I am wondering, since a lot of things can be done using reflection, can I change a private readonly field after the constructor completed its execution?
(note: just curiosity)
public class Foo
{
private readonly int bar;
public Foo(int num)
{
bar = num;
}
public int GetBar()
{
return bar;
}
}
Foo foo = new Foo(123);
Console.WriteLine(foo.GetBar()); // display 123
// reflection code here...
Console.WriteLine(foo.GetBar()); // display 456
You can:
typeof(Foo)
.GetField("bar",BindingFlags.Instance|BindingFlags.NonPublic)
.SetValue(foo,567);
The obvious thing is to try it:
using System;
using System.Reflection;
public class Test
{
private readonly string foo = "Foo";
public static void Main()
{
Test test = new Test();
FieldInfo field = typeof(Test).GetField
("foo", BindingFlags.Instance | BindingFlags.NonPublic);
field.SetValue(test, "Hello");
Console.WriteLine(test.foo);
}
}
This works fine. (Java has different rules, interestingly - you have to explicitly set the Field to be accessible, and it will only work for instance fields anyway.)
I agree with the other answers in that it works generally and especially with the comment by E. Lippert that this is not documented behavior and therefore not future-proof code.
However, we also noticed another issue. If you're running your code in an environment with restricted permissions you might get an exception.
We've just had a case where our code worked fine on our machines, but we received a VerificationException when the code ran in a restricted environment. The culprit was a reflection call to the setter of a readonly field. It worked when we removed the readonly restriction of that field.
You asked why you would want to break the encapsulation like that.
I use an entity helper class to hydrate entities. This uses reflection to get all the properties of a new empty entity, and matches the property/field name to the column in the resultset, and set's it using propertyinfo.setvalue().
I don't want anyone else to be able to change the value, but I don't want to take all the effort to custom code hydration methods for every entity either.
My many of my stored procs return resultsets that don't correspond directly to tables or views, so the code gen ORM's do nothing for me.
Don't do this.
I just spent a day fixing a surreal bug where objects could be not of their own declared type.
Modifying the readonly field worked once. But if you tried to modify it again, you'd get situations like this:
SoundDef mySound = Reflection_Modified_Readonly_SoundDef_Field;
if( !(mySound is SoundDef) )
Log("Welcome to impossible-land!"); //This would run
So don't do it.
This was on the Mono runtime (Unity game engine).
Another simple way to do this using unsafe (or you could pass the field to a C method via DLLImport and set it there).
using System;
namespace TestReadOnly
{
class Program
{
private readonly int i;
public Program()
{
i = 66;
}
private unsafe void ForceSet()
{
fixed (int* ptr = &i) *ptr = 123;
}
static void Main(string[] args)
{
var program = new Program();
Console.WriteLine("Contructed Value: " + program.i);
program.ForceSet();
Console.WriteLine("Forced Value: " + program.i);
}
}
}
The answer is yes, but more importantly:
Why would you want to? Intentionally breaking encapsulation seems like a horrifically bad idea to me.
Using reflection to change a readonly or constant field is like combining the Law of Unintended Consequences with Murphy's Law.
I just want to add that if you need to do this stuff for unit testing, then you can use:
A) The PrivateObject class
B) You will still need a PrivateObject instance, but you can generate "Accessor" objects with Visual Studio. How to: Regenerate Private Accessors
If you are setting private fields of an object in your code outside of unit testing, that would be an instance of "code smell" I think that perhaps the only other reason you would want to do this is if you are dealing with a third party library and you can't change the target class code. Even then, you probably want to contact the 3rd party, explain your situation and see if they won't go ahead and change their code to accomodate your need.
Related
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();
}
}
I'm trying to do something which I feel should be straight forward and simple to do, yet for the life of me, I can't get it to work. Using PostSharp, I want to create an aspect that I apply to a class (or interface) definition and that aspect, at compile time, reflects the type that it has been applied to and then stores that reflection information in a variable ready for extraction at run time.
I know that the CompileTimeInitialize function should be used to generate the reflection information and save it to a variable. This all then gets serialised. However, I can't get the information stored out at run time. The RuntimeInitialize can see the variable and the data in it, but when I then get the attribute from the type elsewhere in my code using GetCustomAttributes, the attribute is empty.
