I have some code like this:
[AcceptVerbs(HttpVerbs.Post)]
public ActionResult Save([Bind(Prefix="")]Person person)
{
String s = person.property;
/* ... */
}
But it throws the error: "Cannot use local variable 'person' before it is declared".
What simple thing am I missing?
It is most likely that you are receiving this error because the same variable is being declared later in the same code block.
According to compiler rules, a variable reference will refer to default by a matching declaration withing the same block EVEN IF THE SAME DECLARATION EXISTS OUTSIDE OF THE BLOCK IN IN LOGICAL SCOPE FLOW.
So in short, check to see if the variable isnt being declared later on(a couple of lines down) in the same application block.
Okay, this is just some really bizarre error - if the variable is named a particular name it does not work, for any other name it does work...
I had the same problem with a declared variable named endingYear.
Declared here:
public ChartData(MetricInfo metricInfo, MetricItem[] metricItems) : this()
{
int endingYear = 0;
Further along in the method this was not a problem:
endingYear = endingDate.Year;
But when I referenced the very same varable in a Case statement I got the "Cannot use local variable before it is declared" error even thou the variable was in intelesense:
case "QRTR_LAST_FULL_QRTR":
if (metricInfo.CalendarType == "CALENDAR")
{
switch (endingDate.Month)
{
case 1:
case 2:
case 3:
loopControl = 4;
endingYear = endingDate.Year - 1;
Based on Matt's result I tried changing the variable name to endYear and the problem went away. Very strange and a waste of a half hour or so. If it was not for this thread of posts it probably would have been a bigger time loss.
Related
This question already has answers here:
C# variable scoping: 'x' cannot be declared in this scope because it would give a different meaning to 'x'
(3 answers)
Closed 2 years ago.
I have a piece of code similar to this
public string GetMessage()
{
if (new Random().Next(10) % 2 == 0 == 0)
{
string message = "bad luck";
return message;
}
string message = "lucky";
return message;
}
This code could surely be improved, but what's puzzling me is that it unexpectedly produces the error
A local or parameter named 'message' cannot be declared in this scope because that name is used in an enclosing local scope to define a local or parameter
Now what's weird about this is that the variable in the smaller scope shouldn't even exist to cause the clash. It's legal in C:
char* GetMessage()
{
if (rand() % 2 == 0)
{
char* message = "bad luck";
return message;
}
char* message = "lucky";
return message;
}
This compiles and works. As soon as the smaller scope ends, the variable inside of it ceases to exist and I'm free to reuse the name. Similar code also works in Swift, so why is this different in C#?
In C#, any variables declared in a block will remain in scope throughout the entire block where they have been declared. This means that if you declare a variable in an if/else block, you cannot then declare a variable with the same name below in the same method block, as C# will see it already exists.
This site explains variable scope in C# really well in my opinion if you would like to check it out.
You had rather good luck with your c sample. In c we expect that the storage for "message" is out of scope after return : but the pointer returned points to where the value of "message" used to be. To correct the c, declare and define a local variable with enough space to take the longest message IN THE CALLING FUNCTION. Pass a pointer to that space to the function, rather than returning a dangling pointer. As a side note: in production code, your c code might work for years then unexpectedly show intermittent failures. Please review the scope rules in both c and c#.
On MSDN, this code is posted at https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/try-catch I am unable to understand why it throws the error:
Use of unassigned local variable 'n'.
static void Main()
{
int n;
try
{
// Do not initialize this variable here.
n = 123;
}
catch
{
}
// Error: Use of unassigned local variable 'n'.
Console.Write(n);
}
Compiler Error CS0165
The C# compiler does not allow the use of uninitialized variables. If
the compiler detects the use of a variable that might not have been
initialized, it generates compiler error CS0165. For more information,
see Fields. Note that this error is generated when the compiler
encounters a construct that might result in the use of an unassigned
variable, even if your particular code does not. This avoids the
necessity of overly-complex rules for definite assignment.
