Is there anyway way to prevent a class having static instances of it created in C#. I don't think there is but it could be useful. E.g just some attribute to prevent it.
something like this
[NoStaticInstances]
public class MyClass {
}
so that
public static MyClass _myClass;
would cause an error?
There's no such thing as a "static instance" - there's only a static variable, which is assigned a value. And there's no way of preventing static variables of a particular type being declared, unless you make the type itself static, which will prevent any instances being created and any variables of that type from being declared.
Imagine if your desired feature did exist... how would you expect the following code to behave?
class Test
{
static object foo;
static void Main()
{
MyClass bar = new MyClass();
foo = bar;
}
}
Which line of that would cause an error, if any? If it's the assignment, imagine this instead:
class Test
{
static object foo;
static void Main()
{
MyClass bar = new MyClass();
object tmp = bar;
foo = tmp;
}
}
In short, I don't think you're going to be able to prevent static variables holding references to instances of your class. Out of interest, why do you want to?
What you can do is the following:
public class MyClass
{
public MyClass()
{
#if DEBUG // Only run in debug mode, because of performance.
StackTrace trace = new StackTrace();
var callingMethod = trace.GetFrames()[1].GetMethod();
if (callingMethod.IsStatic &&
callingMethod.Name == ".cctor")
{
throw new InvalidOperationException(
"You naughty boy!");
}
#endif
}
}
Static fields will 'normally' be created by static constructors. What the above code does is looking at the calling method to see if it is a static constructor and if that's the case, throw an exception.
Note however, that this check is quite fragile and smart users can easily work around this by refactoring the creation of this method to another method. In other words, I agree with every body else that there is no good way to do this.
Such a restriction would not make sense.
What if you write
static object something = new YourClass();
Not really, there is no language or compiler feature that supports this.
No, there's no way to dictate the scope or lifetime of object references in C#.
Related
I am much confused about static variable actually i am executing below program.
class ABC
{
public static int prop { get; set; }
const int i =5;
static int j;
public ABC()
{
prop = 8;
j = 9;
Console.WriteLine("Under ABC class's constructor.");
}
public int getValue()
{
j = 6;
prop = 89;
return j;
}
}
class Program
{
static void Main(string[] args)
{
ABC obj = new ABC();
Console.WriteLine(obj.getValue());
//Console.WriteLine(ABC.j);
Console.ReadLine();
}
}
And its executing without any compile or run time error.
I have following confusions.
can we assign static variable/property inside the non static constructor?
can we assign static variable/property inside the instance method also?
If we can do assignment in above two cases for static variable/property then what is the use of static constructor?
Finally what are the locations inside a class where we can assign/initialize a static variable/property?
can we assign static variable/property inside the non static
constructor?
Yes.
can we assign static variable/property inside the instance method
also?
Yes.
If we can do assignment in above two cases for static
variable/property then what is the use of static constructor?
A static constructor is also called a type initializer. It is responsible for initializing the type it is defined in. You may use it to perform calculations that can be done upfront and are the same for all instances of that type. You'll therefore save some execution time when creating an instance because the calculation has already been done by the type initializer. Note that the type initializer runs before the type is used the first time. So you cannot deterministically tell when it actually runs. You can also not catch exceptions thrown by it because you don't actually invoke the type initializer yourself. You therefore need to be careful not to put error prone operations inside type initializers (f.e. do not do IO operations inside them).
Finally what are the locations inside a class where we can
assign/initialize a static variable/property?
From anywhere. Note that, even across threads, you can read and write to a static member from anywhere. This makes it very hard to find bugs that may occure due to a programm mutation a static somewhere in memory.
As a side note: Try to avoid having mutable static memory to keep your application simpler. If you really need to ... you should consider locking access to the static resource to prevent data races.
First, a general note, what Xamarin calls their "linker" is actually more of a "dead code remover". It is supposed to prevent uncallable code from making it into the compiled app.
