In Visual Studio I used the following code:
private bool answer = true;
Private Buttonclick()
{
if()
{
answer =false
}
}
Compiler gives me a warning that says "answer is assigned but its value is never used".
How do I fix this?
It means you have given answer a value, but not referenced it elsewhere. Meaning you are not using answer elsewhere in your program.
You fix it by referencing answer from another part of your program.
The reason the warning pops up is not because the variable is never assigned, but because it is in fact never used for any sort of comparison in the rest of the code.
Please go through this reference:
http://weblog.west-wind.com/posts/2009/Feb/28/Variable-is-assigned-but-its-Value-is-never-used-Warning
If you want to disable warnings, the error list contains a button that can stop them being shown in the list.
Additionally, a warning is just a warning. They won't stop your program from compiling, or even running. They may, however, declare in advance runtime errors and the like, so don't ignore them completely.
In addition to the answers above, for more deep understanding of the this warning:
This warning will not always pop-up (if you not use assigned variable). Assigning a non-constant expression or method result will NOT generate the warning. It is done intentionally, since such an unassigned variables can be used in debugging.
For example:
var answer = SomeObject.SomePropery; //will not generate CS0219
There are excellent explanation in Microsoft.Docs:
https://learn.microsoft.com/en-us/dotnet/csharp/misc/cs0219
Related
The code:
int num01;
num01 = 20;
Gives:
Warning CS0219 The variable 'num01' is assigned but its value is never used
https://learn.microsoft.com/en-us/dotnet/csharp/misc/cs0219
You're creating a variable named num01, but you're not using it. I'm far from a C# expert, but I assume the easiest way to get rid of this warning would be to either use or remove the offending variable.
This is a warning, not an error. Everything will still compile and run fine.
Assigning a value to a variable is not 'using' it.
Although not recommended, you can suppress this warning by adding the following line above it
#pragma warning disable CS0219
CS0219 is a warning, not an error, so your program should compile. It might seem strange to you, because you are "using" the variable by assigning 20 to it. The warning message should be understood as pointing out that you are not using the variable in any other way besides assigning to it. In other words, the assignment cannot have any effect, other than if you are stepping through the program and looking at values in the debugger. If you add another line to your code using the variable in any way other than an assignment, then the warning will go away. For example, the following will do:
int num01;
num01 = 20;
Console.WriteLine(num01);
I have a bool variable like this:
bool myBool = true;
If I write if (myBool == null) I get the following warning:
The result of the expression is always 'false' since a value of type 'bool' is never equal to 'null' of type 'bool?'.
That's clear to me because it doesn't make sense to check whether a non-nullable variable is null. Visual Studio notices that and marks as warning.
Now I have a string, which is nullable by default, as I know.
Why can I apply a null-conditional operator to a hardcoded string without Visual Studio noticing it? I'm thinking of something like this:
"This is a string"?.AnyStringMethod();
Shouldn't Visual Studio notice that this string isn't null at all?
Warnings are for code that looks right but is actually wrong.
Your code looks wrong but does the right thing anyways.
Therefore: no warning.
Visual Studio must go off the type that the operator is working with.
When you create a bool, there is no way that it could ever be null by the type, until you change that type to bool?.
However, with a hard coded string, even though it has text within the quotes, there's no guarantee that it will stay there. The "variable" that gets created (even as just a plain string) is still of type string, which is able to have a null assigned to it and not change the type.
What you are looking for would be for them to inspect the value of every variable that they are creating. If they were to do that, then why not also check for something like this?
var i = 0;
if (i > 2) // This will always be false!
Update
As InBetween mentioned in the comments, there is a bit of an oversight in here as well. Having a string such as "Some string" that is not assigned in a variable is functionally equivalent to const string s = "Some string";. If you were to declare it like that, the code inspectors will detect if you run a comparison on that, such as the following:
const string s = "Some String";
if (s == null) // This will give a warning that this can't happen
I would attribute that difference in the way that the const is handled versus the way a plain static string is handled could be attributed to different development teams working on different parts at different times. Again, this is such an edge case that doesn't cause huge problems that it doesn't get warned that no one working on it most likely didn't think about it.
Because no one thought about it? Your code is so pointless that probably no one foresaw it would ever be used in production code. I'm pretty sure this scenario didn't even crop up once in the C# design comittees although I'd take that with a grain of salt until someone like Eric Lippert sheds more light on the issue.
C# sharp isn't born with all potential features and then someone decides to prune it. In order for the compiler to give a certain warning someone has to think about it, implement it, test it and document it.