More-so, imagine this situation
int n;
try
{
throw new Exception();
n = 123; // this code is never reached
}
catch
{
}
// oh noez!!! bam!
// The compiler is trying to be nice to you
if(n == 234);
In short, computer says no
Note : when you get a compiler error in visual studio, you can click on the error code and it sometimes (if you are lucky) gives you more concise information about what the error means
I believe, what you're confused about, is that even though the variable n appears to be initialized, why the compiler complains it isn't?
And there's a good reason for that; even though n is initialized at one point, it isn't initialized in all possible paths. In other words, you have to account for each scenario in your code and ensure that in all of them, the initialization occurs.
But in this case, it doesn't satisfy that condition. In your try block, if there was an exception before the program gets to execute the n = 123; line, the program will go to the catch and then following that, will go to your Console.Write(n) line at which point you're trying to print a variable that isn't initialized.
So, the best way to prevent such a scenario is to initialize the variable before the try block. In general it is advised that you always initialize a variable as soon as it is declared.
EDIT
From a beginner's point of view, you might argue that there's only one line of code inside the try block, and therefore there's no way the program will not execute the initialization. But you must look at it from the compiler's perspective; it doesn't understand the intention of your program, it simply verifies (that's what a compiler does) if a program is written according to a predefined set of rules. And in this case, it isn't.
If you look at the article, you'll see the answer:
// Error: Use of unassigned local variable 'n'.
When you write int n; you do not initialize variable and try to use it in Console.Write(n);, so you will get compilation error: https://ideone.com/q3LXwl
This error is because you are using n in Console.Write() function. And suppose if Try block generates an exception then n would not be initialized. Therefore this error occurs.
public class Program
{
public static void main(String[] args)
{
string message = "This is a message";
int number = 6;
object obj = null;
int? nullable = (int?)12;
}
}
The first three variable declarations on this program throw the following warning:
The variable 'X' is assigned but its value is never used
Yet, the last statement:
int? nullable = (int?)12;
doesn't throw anything. Why is that?
The warning that you're seeing is only shown in cases where the compiler can prove that the expression used to initialize the variable can't possibly cause any side effects. When you're just assigning a literal string, integer, or null value to a variable, the compiler knows that none of those things can possibly cause side effects. For your last value you're not just assigning a literal value though; you're using the explicit operator of a type as well, and as far as the compiler is concerned, that operator is just some code that could do anything. It could, for example, cause relevant side effects (it doesn't, but the compiler doesn't know that) that would make the line not superfluous.
Warnings are best practice suggestions, they are not true errors. Visual studio is smart enough to see you created the variables but never used them so they are useless. It is recommending that you get rid of them since you don't use them.
If you actually do something with them then the errors will go away. For example if you said:
number += nullable;
Would get rid of 1 of the errors. If you did something like:
message = message + " and this is more message";
It would get rid of the other error.
I believe it sees the int? as an object and since you are casting it to the nullable variable, it can't figure out if it had been used before hand. I think it has something more to do with the casting just isn't caught as an error because it can't tell if you referenced that variable somewhere else.
I think it would be the equivalent of something like this:
var a = new SomeClass();
var b = a;
Since it can't tell if a has really been used, then it doesn't show an error. If you put that in with a real class it will not show the error also.
It seems like the Warning doesn't show because the line with the nullable actually do an operation before the assignation. As for exemple the following code only generate a warning on the variable named number. This seems to be logical in the meaning that the line with warning are truely useless in the current code. The other lines might do something during the execution and will not be "optimized out". To test it run your code in release mode with the debugger ad you will see that all lines that have a warning are skipped ("optimized")
class Program
{
static void Main(string[] args)
{
string message = ';'.ToString();
int number = 6;
object obj = (object)(new t());
int? nullable = (int?)12;
}
class t
{ }
}
With this being said. The nullable it not optimized because there is a cast from a int '12' to a int?. Since it is a real cast and not a "useless cast" an operation is needed during the run time.