Here is some of the code I have so far:
[Serializable]
[MulticastAttributeUsage(MulticastTargets.Class, AllowMultiple = false, PersistMetaData = true)]
[AttributeUsage(AttributeTargets.Class, AllowMultiple = false, Inherited = false)]
public class MetadataAttribute : TypeLevelAspect {
public MetadataAttribute() { }
private int test;
public override bool CompileTimeValidate(Type type) {
this.test = 11;
return true;
}
public override void CompileTimeInitialize(Type type, AspectInfo aspectInfo) {
this.test = 9;
}
public override void RuntimeInitialize(Type type) {
// When I break here, I can see during debugging, that test is 11 (I would have expected 9 to be honest, but I suspect that CompileTimeInitialize is not executed for a TypeLevelAspect - unless I am mistaken?)
}
}
Here is the console application that I am trying to use to extract the number:
class Program {
static void Main(string[] args) {
var ma = typeof(Test).GetCustomAttribute<MetadataAttribute>();
var test = new Test();
var ma2 = test.GetType().GetCustomAttribute<MetadataAttribute>();
// When I break here, both ma and ma2 have test set to 0.
}
}
[Metadata]
public class Test { }
Thanks in advance for any help, it is much appreciated and goes a long way to making my hair last longer on my head than in my hands :)
When you are using Type.GetCustomAttributes, you are requiring the CLR to build a new instance of custom attributes from their definition stored in metadata, i.e. by calling the constructor and setting fields and properties. Therefore, you are not getting the instance that PostSharp created and initialized, but a fully new instance.
If you want to access the PostSharp instance, you should use RuntimeInitialize to store this instance in some kind of shared repository of aspect instances, then access them from code. Note that PostSharp runs RuntimeInitialize lazily, so you won't be able to access your instance until it has been initialized.
Hi I have a possible design flaw and i need to solve it with an extension method.
Lets say I have a class and it has a property of StringCollection. Example code
public class MyProblematicClass
{
public IDbAccess Db{get;set;}
public StringCollection Errors{get;set;}
public MyProblematicClass(IDbAcces db){ Db=db;}
public int SetItem(Item i)
{
var id = Db.Save(i);
this.Errors = Db.Erros;
return id;
}
}
What I am doing is, in my unit test class I mock IDbAccess. This class validates object according to attributes. If any error occures it doesnt hit to db, it just fills its own Errors collection. For unit test I use another dbclass which just runs validation routines and here is problem i cannot get Error. Let me give you example for further understanding ( I know design is problematic, but for now I want to deal with it without changing anything)
public static class MyDbExtension
{
public static Save(Item i)
{
Validation v = new Validation();
var erros = v.ValidateObject(i);
//Here is problem i cannot pass it to MyProblematicClass
if ( errors.Count > 0 )
return -1;
else
return 1;
/* what I want to is :
var stackTrace = new StackTrace(); get stack trace
var object = stackTrace.GetFrame(1).GetMethod().GetObject() or sth like that. get object
object.GetProperties()[0].SetValue(object,errors,null); find property and set it.
*/
}
}
in my unit test :
public class UnitTest
{
Mock<IDbAccess> _db ;
MyProblematicClass _mpc;
pubic Setup()
{
_db.Setup(x=>x.Save(It.IsAny<Item>).Returns(u =>MyDbExtension.Save(u));
_mpc = new MyProblematicClass(_db.Object);
}
public void SetItem_EmptyObject_Contains3Erros()
{
Item i = new Item();
_mpc.SetItem(i);
//At this point i cannot set _mpc.Errors
}
What I want to achieve is in my DbExtension class can I access caller class and set its Errors property? I tried but it wasn unlikely yet. If anyone has any decent solution I will be appreciative and of course you can comment on design problems.
Edit
I appreciate Alex's answer he just said ignore Save method just mock Erros property and it will be ok. That make sense but what I wonder is in question, is it possible to access Stack Trace and manipulate caller methods object's property?
Thanks in advance.
