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.
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?
Consider this code:
static void Main()
{
int? a = new int?(12); // warning CS0219: The variable 'a' is assigned but its value is never used
int? b = 12; // warning CS0219: The variable 'b' is assigned but its value is never used
int? c = (int?)12; // (no warning for 'c'?)
}
The three variables a, b and c are really equivalent. In the first one, we call the public instance constructor on Nullable<> explicitly. In the second case, we utilize the implicit conversion from T to T?. And in the third case we write that conversion explicitly.
My question is, why will the Visual C# 5.0 compiler (from VS2013) not emit a warning for c the same way it does for the first two variables?
The IL code produced is the same in all three cases, both with Debug (no optimizations) and with Release (optimizations).
Not sure if this warning is covered by the Language Specification. Otherwise, it is "valid" for the C# compiler to be inconsistent like this, but I wanted to know what the reason is.
PS! If one prefers the var keyword a lot, it is actually plausible to write var c = (int?)12; where the cast syntax is needed to make var work as intended.
PPS! I am aware that no warning is raised in cases like int? neverUsed = MethodCallThatMightHaveSideEffects();, see another thread.
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 piece of code:
class Foo
{
public Foo()
{
Bar bar;
if (null == bar)
{
}
}
}
class Bar { }
Code gurus will already see that this gives an error. Bar might not be initialized before the if statement.
What is the value of bar? Shouldn't it be null? Aren't they set to null? (null pointer?)
No, local variables don't have a default value1. They have to be definitely assigned before you read them. This reduces the chance of you using a variable you think you've given a sensible value to, when actually it's got some default value. This can't be done for instance or static variables because you don't know in what order methods will be called.
See section 5.3 of the C# 3.0 spec for more details of definite assignment.
Note that this has nothing to do with this being a reference type variable. This will fail to compile in the same way:
int i;
if (i == 0) // Nope, i isn't definitely assigned
{
}
1 As far as the language is concerned, anyway... clearly the storage location in memory has something in it, but it's irrelevant and implementation-specific. There is one way you can find out what that value is, by creating a method with an out parameter but then using IL to look at the value of that parameter within the method, without having given it another value. The CLR doesn't mind that at all. You can then call that method passing in a not-definitely-assigned variable, and lo and behold you can detect the value - which is likely to be the "all zeroes" value basically.
I suspect that the CLI specification does enforce local variables having a default value - but I'd have to check. Unless you're doing evil things like the above, it shouldn't matter to you in C#.
Fields (variables on classes / structs) are initialized to null/zero/etc. Local variables... well - since (by "definite assignment") you can't access them without assigning there is no sensible way of answering; simply, it isn't defined since it is impossible. I believe they happen to be null/zero/etc (provable by hacking some out code via dynamic IL generation), but that is an implementation detail.
For info, here's some crafy code that shows the value of a formally uninitialised variable:
using System;
using System.Reflection.Emit;
static class Program
{
delegate void Evil<T>(out T value);
static void Main()
{
MakeTheStackFilthy();
Test();
}
static void Test()
{
int i;
DynamicMethod mthd = new DynamicMethod("Evil", null, new Type[] { typeof(int).MakeByRefType()});
mthd.GetILGenerator().Emit(OpCodes.Ret); // just return; no assignments
Evil<int> evil = (Evil<int>)mthd.CreateDelegate(typeof(Evil<int>));
evil(out i);
Console.WriteLine(i);
}
static void MakeTheStackFilthy()
{
DateTime foo = new DateTime();
Bar(ref foo);
Console.WriteLine(foo);
}
static void Bar(ref DateTime foo)
{
foo = foo.AddDays(1);
}
}
The IL just does a "ret" - it never assigns anything.
Local variables do not get assigned a default value. You have to initialize them before you use them. You can explicityly initialize to null though:
public Foo()
{
Bar bar = null;
if (null == bar)
{
}
}
Local variables are not assigned a default value, not even a null.
The value of bar is undefined. There's space allocated for it on the stack, but the space isn't initialised to any value so it contains anything that happened to be there before.
(The local variable might however be optimised to use a register instead of stack space, but it's still undefined.)
The compiler won't let you use the undefined value, it has to be able to determine that the variable is initialised before you can use it.
As a comparison, VB does initialise local variables. While this can be practical sometimes, it can also mean that you unintenionally use a variable before you have given it a meaningful value, and the compiler can't determine if it's what you indended to do or not.
It doesn't matter because no such code should be compilable by any compiler that implements C#.
If there was a default value, then it would be compilable. But there is none for local variables.
Besides "correctness", local variable initialization is also related to the CLR's verification process.
For more details, see my answer to this similar question: Why must local variables have initial values?
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.