Is there a way in C# to create a field which is a reference to another field which is a value type?
class myClass
{
bool b1;
public void method1(ref bool b)
{
b1 = b;
}
}
I want b1 to reference the value of b, just as b references the value of the original argument, so that changes to b1 will affect the original argument.
EDIT:
What I’m trying to achieve is a myCheckBox class which automatically updates a field. See: How do I change a value argument from within an event handler?
Sure! Take a look at Eric's answer to this question:
Setting a ref to a member field in C#
As others have pointed out, you cannot store a reference to a variable
in a field in C#, or indeed, any CLR language.
Of course you can capture a reference to a class instance that
contains a variable easily enough
Well... there is a very contort way :) of course.
That is, using reflection!
You cannot get the address of a field, but we can use reflection.
Reflection is slower than accessing directly a field, i warn you.
And really, accessing private fields of other classes is a really bad practice!
Is however useful sometime for some dirty hacks when you don't have control of code written by other people.
Here the example, but i keep saying, it is not a good practice, is here only for curiosity and for educational purposes!
Fine another way to access your field, using properties or using a class that modify your properties.
// Our FieldReference class that internally uses reflection to get or set a field value.
public class FieldReference<T>
{
private object ownerObject;
private FieldInfo fieldInfo;
public FieldReference(object ownerObject, string fieldName)
{
this.ownerObject = ownerObject;
this.fieldInfo = ownerObject.GetType().GetField(fieldName, BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance);
}
public FieldReference(object ownerObject, FieldInfo fieldInfo)
{
this.ownerObject = ownerObject;
this.fieldInfo = fieldInfo;
}
public T Value
{
get { return (T)this.fieldInfo.GetValue(this.ownerObject); }
set { this.fieldInfo.SetValue(this.ownerObject, value); }
}
}
// Our dummy class
public class MyClass
{
// Our field we want to expose.
private int myField;
public MyClass(int value)
{
this.myField = value;
}
// Just a function we use to print the content of myField.
public override string ToString()
{
return this.myField.ToString();
}
}
class Program
{
public static void Main()
{
// We create our class.
MyClass mc = new MyClass(5);
// We print field value, should be 5 :)
Console.WriteLine(mc.ToString());
// We create our field reference
FieldReference<int> fieldref = new FieldReference<int>(mc, "myField");
// We set the value using field reference.
// Note, we accessed a private field :)
fieldref.Value = 100;
// Now we print the value, should be 100!
Console.WriteLine(mc.ToString());
Console.ReadLine();
}
}
Looks like something that is better solved using delegates/events.
Instead of trying to do the impossible (force value types to behave as reference types), use an event and fire it whenever this value is changed.
Subscribe to this event from the caller/s and you are good to go.
Not knowing what you would want this for you could use a delegate for this, it does sound like a code smell though:
class myClass
{
Action<bool> modify;
public void method1(Action<bool> modify)
{
this.modify = modify;
}
public void ModifyIt()
{
modify(false);
}
}
bool b1 = true; //b1 is true
var m = new myClass();
m.method1(val => { b1 = val; });
m.ModifyIt(); //b1 is false now
Related
I have a C# class that looks like:
public class MyClass {
private Member member;
public MyClass() {
member = new Member();
}
//.. other properties and methods here.
}
public class Member {
public String property1 { get; set; }
public bool isSet { get; set; }
// other things to do.
}
Now, normally I want Member to be exposed, like this:
public class MyClass {
public Member member;
public Member Property {
get { return this.member; }
set { this.member = value; }
}
}
to get it done and over with. However, in the actual implementation, exposing Member object would be a security risk and is something that is usable only internally by MyClass' processing. My team prefers that it is hidden out of use and not publicly consumable. Given that, I was looking through Reflection on how to do it. I was going to be using the SetValue() to make a MyClassExtension, to make a cleaner implementation, but I get InvalidOperationExceptions about accessing/modifying fields.
Can someone help?
You can use BindingFlags to access non-public members:
var v = new MyClass();
var memberField = v.GetType().GetField("member",
BindingFlags.NonPublic | BindingFlags.Instance);
var member = memberField.GetValue(v);
// no flags necessary for a public property
var property1Property = member.GetType().GetProperty("property1");
property1Property.SetValue(member,"test");
Although be aware that if exposing Member is a security risk, then accessing it through reflection is just as risky - in fact more so, because any errors won't show up until run-time.
It would be better to know exactly what you're trying to accomplish - there may be another way that doesn't involve reflection.
My problem, narrowed down to a simple explaination, is the following:
I have a class which needs to work with a number (without changing it) which is subject to change. This number doesn't necessarily come from another class, and it can be anything.
But I'd like to only "give" it to the class once, instead of constantly having to call update methods or having to create a wrapper (since again, as I said, this should work with any kind of number and having to wrap up everything is kind of unpratical).
