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.
Related
The C# compiler is a bit ... old fashioned ... and won't do static analysis. So it breaks on seemingly correct code like this:
MyStruct s;
bool inited = false;
foreach( Something foo in ACollection )
{
if( foo.Test() )
continue;
if( inited )
s.DoSomething();
else
{
s = foo.GetMeAnS();
inited = true;
}
}
Note: the unusual problem is that "s" is a struct. If it were a class, I'd simply init it to null. This struct has no meaningful "uninited" state, and I don't want to pay the performance cost of initing something I immediately throw away, just to satisfy a weak compiler.
The code (should be) fully correct: it's impossible to access s until s has been inited. (I've copy/pasted from actual code, but edited-out long method names for simplicity).
C# compiler in Mono used to allow this, but now it doesn't. Nothing has changed except the compiler, which now gives an error on unassigned variable.
Is there a code way to tell it to shut up and mind its own business? :) I don't want to fiddle with changing compiler-settings (if possible) because the code is compiled by other people/orgs - I'd prefer a code way of fixing the problem.
Is there a code way to tell it to shut up and mind its own business?
The compiler's business is implementing the C# specification. The code you've written should not compile according to the C# specification. The s.DoSomething() call is reachable without s being definitely assigned, therefore your code is broken. That's not the compiler's fault. If the Mono compiler used to allow it, that was a bug which has apparently now been fixed.
The simplest way of fixing it is to definitely assign the value, of course:
MyStruct s = new MyStruct(); // Value will never actually be used
There are plenty of cases where we (as humans) can tell that something will never happen, but the compiler can't. Here's another example:
public int Foo(int input)
{
if (input >= 0)
{
return input;
}
else if (input < 0)
{
return -input;
}
// This is still reachable...
}
We know that every int input will go into one of those if bodies, but the compiler will still (correctly) give a compilation error on the above code, because the closing brace is reachable and it's a non-void method.
Your claim that "The code (should be) fully correct" is according to your reasoning, not the C# specificiation... and the compiler is only meant to care about the latter.
One thing to note: the specification doesn't even care about the fact that we do actually set inited to true in some cases. Even if it always had the value of false, it's still just a local variable, not a constant expression. Here's a simple example demonstrating that with no loop:
static void Main()
{
int x;
bool condition = false;
if (condition)
{
Console.WriteLine(x);
}
}
This still gives an error: "error CS0165: Use of unassigned local variable 'x'"
From section 8.7.1 of the C# 5 specification:
The first embedded statement of an if statement is reachable if the if statement is reachable and the boolean expression does not have the constant value false.
Here the expression is condition, which is a local variable. A local variable is not a constant expression in technical terms, even if it will never change. If you make it a local constant instead, it will compile:
static void Main()
{
int x;
const bool condition = false;
if (condition)
{
Console.WriteLine(x);
}
}
Now there's a warning about the body of the if statement being unreachable - but there's no error.
The C# specification was designed such that you should never be able to use an uninitialized variable. This is not an old fashioned concept. The old fashioned way to deal with this (in C++) was to say, "This is undefined behaviour, anything can happen".
Strangely enough lots of bugs were caused by this attitude, which gave rise to many compilers automagically initing (in debug mode) variables into such gems as 0xDEADBEEF.
As for why the C# compiler doesn't do code analysis to find if the variable is inited when it reaches that code?
Halting Problem
The problem could be rewritten like this.
bool inited = false;
MyStruct? s;
while (true)
{
foreach( Something foo in ACollection )
foo.Test();
if( inited && false == s.HasValue )
return;
}
This is only slightly changed code. But you can see, I have converted your problem into the Halting Problem, where the inputs are, the state of each Something and the implementation of foo.Test().
This is proven to be undecidable on a Turing Machine, which your CLR VM certainly is.
In short, you are asking, why hasn't microsoft broken the laws of Mathematics and Computer Science when they wrote the C# compiler.
Or you are asking, shouldn't the Microsoft C# Compiler try hard before giving up on the Halting Problem. To which my response is, how hard should they try?
I have a two methods that are equivalent to this (pardon the contrived example):
public void WithResource(Action<Resource> action) {
using (var resource = GetResource()) {
action(resource);
}
}
public void Test() {
int id;
SomeObject someObject;
WithResource((resource) => {
id = 1;
someObject = SomeClass.SomeStaticMethod(resource);
});
Assert.IsNotNull(someObject);
Assert.AreEqual(id, someObject.Id);
}
(There's some more logic in the WithResource call I'm trying to factor out.)
I'm getting Use of unassigned local variable compile-time errors because the assertions
are... using unassigned variables. I'm currently avoiding the issue by assigning them -1 and null respectively.
Initializing to null doesn't feel bad, but I'd like to avoid putting the -1 in there... I would really like to tell the compiler "trust me, this will become initialized". Since it's a test, I don't actually care too much if it bombs, because that only means I'll have to fix the test.
I'm tempted to ask if there's a way to give that hint to the compiler, but have the feeling that's even uglier. Does such a hint exist or should I just initialize the variables like I'm doing now?
Compiler is not smart enough to determine if the assignment would be made and hence the error.