Based on this recent question, I don't understand the answer provided. Seems like you should be able to do something like this, since their scopes do not overlap
static void Main()
{
{
int i;
}
int i;
}
This code fails to compile with the following error:
A local variable named 'i' cannot be declared in this scope because it would give a different meaning to 'i', which is already used in a 'child' scope to denote something else
I don't think any of the answers so far have quite got the crucial line from the spec.
From section 8.5.1:
The scope of a local variable declared in a local-variable-declaration is the block in which the declaration occurs. It is an error to refer to a local variable in a textual position that precedes the local-variable-declarator of the local variable. Within the scope of a local variable, it is a compile-time error to declare another local variable or constant with the same name.
(Emphasis mine.)
In other words, the scope for the "later" variable includes the part of the block before the declaration - i.e. it includes the "inner" block containing the "earlier" variable.
You can't refer to the later variable in a place earlier than its declaration - but it's still in scope.
"The scope of local or constant variable extends to the end of the current block. You cannot declare another local variable with the same name in the current block or in any nested blocks." C# 3.0 in a Nutshell, http://www.amazon.com/3-0-Nutshell-Desktop-Reference-OReilly/dp/0596527578/
"The local variable declaration space of a block includes any nested blocks. Thus, within a nested block it is not possible to declare a local variable with the same name as a local variable in an enclosing block." Variable Scopes, MSDN, http://msdn.microsoft.com/en-us/library/aa691107%28v=vs.71%29.aspx
On a side note, this is quite the opposite that of JavaScript and F# scoping rules.
From the C# language spec:
The local variable declaration space of a block includes any nested blocks. Thus, within a nested block it is not possible to declare a local variable with the same name as a local variable in an enclosing block.
Essentially, it's not allowed because, in C#, their scopes actually do overlap.
edit: Just to clarify, C#'s scope is resolved at the block level, not line-by-line. So while it's true that you cannot refer to a variable in code that comes before its declaration, it's also true that its scope extends all the way back to the beginning of the block.
This has been a rule in C# from the first version.
Allowing overlapping scopes would only lead to confusion (of the programmers, not the compiler).
So it has been forbidden on purpose.
For C#, ISO 23270 (Information technology — Programming
languages — C#), §10.3 (Declarations) says:
Each block, switch-block, for-statement, foreach-statement, or
using-statement creates a declaration space for local variables and
local constants called the local variable declaration space. Names are
introduced into this declaration space through local-variable-declarations
and local-constant declarations.
If a block is the body of an instance
constructor, method, or operator declaration, or a get or set accessor for
an indexer declaration, the parameters declared in such a declaration are
members of the block’s local variable declaration space.
If a block is the
body of a generic method, the type parameters declared in such a declaration
are members of the block’s local variable declaration space.
It is an error
for two members of a local variable declaration space to have the same name.
It is an error for a local variable declaration space and a nested local
variable declaration space to contain elements with the same name.
[Note: Thus, within a nested block it is not possible to declare a local
variable or constant with the same name as a local variable or constant
in an enclosing block. It is possible for two nested blocks to contain
elements with the same name as long as neither block contains the other.
end note]
So
public void foobar()
{
if ( foo() )
{
int i = 0 ;
...
}
if ( bar() )
{
int i = 0 ;
...
}
return ;
}
is legal, but
public void foobar()
{
int i = 0 ;
if ( foo() )
{
int i = 0 ;
...
}
...
return ;
}
is not legal. Personally, I find the restriction rather annoying. I can see issuing a compiler warning about scope overlap, but a compilation error? Too much belt-and-suspenders, IMHO. I could see the virtue of a compiler option and/or pragma , though ( perhaps -pedantic/-practical, #pragma pedantic vs #pragma practical, B^)).
It's not a question of overlapping scopes. In C# a simple name cannot mean more than one thing within a block where it's declared. In your example, the name i means two different things within the same outer block.