I have a type in my app. When I use reflection to get its constructors, I see zero constructors:
private static int GetConstructorCount(Type type) {
ConstructorInfo[] constructors = type.GetConstructors();
return constructors.Count();
}
Yet when I use reflection to see its instance members, I see many:
private static void LogMemberInfo(Type type) {
int constructorCount = GetConstructorCount(type);
MyLoggingMethod(constructorCount, "Constructors");
MemberInfo[] members = type.GetMembers();
List<string> willLog = new List<string>();
foreach(MemberInfo member in members) {
if (member.DeclaringType == type) {
willLog.Add(member.Name);
}
}
willLog.Sort();
foreach (string str in willLog) {
MyLoggingMethod.LogLine(str);
}
}
Output from the above is:
0 Constructors
lots of surviving members, including instance members
This is a problem, because the type is a gateway to a whole lot of other types. I was hoping that by getting rid of all the constructors, all the instance members would disappear. They don't.
Is this a bug in the linker? Or is there a reason why it might still not want to get rid of instance members?
I do access members of the type via casting. Perhaps this is the problem?
public class MySuperclass {
public static MySuperclass Instance {get; set;}
}
public MyClass: MySuperclass {
public static SomeMethod() {
MySuperclass object = MySuperclass.Instance;
MyClass castObject = object as MyClass; // castObject will always be null, as no constructors survived the linking process. But maybe the linker doesn't realize that?
if (castObject!=null) {
castObject.InstanceMethod();
}
}
}
UPDATE: Getting rid of all the casts did not solve the problem. I am calling virtual members of superclass objects in lots of places; that's my next guess, but if that's the problem, fixing will be messy.
At least in my case, calling any static method on a type leads to preservation of lots of instance members. I literally tried this:
public class MyType() {
public static bool DummyBool() {
return true;
}
// instance members here
}
Once the type was getting removed by the linker, I put in a call to MyType.DummyBool(). This led to a lot of instance members being preserved.
This may not be the case for everyone. But it was the case for me.
Another insidious thing to watch out for is that if a static class has any properties that are initialized on startup, and the class as a whole is preserved, then those properties are preserved, even if they are never called:
public static class StaticClass {
public static Foo FooProperty {get;} = new Foo(); // if any code that is not removed calls StaticClass.SomeString, then Foo will be preserved.
public static string SomeString {
get {
return "Hello";
}
}
}
I am also seeing at least one case where code in a class that is removed by the linker nonetheless causes another class not to be removed. I assume this is a bug; however, my example is rather involved, and my attempts to get a simple repro have failed.
Have you tried using the Preserve attribute? The linker will not "optimize" code decorated with it:
[Xamarin.iOS.Foundation.Preserve]
For more information see the Xamarin documentation here
I am working on some code which is something like this:
class A
{
static SomeClass a = new Someclass("asfae");
}
Someclass contains the required constructor.
The code for this compiles fine without any warning. But I get a code hazard in system:
"The Someclass ctor has been called from static constructor and/or
static initialiser"
This code hazard part of system just to make it better by warning about possible flaws in the system or if system can get into bad state because of this.
I read somewhere on the web that static constructor/initialiser can get into deadlock in c# if they wait for a thread to finish. Does that have something to do with this?
I need to get rid of this warning how can i do this.
I can't make the member unstatic as it's used by a static function.
What should I do in this case , Need help.
You could hide it behind a property and initialize it on first use (not thread-safe);
class A
{
static SomeClass aField;
static SomeClass aProperty
{
get
{
if (aField == null) { aField = new Someclass("asfae"); }
return aField;
}
}
}
or use Lazy (thread-safe):
class A
{
static Lazy<SomeClass> a = new Lazy<SomeClass>(() => new Someclass("asfae"));
}
...or this very verbose thread safe version :)
class A
{
static SomeClass aField;
static object aFieldLock = new object();
static SomeClass aProperty
{
get
{
lock (aFieldLock)
{
if (aField == null) { aField = new Someclass("asfae"); }
return aField;
}
}
}
}
By initialising it as a static field, it behaves as it would in a static constructor, i.e. it probably gets initialised the first time an instance of your class is instantiated, but might happen earlier. If you want more control over exactly when the field is initialised, you could use Lazy<T>, e.g.:
{
static Lazy<SomeClass> a = new Lazy<SomeClass>(() => new Someclass("asfae"));
}
This way, you know that the initialisation of SomeClass will only happen the first time the field is accessed and its Value property called.
