I'm trying to make a mod for a game in c# and I'm wondering if there's a way to change the value of a read only property using reflections.
In general, no.
Three examples:
public int Value { get { return _value + 3; } } // No
public int Value { get { return 3; } } // No
public int Value { get; private set; } // Yes
So, you can change the value of the property while this property has corresponding private, protected or internal field.
Try this:
typeof(foo).GetField("bar", BindingFlags.Instance|BindingFlags.NonPublic).SetValue(foo,yourValue)
You can in both those scenarios:
readonly int value = 4;
and
int value {get; private set}
using
typeof(Foo)
.GetField("value", BindingFlags.Instance)
.SetValue(foo, 1000); // (the_object_you_want_to_modify, the_value_you_want_to_assign_to_it)
You cannot modify
int value { get { return 4; } }
though.
If it returns a calculated value like
int value { get { return _private_val + 10; } }
you would have to modify _private_val accordingly.
Yes, this is absolutely possible. Whether it is good practice or helpful to your purpose, I do not know. Going off of #ske57's great advice, here is a sample program that demonstrates reflection. The initial field value of 5 and the reflected field value of 75 are written to the console.
using System;
using System.Reflection;
namespace JazzyNamespace
{
class Program
{
static void Main()
{
var reflectionExample = new ReflectionExample();
// access the compiled value of our field
var initialValue = reflectionExample.fieldToTest;
// use reflection to access the readonly field
var field = typeof(ReflectionExample).GetField("fieldToTest", BindingFlags.Public | BindingFlags.Instance);
// set the field to a new value during
field.SetValue(reflectionExample, 75);
var reflectedValue = reflectionExample.fieldToTest;
// demonstrate the change
Console.WriteLine("The complied value is {0}", initialValue);
Console.WriteLine("The value changed is {0}", reflectedValue);
Console.ReadLine();
}
}
class ReflectionExample
{
public readonly int fieldToTest;
public ReflectionExample()
{
fieldToTest = 5;
}
}
}
As Mark said there could be scenarios where you cannot as . Think that the property itself could be a function derived from other properties, members.
However you may want to try the mechanisms explained here:
Is it possible to set private property via reflection?
Related
As the title says, I would like to set the maximum value of the skill, stam and luck integers to the value of the related *Max integers. The *Max int values are set randomly during the start up of the program and the regular values are changed throughout the running of the program. There may be a few instances where the *Max value gets increased or decreased during play.
public static int skillMax = 0;
public static int stamMax = 0;
public static int luckMax = 0;
public static int skill = skillMax;
public static int stam = stamMax;
public static int luck = luckMax;
As my knowledge of C# is still in its infancy, I have not tried much. However I have searched far and wide on the internet however and not been able to find anything except for the MinValue and MaxValue fields and this piece of code with no explanation:
protected int m_cans;
public int Cans
{
get { return m_cans; }
set {
m_cans = Math.Min(value, 10);
}
}
Thanks in advance for any advice you throw my way!
Explanation for the code: Cans is a property. Properties provide controlled access to class or struct fields (variables). They consist of two methods called get to return a value and set to assign the value. A property can also have only a getter or only a setter.
The property Cans stores its value in a so called backing field. Here m_cans. The setter gets the new value through the keyword value.
Math.Min(value, 10) returns the minimum of the two parameters. I.e., for example, if value is 8, then 8 is assigned to m_cans. If value is 12, then 10 is assigned to m_cans.
You can use this property like this
var obj = new MyCalss(); // Replace by your real class or struct name.
obj.Cans = 20; // Calls the setter with `value` = 20.
int x = obj.Cans; // Calls the getter and returns 10;
Properties help to implement the principle of Information hiding.
You can easily adapt this example your variables. Often class level variables (fields) are prepended with _ to differentiate them from local variables, i.e. variables declared in methods. Properties are written in PascalCase.
private static int _skillMax; // Fields are automatically initialized to the default
// value of their type. For `int` this is `0`.
public static int SkillMax
{
get { return _skillMax; }
set {
_skillMax = value;
_skill = _skillMax; // Automatically initializes the initial value of Skill.