In case of myBool == null, the warning is justified becuase its a plausible error that could potentially make it into production code and its clearly a bug in the program's logic. The second scenario is completely harmless even if it ends up making it into production, so the warning really doesn't make much sense.
Because bool is of a value type while string is a reference type
Value types cannot be null, but reference types are automaticly nulled through Default
The reason Visual Studio doesn´t notice, is because it is of no importancable really.. it is like asking if blue is more of a color than green
A string literal is slightly different than a string object. I believe the string literal is basically like a constant, it is immutable, and will not ever be null.
When you assign a string literal to a variable, you are creating a reference to the string in a memory location. That reference can be null. If you try to concatenate your string variable with another string and store back in your original string variable, the original string in memory is destroyed and a new string is created that is the concatenated string. This is because strings are always immutable.
Just curious, I'm not trying to solve any problem.
Why only local variables should be assigned?
In the following example:
class Program
{
static int a;
static int b { get; set; }
static void Main(string[] args)
{
int c;
System.Console.WriteLine(a);
System.Console.WriteLine(b);
System.Console.WriteLine(c);
}
}
Why a and b gives me just a warning and c gives me an error?
Addionally, why I can't just use the default value of Value Type and write the following code?
bool MyCondition = true;
int c;
if (MyCondition)
c = 10;
Does it have anything to do with memory management?
Tim gives a good answer to your first question but I can add a few more details.
Your first question is basically "locals are required to be definitely assigned, so why not make the same restriction on fields?" Tim points out that Jon points out that it is actually quite difficult to do so. With locals it is almost always crystal clear when a local is first read and when it is first written. In the cases where it is not clear, the compiler can make reasonable conservative guesses. But with fields, to know when a first read and a first write happens, you have to know all kinds of things about which methods are called in what order.
In short, analyzing locals requires local analysis; the analysis doesn't have to go past the block that contains the declaration. Field analysis requires global analysis. That's a lot of work; it's easier to just say that fields are initialized to their default values.
(Now, that said, it is of course possible to do this global analysis; my new job will likely involve doing precisely this sort of analysis.)
Your second question is basically "Well then, if automatic assignment of default values is good enough for fields then why isn't it good enough for locals?" and the answer is "because failing to assign a local variable and accidentally getting the default value is a common source of bugs." C# was carefully designed to discourage programming practices that lead to irritating bugs, and this is one of them.
Because other variables are initialized with their default value.
Jon Skeet has already found some interesting words on this issue:
For local variables, the compiler has a good idea of the flow - it can
see a "read" of the variable and a "write" of the variable, and prove
(in most cases) that the first write will happen before the first
read.
This isn't the case with instance variables. Consider a simple
property - how do you know if someone will set it before they get it?
That makes it basically infeasible to enforce sensible rules - so
either you'd have to ensure that all fields were set in the
constructor, or allow them to have default values. The C# team chose
the latter strategy.
and here's the related C# language specification:
5.3 Definite assignment
At a given location in the executable code of a function member, a
variable is said to be definitely assigned if the compiler can prove,
by a particular static flow analysis (§5.3.3), that the variable has
been automatically initialized or has been the target of at least one
assignment.
5.3.1 Initially assigned variables
The following categories of variables are classified as initially
assigned:
Static variables.
Instance variables of class instances.
Instance variables of initially assigned struct variables.
Array elements.
Value parameters.
Reference parameters.
Variables declared in a catch clause or a foreach statement.
5.3.2 Initially unassigned variables
The following categories of variables are classified as initially
unassigned:
Instance variables of initially unassigned struct variables.
Output parameters, including the this variable of struct instance
constructors.
Local variables, except those declared in a catch clause or a
foreach statement.
The CLR provides a hard guarantee that local variables are initialized to their default value. But this guarantee does have limitations. What is missing is its ability to recognize scope blocks inside the method body. They disappear once the compiler translates the code to IL. Scope is a language construct that doesn't have a parallel in the CLI and cannot be expressed in IL.
You can see this going wrong in a language like VB.NET for example. This contrived example shows the behavior:
Module Module1
Sub Main()
For ix = 1 To 3
Dim s As String
If ix = 2 Then s = "foo"
If s Is Nothing Then Console.WriteLine("null") Else Console.WriteLine(s)
Next
Console.ReadLine()
End Sub
End Module
Output:
null
foo
foo
Or in other words, the local variable s was initialized only once and retains its value afterwards. This has a knack for creating bugs of course. The VB.NET compiler does generate a warning for it and has simple syntax to avoid it (As New). A managed language like C++/CLI has the same behavior but doesn't emit a diagnostic at all. But the C# language gives a stronger guarantee, it generates an error.