Here's some code, hoping it helps:
public class SimpleExample
{
int value;
public SimpleExample(int variableOfWhichINeedAReference)
{
//Of course this won't work, but I'll keep it simple.
value = variableOfWhichINeedAReference;
}
public void DisplayValue()
{
print(value);
}
}
public class RandomClass
{
int myValue = 10;
SimpleExample s = new SimpleExample(myValue);
public void WorkWithValue()
{
myValue++;
}
public void Display()
{
print(foo);
print(bar);
s.DisplayValue();
}
}
Now, the problem seems pretty obvious: If I instantiate a SimpleExample and give it a variable as a parameter, it will get its value rather than a reference to it.
Is there a simple enough way that can avoid me the creation of a wrapper? Thanks.
Make a really simple class:
class Ref<T>
{
public T Value;
public Ref<T>()
{
}
public Ref<T>(T value)
{
this.Value = value;
}
}
Then use it like this:
class A
{
Ref<int> x;
public A(Ref<int> x)
{
this.x = x;
}
public void Increment()
{
x.Value++;
}
}
...
Ref<int> x = new Ref<int>(7);
A a = new A(x);
a.Increment();
Debug.Assert(x.Value == 8);
Note that the Ref<T> class here is a reference to a value - not a reference to a variable. If you want a reference to a variable, use Eric Lippert's solution (as pointed out by Filip).
So what you want is not an int, but rather a way of getting an int at some point in time. There are several ways of doing this, one of which is to have your object accept a Func<int>. Then the code can pass in a method that returns the current value of...whatever, rather than the value at the time SimpleExample is created. Using a lambda to close over a variable makes doing this much easier as well.
public class SimpleExample
{
Func<int> func;
public SimpleExample(Func<int> func)
{
this.func = func;
}
public void DisplayValue()
{
print(func());
}
}
public class RandomClass
{
int myValue = 10;
SimpleExample s;
public RandomClass()
{
s = new SimpleExample(() => myValue);
}
public void WorkWithValue()
{
myValue++;
}
public void Display()
{
print(foo);
print(bar);
s.DisplayValue();
}
}
There is no standard wrapper for the purpose you seek, though a single-element array could be used for that purpose. Alternatively, one could define a simple wrapper type:
public class ExposedValueHolder<T> { public T Value; } // Really simple class, eh?
and then use an ExposedValueHolder<YourStructType> to wrap your object. It's not possible in general to capture something passed as an arbitrary ref parameter, since objects may live indefinitely but byrefs (the things which are actually passed when using ref parameters) may die any time after function they're passed to goes out of scope.
Is it possible to have one variable point to another variable for shortcut purposes? For instance, let's say I have a variable in one class that is called SharedResources.proxTelescope, how do I get a variable in another class called prox to point to the first variable, in a sort of shortcut type of thing. I could just do var prox = SharedResources.proxTelescope;, but if proxTelescope changes, it won't reflect on prox, will it? How should I do it instead.
I would set things up as a property.
private <type> prox
{
get { return SharedResources.proxTelescope; }
set { SharedResources.proxTelescope = value }
}
You can create a property, something like:
public YourTypeHere prox
{
get { return SharedResources.proxTelescope; }
set { SharedResources.proxTelescope = value; }
}
If the variables are classes then they are reference type, so any change in one of them will be reflected in the other.
If the variables are structs, then they are value types and you have a problem, you can make some function(or property) to get the value.
If the value should be 'shared' among all instances, then it would probably be best to have the variable exist as a static member of the class it is on, and create an instance property that retreives the value of the static variable.
Another approach would be to use a Func<T>:
public class MyClass
{
public Func<DesiredType> ValueGetter {get;set;}
public DesiredType Value { get { return ValueGetter(); } }
}
This can then by used this way:
var myClass = new MyClass();
myClass.ValueGetter = () => SharedResources.proxTelescope;
var value = myClass.Value;
Consider the class:
public class foo
{
public object newObject
{
get
{
return new object();
}
}
}
According to MSDN:
Properties are members that provide a flexible mechanism to read,
write, or compute the values of private fields. Properties can be used
as though they are public data members, but they are actually special
methods called accessors. This enables data to be accessed easily
And:
Properties enable a class to expose a public way of getting and
setting values, while hiding implementation or verification code.
A get property accessor is used to return the property value, and a
set accessor is used to assign a new value. These accessors can have
different access levels. For more information, see Accessor
Accessibility.
The value keyword is used to define the value being assigned by the
set indexer.
Properties that do not implement a set method are read only.
while still providing the safety and flexibility of methods.
Does this therefore mean that at some point in time the value of the newObject property has a reference to the returned new object?
edit removed readonly from property
edit2 also would like to clarify that this is not the best use for a property but its done to try and illustrate the question more effectively.