I think to understand your problem you need to know the difference between static constructors and type initializers, there is a great article from Jon Skeet about this issue:
http://csharpindepth.com/Articles/General/Beforefieldinit.aspx
The point is that following constructions are not the same, and there are difference in the behavior:
class Test
{
static object o = new object();
}
class Test
{
static object o;
static Test()
{
o = new object();
}
}
In any case, you could try to create a static constructor for your class to be able to have more control on this initialization, and maybe the warning will disappear.
If the member is only used by a static method, and only by this one, I would recommend you to put it in the scope if this static method and not as class member.
I am working with WF4 and need to use Types I created before, in a Workflow, but I'm not sure of my strategy.
I have a class:
class MyClass
{
public MyClass()
{
//Constructor Logic
}
public void Connect()
{
//Connect to a TCP/Device for example
}
public void Disconnect()
{
//Disconnect from a TCP/Device for example
}
}
and i want to use it in a WF4 Flowchart or StateMachine.
Then i have my main application:
class Program
{
private MyClass myObject;
WorkflowApplication WorkflowApplicationHoster;
static void Main(string[] args)
{
myObject = new MyClass;
IDictionary<string,object> input = new Dictionary<string,object>() {{"MyClassInstance",myObject} };
WorkflowApplicationHoster = new WorkflowApplication(new MyWorkflow,input);
WorkflowApplicationHoster.Run();
}
}
In my Workflow i have the "InArgument" -> "MyClassInstance" which is a MyClass Type and i use it for the whole workflow.
This doesn't feel correct. How to use own classe with the WF4?
OK -- so if I'm understanding this properly what you're trying to understand is how to get a new instance of your type into the workflow so it can be used. Generally speaking I've always been able to simply declare a variable and initialize it in some manner, but the question becomes what kind of initialization do you need?
If you just need to create a new instance of it, like shown above, then declare a variable of your type and in the Default Value issue the New {TypeName}() to create a new instance.
However, you're going to need to provide a lot more information if this doesn't help.
You want to use that MyClass instance in global scope; is how I read this.
One popular way is to create it as a Singleton. Generally this means you have a private/protected constructor and a public Instance method that ensures that one and only one instance is ever created.
Another way is to make the class, and thus all it's methods, static.
There are multiple threads in StackOverflow on the topic of these approaches. Additionally, it seems the real argument is whether to have something in global scope or not, not necessarily how that's implemented.
I am getting into C# and I am having this issue:
namespace MyDataLayer
{
namespace Section1
{
public class MyClass
{
public class MyItem
{
public static string Property1{ get; set; }
}
public static MyItem GetItem()
{
MyItem theItem = new MyItem();
theItem.Property1 = "MyValue";
return theItem;
}
}
}
}
I have this code on a UserControl:
using MyDataLayer.Section1;
public class MyClass
{
protected void MyMethod
{
MyClass.MyItem oItem = new MyClass.MyItem();
oItem = MyClass.GetItem();
someLiteral.Text = oItem.Property1;
}
}
Everything works fine, except when I go to access Property1. The intellisense only gives me "Equals, GetHashCode, GetType, and ToString" as options. When I mouse over the oItem.Property1, Visual Studio gives me this explanation:
MemberMyDataLayer.Section1.MyClass.MyItem.Property1.getcannot be accessed with an instance reference, qualify it with a type name instead
I am unsure of what this means, I did some googling but wasn't able to figure it out.
In C#, unlike VB.NET and Java, you can't access static members with instance syntax. You should do:
MyClass.MyItem.Property1
to refer to that property or remove the static modifier from Property1 (which is what you probably want to do). For a conceptual idea about what static is, see my other answer.
You can only access static members using the name of the type.
Therefore, you need to either write,
MyClass.MyItem.Property1
Or (this is probably what you need to do) make Property1 an instance property by removing the static keyword from its definition.
Static properties are shared between all instances of their class, so that they only have one value. The way it's defined now, there is no point in making any instances of your MyItem class.
I had the same issue - although a few years later, some may find a few pointers helpful:
Do not use ‘static’ gratuitously!