You can't do anything about it, You have to assign it some default value, probably 0,-1, default(int) or int.MinValue
You should initialize the variables. The compiler will never trust you ;)
The other answers are correct. The easiest way to solve this problem is to initialize the locals.
I assume that you understand why the error is being produced: the compiler has no ability to know that the method called actually runs the lambda, and therefore no knowledge that the locals are initialized.
The only way to trick the compiler into not checking whether a variable is assigned is to make the variable non-local:
public void Test() {
int[] id = new int[1];
SomeObject[] someObject = new SomeObject[1];
WithResource((resource) => {
id[0] = 1;
someObject[0] = SomeClass.SomeStaticMethod(resource);
});
Assert.IsNotNull(someObject[0]);
Assert.AreEqual(id[0], someObject.Id);
}
Now you might say, well here I've clearly assigned id. Yes, but notice that the compiler does not complain that you've used id[0] before initializing it! The compiler knows that array element variables are initialized to zero.
I have the following code:
var ad = product.AttributesDictionary;
var ddl = product.DetailedDescriptionList;
var rdl = product.RelatedProductList;
I have read that the C#/JIT compiler can do a lot of optimizations. Since the variable are not used after the assignment. I fear the C#/JIT compiler might omit these lines for optimizations.
Please don't tell me to remove this line.
Update: I think no one is able to understand what I mean. My question is that if you assign a property to a variable and then don't use this variable, will the C#/JIT compiler omit this code or not?
(This is not specific to the C# compliler, it's just a general observation)
Assigning a property to a variable has actually two parts:
Executing the get method of the property
Assigning the return value of the method to the variable.
Since the first part can be as complex as any method, and have plenty of side-effects, no compiler ever will optimize it away.
If the variable is not used after the assignment the second part can be optimized, if the compiler is optimizing aggressively.
However, there is another kind of optimizer that behaves way less predictably - the next developer that sees that code.
Those line look like they don't do anything, so if the next person that sees them is a pedant that hates superfluous code (as most developers are), those lines could be deleted without a second glance.
Try to find a better - more explicit - way to accomplish whatever you are trying to accomplish with those lines;
If you are trying to initialize a flyweight cache forcibly with a dummy call, then you can try ToString(), like product.AttributesDictionary.ToString(); I believe compiler won't omit that line. You may need to check if product.AttributesDictionary is null or not depending on cache's behavior.
Well, let's see what the spec says about this:
Section 3.10: Execution order
Execution of a C# program proceeds such that the side effects of each
executing thread are preserved at critical execution points. A side
effect is defined as a read or write of a volatile field, a write to a
non-volatile variable, a write to an external resource, and the
throwing of an exception.
[..]
Additionally, the execution environment need not evaluate part of an
expression if it can deduce that that expression’s value is not used
and that no needed side effects are produced (including any caused by
calling a method or accessing a volatile field).
Since it is evidently possible that your code throws an exception, it would be a compiler bug if the accesses were optimized away.
So this code is technically safe. Throwing exceptions from an accessor agrees not only with the letter of the law, but also with the spirit: accessors are methods, and methods can certainly be expected to throw depending on the particulars.
However, on the human-facing side of this code things don't look good. If there is a reasonable expectation that you would want to check something, your code should enable this:
obj.TrySomething()
It should not require you to do this:
try { obj.DoSomething(); }
catch (SomeException e) { ... }
You can find the answer by compiling this code into an assembly (DLL or EXE) and then using ILDASM to convert it into plain text IL.
This will still not tell you exactly what happens at runtime because the JIT turns the IL into x86/x64 machine code. But the Visual Studio debugger shows you disassembly for that, allowing you to dig further.
I have tried it, with this code:
public class Example
{
public string SomeProperty { get; set; }
public static void DoSomething(Example instance)
{
var value = instance.SomeProperty;
}
}
The method DoSomething compiles to the following IL (courtesy of ILSpy):
.method public hidebysig static
void DoSomething (
class Example 'instance'
) cil managed
{
// Method begins at RVA 0x2061
// Code size 8 (0x8)
.maxstack 8
IL_0000: ldarg.0
IL_0001: callvirt instance string Example::get_SomeProperty()
IL_0006: pop
IL_0007: ret
} // end of method Example::DoSomething
You will see that the property accessor, get_SomeProperty() is called, even in the case of the most basic property accessor that one can write, and when the type is defined in the same assembly.
Why does taking the address of a variable eliminate the "Use of unassigned local variable" error?
(Why can we take the address without initialization in the first place?)
static unsafe void Main()
{
int x;
int* p = &x; //No error?!
x += 2; //No error?!
}
C# Language spec, section 18.5.4:
The & operator does not require its argument to be definitely assigned, but following an & operation, the variable to which the operator is applied is considered definitely assigned in the execution path in which the operation occurs. It is the responsibility of the programmer to ensure that correct initialization of the variable actually does take place in this situation.
...
The rules of definite assignment for the & operator exist such that redundant initialization of local variables can be avoided. For example, many external APIs take a pointer to a structure which is filled in by the API. Calls to such APIs typically pass the address of a local struct variable, and without the rule, redundant initialization of the struct variable would be required.
I think because, once you've taken a pointer to the variable, there's no way for the compiler to analyze whether a value is assigned via that pointer, so it's excluded from the definite assignment analysis.
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.