You return a new object on each access to the property and that is not the expected behavior of properties. Instead you should return the same value each time (e.g. a value stored in a field). A property getter is simply glorified syntax for a method that returns a value. Your code compiles into something like this (the compiler creates a getter by prefixing the property name with get_ which is then emitted as IL):
public class foo
{
public object get_newObject()
{
return new object();
}
}
Each call to the getter will create a new object that foo doesn't know about or has access to.
Does this therefore mean that at some point in time the value of the newObject property has a reference to the returned new object?
No.
Property using a backing field:
class Foo {
readonly Object bar = new Object();
public Object Bar { get { return this.bar; } }
}
Using automatic properties:
class Foo {
public Foo() {
Bar = new Object();
}
public Object Bar { get; private set; }
}
A property is accessed using the same easy syntax as a public field. However, by using a property you can add code to the getter and the setter allowing you to do stuff like lazy loading in the getter or validation in the setter (and much more).
Under the hood, your property will simply be calling a function named get_newObject() that looks like this:
public object get_newObject()
{
return new object();
}
Since that is the case, it will always return a new object every time it is invoked.
If you want to retain a reference to the object, then I would recommend creating a private field to hold the data and having the property access that field, like so:
private object myObject;
public object newObject
{
if(myObject == null)
{
myObject = new object();
}
return myObject;
}
Since your property doesn't define set, and your field is private, newObject is basically eradonly outside of the containing class.
Properties in C# are "syntactic sugar". The code within the get block of a property is in fact put into a hidden get_PropertyName() method, and the set block into a hidden set_PropertyName() method. In the case of your code, the following method will be created:
public object get_newObject()
{
return new object();
}
You can see these hidden methods if you view the compiled assembly using Reflector, or ildasm.
When the property is used, the C# compiler converts any "get" accesses of your property into calls of the get_newObject() method. As an example:
If you were to write the following:
var foo = new foo();
var aNewObject = foo.newObject;
The compiler would convert that to:
var foo = new foo();
var aNewObject = foo.get_newObject();
So, in answer to your other question, the newly created object returned when someone "gets" the property won't be stored within your foo instance, the caller will simply get a new object every time.
Not exactly. Properties are just syntactic sugar so that you don't have to write accessor methods (like Java).
So this:
private int _myInteger;
public int MyInteger
{
get { return _myInteger; }
set { _myInteger = value; }
}
is equivilant to this:
private int _myInteger;
public int GetMyInteger()
{
return _myInteger;
}
public void SetMyInteger(int value)
{
_myInteger = value;
}
and it gets better with this, which is also equivilant:
public int MyInteger { get; set; }
like in java I have:
Class.getSuperClass().getDeclaredFields()
how I can know and set private field from a superclass?
I know this is strongly not recommended, but I am testing my application and I need simulate a wrong situation where the id is correct and the name not. But this Id is private.
Yes, it is possible to use reflection to set the value of a readonly field after the constructor has run
var fi = this.GetType()
.BaseType
.GetField("_someField", BindingFlags.Instance | BindingFlags.NonPublic);
fi.SetValue(this, 1);
EDIT
Updated to look in the direct parent type. This solution will likely have issues if the types are generic.
Yes, you can.
For fields, use the FieldInfo class. The BindingFlags.NonPublic parameter allows you to see private fields.
public class Base
{
private string _id = "hi";
public string Id { get { return _id; } }
}
public class Derived : Base
{
public void changeParentVariable()
{
FieldInfo fld = typeof(Base).GetField("_id", BindingFlags.Instance | BindingFlags.NonPublic);
fld.SetValue(this, "sup");
}
}
and a small test to prove it works:
public static void Run()
{
var derived = new Derived();
Console.WriteLine(derived.Id); // prints "hi"
derived.changeParentVariable();
Console.WriteLine(derived.Id); // prints "sup"
}
This class will let you do it:
http://csharptest.net/browse/src/Library/Reflection/PropertyType.cs
Usage:
new PropertyType(this.GetType(), "_myParentField").SetValue(this, newValue);
BTW, It will work on public/non-public fields or properties. For ease of use you can use the derived class PropertyValue like this:
new PropertyValue<int>(this, "_myParentField").Value = newValue;
Like JaredPar suggests, I did the follow:
//to discover the object type
Type groupType = _group.GetType();
//to discover the parent object type
Type bType = groupType.BaseType;
//now I get all field to make sure that I can retrieve the field.
FieldInfo[] idFromBaseType = bType.GetFields(BindingFlags.NonPublic | BindingFlags.Instance);
//And finally I set the values. (for me, the ID is the first element)
idFromBaseType[0].SetValue(_group, 1);
Thanks to all.