This rule is called "definite assignment". The exact rules are explained in detail in the C# Language Specification, chapter 5.3.3
Definite assignment checking has its limitations. It works well for local variables since their scope is very limited (private to the method body) and you can't get to them with Reflection. Much harder to do with fields of a class, it requires whole-program analysis that may need to reach beyond what the compiler can see. Like code in another assembly. Which is why the C# compiler can only warn about it but can't reject the code out-right.
Just a quick Question to the compiler of VisualStudio 2008.
We do have enabled that compiler-warnings are treated as errors which works fine, but today i recognized, that following behaviour:
static void Main(string[] args)
{
int number = 0;
DateTime dateTime = DateTime.Now;
}
Compiling this snippet results in only one warning: "The variable 'number' is assigned, but its value is never used".
Can anyone explain the difference to me, why the variable number results to the error, but not the dateTime variable?
Ok, it seems it has something to do with literals. Taking the following code in account:
static void Main(string[] args)
{
string str1 = "Foo";
string str2 = str1;
}
Compiling with both lines leads to no warning, although the variable "str2" is never referenced. If you comment out the line string str2 = str1; the warning shows up for variable "str1" is never used.
It is because DateTime.Now is a property, not a literal. Property getters can have side-effects, simply calling one can be useful by itself. Not that this is a good idea, it is however not verboten to do so and the compiler isn't smart enough to tell whether or not it does. It can't anyway, it ultimately calls operating system code to obtain the current system time.
My guess is that the statement int number = 0; is completely side-effect free and can be identified as superfluous by the compiler. DateTime dateTime = DateTime.Now; on the other hand, is an evaluation of a static property and could potentially execute other code, thus compiler does not identify it as an unused variable. In other words, while the variable might be unused, the act of assigning it could potentially do something else.
Eric Lippert wrote an article on this, so I'll leave it to him to explain:
Normally C# warns on all variables and
fields which are never read, never
written, etc. But in this case we
suppress the warning on purpose if the
assignment is not a constant
expression.
This is because there is no good way
in the Visual Studio debugger to say
"show me the return value of the last
function call". Though I would agree
were you to sensibly point out that
the way to solve this is to fix the
debugger, given that I have no ability
to fix it, we need a solution in C#
for our customers.
See article for further explanation.
DateTime.Now is a reference. I might be wrong since i have no experience with C# but i don't think you're creating a new object when using DateTime.Now.
if variable is not assigned, then it takes the default value at run time.
for example
int A1;
if i will check the value of A1 at runtime it will be 0;
then why at compile time it throws a error of unassigned value;
why CLR don't use to a lot the default value at runtime;
int A1;
int B1 = A1+10;
it should be 11 as the default value of A1 is 0;
there project property where i can check for "assign default values for unassigned variable";
Can anybody tell me where i can find it?
Your statement
if variable is not assigned,then it takes the default value at run time
is only true for member variables in a class.
For local variables inside a function, this is wrong. Local variables inside a function always require initialization.
it should be 11 as the default value of A1 is 0;
This is exactly the reason that the C# compiler won't let you get away with using uninitialized variables. The result would be 10, not 11. After a good 30 years of experience with C and C++, languages that allow you to use uninitialized variables, the C# team decided that this was a major source of bugs and to not allow this in a C# program.
There are lots of little tweaks like this. Another great example is not allowing fall through to another case in a switch statement. Forgetting to write break is such a classic bug. Outlawing these C-isms is rather an excellent idea and a big part of why C# is such a great language. Unless you dislike the idea of a compiler as a police officer.
Fwiw: using an uninitialized variable is permitted in VB.NET.
Default value is true for class members, but not for function locals. Whatever code you put directly into an as[pc]x file, the code generator will put it into a function.
Most of the time this happens is because the variable is an Object and before you can use it you need to Instantiate it.
When you assign a String = "" this is instantiated for you
You are talking about local variables or class level variables ? The rules are different for both. Check our Jon Skeet's reply at this:
Initialization of instance fields vs. local variables
The heap (reference classes) and the constructor for structs zero's the data.
Simple value-types like int, but also references (=pointers) to objects, do not get a default value on the stack. You should always set it. If this was not mandatory, specially with object-pointers, this could be a major security breach, because you are pointing to unknown locations.
Any default value (like 0) would probably be wrong 50% of the time.