you get the Type of a null object?
(That link was posted by someone else - not me - as a comment to your original post!)
So you want to know if there is a direct method to check of an object type whose value is set to NULL
In The simple word the answer is NO.
A null reference does not point to any Storage Location, so there is no metadata from which it can make that determination.
Although if you already know it is of type String , you can use following two functions for checking null values
String.IsNullOrWhiteSpace(stringObject);
and
String.IsNullOrEmpty(stringObject)
The best that you could do to set a value to unknown type is use
Convert.ChangeType
e.g. as given in .NET : How do you get the Type of a null object?
public void GetObjectValue<T>(out T destination)
{
object paramVal = "Blah.Blah.";
destination = default(T);
destination = Convert.ChangeType(paramVal, typeof(T).GetType());
}
The type of T can be inferred, so you shouldn't need to give a type parameter to the method explicitly.
Here it is
String.IsNullOrEmpty(s);
If you want to tell whether the actual value is a string, you can't do that with null, as null doesn't have a type.
So it seems you want to determine the actual type of the variable (the one it was declared as):
If the variable is of type string, then you know it at compile time (you declared it as string, after all).
If the variable is generic (like in generic types or a generic method), you can test it via typeof(T), assuming T is your type parameter.
If you got the variable as object though (e.g. as argument of a method), then there is no way to determine its original type if its value is null.
Use the String.IsNullOrEmpty method to check it.
string b = null;
bool c = String.IsNullOrEmpty(b);
See this MSDN link for further details.
IsNullOrWhiteSpace(variableName)
string.IsNullOrEmpty()
var.Trim().Length < 1
The third one is the one I personally use, as it is version-independent.
When I was writing C# code a few days ago I noticed that the compiler complained that I had to cast null to a specific object.
Does this mean that null is simply an uninstantiated version of the type? Or is it a singleton value of a Null class like Java (even though it has special privileges)?
EDIT: an example of the code giving the error would be:
public String duplicate(String toDuplicate)
{
return toDuplicate + toDuplicate;
}
public String duplicate(int? toDuplicate)
{
String asString = toDuplicate.toString();
return asString + asString;
}
public static int Main(string[] args)
{
//This needs to be cast:
duplicate(null);
//To:
duplicate((string)null);
}
The reason I commented on null in Java was after reading this:
There is also a special null type, the
type of the expression null, which has
no name. Because the null type has no
name, it is impossible to declare a
variable of the null type or to cast
to the null type. The null reference
is the only possible value of an
expression of null type. The null
reference can always be cast to any
reference type. In practice, the
programmer can ignore the null type
and just pretend that null is merely a
special literal that can be of any
reference type.
Found here: Is null an Object?
I get the error you refer when i have overloaded methods and the compiler can't resolve which method to call at compile time. Is that it?
According to the MSDN description:
The null keyword is a literal that represents a null reference, one that does not refer to any object. null is the default value of reference-type variables. Ordinary value types cannot be null. However, C# 2.0 introduced nullable value types.
null is the "uninstanciated reference" for any type. It is not a value. null has no defined type.
No - the null is just a literal for the null reference.
The reason you need to "cast" it in this way (I put the word cast in quotes because you are not really casting an instance of anything), is purely to help the compiler resolve which overload of a method you are calling, or the method you are calling even exists.
In your example, you need to specify that you are calling the method "duplicate" that takes a single string argument. If you omit the cast, then the compiler only knows that the method you intended to call is called "duplicate" and has a single argument, but can't tell what type the argument is - did you mean to call duplicate(string) or did you mean to call duplicate(some other type)? Should it compile, or should it error telling you the method you are trying to call does not exist?
You will also get the same issue if you had a duplicate(byte[]) defined, because now your call is ambiguous without the explicit cast.
No its not an object. null is the default value of reference-type variables. Ordinary value types cannot be null. but there is another set called nullable types.
You are probably referring to the fact that the following leads to a compiler error:
int? nullableInt = (somecondition) ? value : null;
Indeed you need to add a cast here:
int? nullableInt = (somecondition) ? value : (int?)null;
Even though I'm not able to explain this in detail, I'd suspect the following:
int? is actually a short form for Nullable<int>, so it is basically an object instance. When assigning an int value to nullableInt, a property of Nullable<int> will be set internally. Directly assigning null would also be ok.
The conditional assignment however, would return two different types: int in case somecondition is true and object (null) otherwise.
Now the compiler doesn't know how to handle this, as the ternary operator needs to return values of the same type. So you need to specify the desired "type for the null value".
Sorry if this is not a very deep technical explanation - I'm sure there's somebody who can elaborate this better, but it might help understand this better.
My question is: can I check whether a variable (string or int/double type) or an array (string or int/double type) is initialized in C#?
Thanks in advance.
You are guaranteed some sort of initialization. For any static or instance members, all variables are automatically initialized when the type or instance is constructed, either explicitly or implicitly (in which case default(Type) is the value, so 0 for numeric types, null for strings and other reference types, etc.).
For local variables, they cannot be used before declaration, so if you can check it, it's been initialized.
Yes you can.
For types that require instances (string or arrays, as you asked), you can verify if they are null.
You could do this many ways but one way is :
if (myObject == null)
{
//initialize it here
}
Primitive data types do not require instancing. For example:
int i;
wont be equal to null, it will be equal to 0.
Try This, :
If var = NULL Then
MsgBox ('Not initialized')
End If
C# requires that all variables be initialized to some value before you read them.
The code block:
int i;
if(i == 0)
{
// something...
}
Will generate a compile-time error because you're trying to access the value of i before assigning it. This also applies to objects (although you can initialize them to null to begin with).
If you are wanting to know if you have modified from your initial assignment, then no, there is no way of telling that directly unless the initial assignment is to a sentinel value that will not be repeated by a subsequent assignment. If this is not the case you will need an extra bool to track.
No. However, you will have a compiler error if it is a local variable. If it is a class member then it is automatically initialized to the default (0 for ints, null for objects, etc.)
tongue in cheek, but accurate answer
Scan your source code and find all usages and declarations of the variable to verify that it is initialized either at declaration, or else somewhere guaranteed before using it.
Just curious.
If you go:
string myString;
Its value is null.
But if you go:
int myInt;
What is the value of this variable in C#?
Thanks
David
Firstly, note that this is only applicable for fields, not local variables - those can't be read until they've been assigned, at least within C#. In fact the CLR initializes stack frames to 0 if you have an appropriate flag set - which I believe it is by default. It's rarely observable though - you have to go through some grotty hacks.
The default value of int is 0 - and for any type, it's essentially the value represented by a bit pattern full of zeroes. For a value type this is the equivalent of calling the parameterless constructor, and for a reference type this is null.
Basically the CLR wipes the memory clean with zeroes.
This is also the value given by default(SomeType) for any type.
default of int is 0
The default value for int is 0.
See here for the full list of default values per type: http://msdn.microsoft.com/en-us/library/83fhsxwc.aspx
Here is a table of default values for value types in C#:
http://msdn.microsoft.com/en-us/library/83fhsxwc.aspx
Reference types default value is usually null.
String is a reference type. Int is a value type. Reference types are simply a pointer on the stack directed at the heap, which may or may not contain a value. A value type is just the value on the stack, but it must always be set to something.
The value for an unitialized variable of type T is always default(T). For all reference types this is null, and for the value types see the link that #Blorgbeard posted (or write some code to check it).