In other words, you should be able to move a variable declaration around to any place within the block where it was declared without causing scopes to overlap. Since changing your example to:
static void Main()
{
int i;
{
int i;
}
}
would cause the scopes of the different i variables to overlap, your example is illegal.
I just compiled this in GCC both as C and as C++. I received no error message so it appears to be valid syntax.
Your question is tagged as .net and as c. Should this be tagged as c#? That language might have different rules than C.
In C you need to put all variable declaration at the very beginning of a block. They need to come all directly after the opening { before any other statements in this block.
So what you can do to make it compile is this:
static void Main()
{
{
int i;
}
{
int i;
}
}
Here's your answer from MSDN .NET Documentation:
...The local variable declaration space of a block includes any nested blocks. Thus, within a nested block it is not possible to declare a local variable with the same name as a local variable in an enclosing block.
int? test;
try
{
test = (int?) Int32.Parse ("7");
} catch {}
if (test == null)
Console.WriteLine("test is null!");
else
Console.WriteLine("test = {0}", test);
I am have some code that does something VERY similar to this, same idea really... Creating a variable, trying to initialize it, then test to see if the initialization was a success.
Visual Studios is giving me an error saying " Use of unassigned local variable 'test' ", which is kind of annoying, this is easily fixed by setting the first line to:
int? test = null;
but I am curious what the difference between the two lines are, because the compiler really seems to care. And to the best of my knowledge, the two lines do the same thing.
The problem is the catch block. The compiler must assume the Int32.Parse code can throw and hence hit your catch block. In the case that happens the Int32.Parse line does not complete and hence test is never assigned a value. That means the "if" line is attempting to use an uninitialized value.
You can fix this by
Assigning test a value in the catch block
Initializing it to null at the start of the method
you're confusing the difference between what is a variable declaration and what is variable initialization
int? test;
simply states that you have a variable named test that is a nullable int
but
int? test = null;
states that you have a variable named test that is a nullable int and its value is null
In VB there is no distinction, but in c# there is a difference. That's why the compiler is complaining, because if something fails in your try block then your test variable would never have been initialized.
You can avoid (int?) cast, to save 7 bytes for " = null" string :)
test = Int32.Parse ("7");
This is an error that always occurs for locally defined variables (inside a method or property as opposed to directly within the class). Although the fact remains that the compiler need not generate this error in order to work, it does it specifically for the purpose of helping you identify potentialy unexpected results in the case of not always assigning your variables. (Someone correct me if I'm wrong, but at least some previous versions of the C# compiler didn't check for unassigned variables in some/all cases.)
Equivalently (instead of assigning test = null in the declaration), you could eliminate the error by assining test = null in the catch block, since this would mean that whatever path the code takes, the variable test gets assigned. However, I think the resolution that you have stated (assigning to null in the declaration) is the correct one - you'll see it very often in C# code that brances a lot (via try-catch statements, if statements, or whatever else) - and to be honest it's only helping you to realise to what and when you are assigning your variables, even though it may seem like a minor irritance.
They do the same thing, your are correct, however the variable requires the explicit assignment of null to get rid of the 'unassigned value 'error, if you want null to be considered an intentional 'not set' variable that wont throw warnings. Beyond that, Jaredpar's answer is right on target.
I believe this style is much cleaner:
try
{
int test = Int32.Parse("7");
Console.WriteLine("Success!");
// add return statement for successful execution here, if any
}
catch
{
Console.WriteLine("Disaster!");
// other return statement here, if any
}
As for the compiler error: Any local field must be explicitly initialized on a code path before reading it. It is a common error not to initialize local fields, that's why it is an error in C#. C/C++ only warns about this, and it can yield *funny* results if it's not initialized and the value reflects the bytes that already were on the call stack.
I can only speculate on this, but it could be a performance aspect of explicitly initializing local fields, contrary to class fields: When an object is instantiated it is less costly for the run-time to initialize the object memory stream once, however initializing a local field multiple times on every method call is redundant.