Understand what ‘static’ implies in terms of both run-time and compile time semantics (behavior) and syntax.
A static entity will be automatically constructed some time before
its first use.
A static entity has one storage location allocated, and that is
shared by all who access that entity.
A static entity can only be accessed through its type name, not
through an instance of that type.
A static method does not have an implicit ‘this’ argument, as does an
instance method. (And therefore a static method has less execution
overhead – one reason to use them.)
Think about thread safety when using static entities.
Some details on static in MSDN:
Static Classes in C#
Static Constructors in C#
This causes the error:
MyClass aCoolObj = new MyClass();
aCoolObj.MyCoolStaticMethod();
This is the fix:
MyClass.MyCoolStaticMethod();
Explanation:
You can't call a static method from an instance of an object. The whole point of static methods is to not be tied to instances of objects, but instead to persist through all instances of that object, and/or to be used without any instances of the object.
No need to use static in this case as thoroughly explained. You might as well initialise your property without GetItem() method, example of both below:
namespace MyNamespace
{
using System;
public class MyType
{
public string MyProperty { get; set; } = new string();
public static string MyStatic { get; set; } = "I'm static";
}
}
Consuming:
using MyType;
public class Somewhere
{
public void Consuming(){
// through instance of your type
var myObject = new MyType();
var alpha = myObject.MyProperty;
// through your type
var beta = MyType.MyStatic;
}
}
cannot be accessed with an instance reference
It means you're calling a STATIC method and passing it an instance. The easiest solution is to remove Static, eg:
public static void ExportToExcel(IEnumerable data, string sheetName)
{
Remove the static in the function you are trying to call. This fixed the problem for me.
I got here googling for C# compiler error CS0176, through (duplicate) question Static member instance reference issue.
In my case, the error happened because I had a static method and an extension method with the same name. For that, see Static method and extension method with same name.
[May be this should have been a comment. Sorry that I don't have enough reputation yet.]
I know this is an old thread, but I just spent 3 hours trying to figure out what my issue was. I ordinarily know what this error means, but you can run into this in a more subtle way as well. My issue was my client class (the one calling a static method from an instance class) had a property of a different type but named the same as the static method. The error reported by the compiler was the same as reported here, but the issue was basically name collision.
For anyone else getting this error and none of the above helps, try fully qualifying your instance class with the namespace name. ..() so the compiler can see the exact name you mean.
Check whether your code contains a namespace which the right most part matches your static class name.
Given the a static Bar class, defined on namespace Foo, implementing a method Jump or a property, chances are you are receiving compiler error because there is also another namespace ending on Bar. Yep, fishi stuff ;-)
If that's so, it means your using a Using Bar; and a Bar.Jump() call, therefore one of the following solutions should fit your needs:
Fully qualify static class name with according namepace, which result on Foo.Bar.Jump() declaration. You will also need to remove Using Bar; statement
Rename namespace Bar by a diffente name.
In my case, the foollowing compiler error occurred on a EF (Entity Framework) repository project on an Database.SetInitializer() call:
Member 'Database.SetInitializer<MyDatabaseContext>(IDatabaseInitializer<MyDatabaseContext>)' cannot be accessed with an instance reference; qualify it with a type name instead MyProject.ORM
This error arouse when I added a MyProject.ORM.Database namespace, which sufix (Database), as you might noticed, matches Database.SetInitializer class name.
In this, since I have no control on EF's Database static class and I would also like to preserve my custom namespace, I decided fully qualify EF's Database static class with its namepace System.Data.Entity, which resulted on using the following command, which compilation succeed:
System.Data.Entity.Database.SetInitializer<MyDatabaseContext>(MyMigrationStrategy)
Hope it helps
YourClassName.YourStaticFieldName
For your static field would look like:
public class StaticExample
{
public static double Pi = 3.14;
}
From another class, you can access the staic field as follows:
class Program
{
static void Main(string[] args)
{
double radius = 6;
double areaOfCircle = 0;
areaOfCircle = StaticExample.Pi * radius * radius;
Console.WriteLine("Area = "+areaOfCircle);
Console.ReadKey();
}
}