You need to setup the return value of _db.Errors, something like this:
public class UnitTest
{
Mock<IDbAccess> _db ;
MyProblematicClass _mpc;
StringCollection errors;
pubic Setup()
{
_db.Setup(x=>x.Save(It.IsAny<Item>).Returns(u =>MyDbExtension.Save(u));
_db.Setup(x=>x.Errors).Returns(errors);
_mpc = new MyProblematicClass(_db.Object);
}
public void SetItem_EmptyObject_ContainsError()
{
errors.Add("Expected Error!");
Item i = new Item();
_mpc.SetItem(i);
Assert.AreEqual("Expected Error!", _mpc.Errors[0]);
}
}
I must admit I don't really follow your design, why are you using a static method for save? You could just as easily have the line:
_db.Setup(x=>x.Save(It.IsAny<Item>).Returns(-1);
Then test IDbAccess.Save() independently.
In your 'extension' class the save method has no return value, and MyProblematicClass does not inspect the return value before assigning errors.
Not sure to fully understand the question, but you cannot access the parameters on the stack from a normal program. Runtime metadata is only about static information (method, properties, constants, etc...).
I believe only a debugger (which is considered as a special beast of its own) can do this without changing the program/source, and this has serious performance cost. As a side note, here is a link that explain how to build your own managed debugger (.NET 4): CLR Managed Debugger (mdbg) Sample 4.0
Another solution is to instrument your code (automatically or using a tool) to add some tracing call that can capture the list of parameters on each traced methods. Tools like PostSharp can do this. Here is another link: Non-Invasive Tracing & Logging
You could use unmanaged debugging API to access the call stack and get the object previous function on the stack was called on.
The problem is, the stack may not contain the method you are expecting. In cases such as inlining and tail call optimization, the call stack doesn't contain the previous method called, which means you can't reliably do what you want.
For more information see this answer by Eric Lippert.
This doesn't use the call stack, but might get you some mileage:
class CalledClass
{
public static void PokeCaller()
{
Program._this.Error = "Error!!!";
}
}
class Program
{
public string Error = null;
[ThreadStatic] public static Program _this;
public void Run()
{
_this = this;
CalledClass.PokeCaller();
Console.WriteLine(Error);
Console.ReadKey();
}
static void Main(string[] args)
{
Program p = new Program();
p.Run();
}
}
Making Errors be [ThreadStatic] might be a more direct way to do it... or some other variation on that theme. You might also combine it with stack trace checking to see if you were actually called by something that has "Errors" attribute before setting it...
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();
}
}
I'm creating a class that will house election results. I have a results class that has a static method that will parse a file and return a results class with the results from the file.
I want to make sure that only the static method can modify the results, so i've been using the internal modifier (Precinct.InternalCandidates) (The prevents instances of my class outside of the dll from accessing the methods, right?).
Anyway, I need to expose the candidates as a read only list to the instantiated version of my class, but I'll obviously need to be able to add candidates during the population process. So, I've created another parameter in the Precinct Class called Precinct.Candidates that exposes a read only version of InternalCandidates
Here's how I'd envision it to work:
Results r = Results.ParseResultsFile("PathToFile.txt");
r.Candidates.Add(new Candidate) // Should error here
Console.WriteLine(r.Candidates[0].Name) // Should work
Here's what I have for my class stubs:
public class Results {
private List<Precinct> precincts = new List<Precinct>();
public ReadOnlyCollection<Precinct> Precincts {
get { return this.precincts.AsReadOnly(); }
}
public Results() {}
public static Results ParseResultsFile(string filePath) { ... }
}
public class Precinct {
internal List<Contest> InternalContests { get; set; }
public ReadOnlyCollection<Contest> Contests {
get { return this.InternalContests.AsReadOnly(); }
}
public Precinct {
this.InternalContests = new List<Contest>();
}
}
Is there a better way to accomplish this?
I'm afraid I have a little bit of bad news Rob... using Reflection, one can completely circumvent access modifiers. They help to protect a team from themselves, but are not suited to providing security.
You will need to ensure the physical security of the code and ensure that nobody can load your DLL into an app domain of their own creation.
UPDATE:
I stand corrected by myself. You can set an attribute that prevents reflection UNLESS THE CALLER HAS FULL TRUST (update from Leppie). See how.
You can prevent callers without full trust from accessing your private/internal methods and fields but a full trust caller cannot be prevented from using reflection.
Again. Cleaning up my old questions... I ended up just rolling my own Collection.
Worked out wonderfully..