// At program start up you only need to set `SkillMax`.
}
}
private static int _skill;
public static int Skill
{
get { return _skill; }
set { _skill = Math.Min(value, _skillMax); }
}
Create methods to update values
private static void UpdateSkill(int newValue)
{
skill = newValue;
skillMax = newValue > skillMax ? newValue : skillMax;
}
Read only properties can be assigned in a constructor. But when I try to explicitly implement get method, compiler shows an error (Property cannot be assigned to -- it is read only.) Can I implement getter or it's supposed to be without implementation?
public class PersonalIncome
{
private decimal _anualRate;
public decimal AnualRate
{
get { return _anualRate != 0 ? _anualRate : 40_000; }
}
public PersonalIncome(decimal paymentRate)
{
switch (paymentRate)
{
case var rate when (rate > 300):
AnualRate = rate; // **Property can not be assigned to -- it is read only.**
break;
default:
break;
}
}
}
You can implement the getter, but then you can only assign values to the backing field directly:
_anualRate = rate;
Once you decide against using the convenience of the auto-property, you have to do everything by yourself.
Your class could be rewritten like this:
public class PersonalIncome
{
public decimal AnualRate { get; private set; }
public PersonalIncome(decimal paymentRate)
{
AnualRate = paymentRate > 300 ? paymentRate : 40_000;
}
}
You refer to a property with a readonly backing-field.
That´s exactly what the compiler also generates from C#6 upwards when using an auto-implemented property with a default-value:
int MyProperty { get; } = -1;
This will be translated to the following:
readonly int _myProperty = -1;
int MyProperty { get { return this._myProperty; } }
Now the compiler replaces every call to your property by the backing-field. However this only works for auto-properties that do not have a body defined. In your case you already have one, which is why the compiler can´t replace that one. As a property itself is nothing but a get- and a set-method, what you want to do is the following, which is obvious non-sense:
int get_MyProperty() { return this._MyProperty; }
...
this.get_MyProperty() = 1;
The reason this works for an auto-property is that the compiler knows how to replace the call to the property. However suppose your own getter was more complex:
get
{
DoSomething();
return this._myProperty + 5;
}
Now the compiler can´t replace the call to the property.
So the only way to have your own get-implementation tigether with a property which is get-only, is to use the backing-field:
this._myProperty = 1;
I am a bit confused with the get set property in C#.
I have the simple code below:
using System;
class Example
{
int _number;
public int Number
{
get
{
return this._number;
}
set
{
this._number = value;
}
}
}
class Program
{
static void Main()
{
Example example = new Example();
example.Number = 5; // set { }
Console.WriteLine(example.Number); // get { }
}
}
The code above using get set properties. However, if I delete the get set code like below code, the results stay the same.
using System;
class Example
{
int _number;
public int Number;
{
}
}
class Program
{
static void Main()
{
Example example = new Example();
example.Number = 5; // set { }
Console.WriteLine(example.Number); // get { }
}
}
My query is, what is the get set code used for? In the above program, the results are same. Can you give me some simple code which show the get set usage?
In your code, Number is simply a public field, as evidenced by the semicolon (;) at the end.
public int Number;
It is not a property, you just have an empty set of brackets right underneath which led to your confusion. If you were to remove the ; then you would actually have a property that is missing it's get, and would not compile at all.
All properties need to have a getter (setters are optional). If you want to avoid writing them, you can use auto properties, which take care of the backing field without you having to get involved:
public int Number { get; set; } // No field required
Note: A common usage pattern you'll see involving auto properties is the following:
public int Number { get; private set; }
This allows for properties that can be read from anywhere, but can only be modified from within the class they belong to.
EDIT: To answer your question, the main difference between fields and properties is in encapsulation. You can read more about the general differences between fields and properties here.
However, the example you have given has one additional difference, the private set. A normal field can be written from and to throughout the program. A property with a private setter however can only be modified from inside the class it belongs to.
Example:
public class Foo
{
public int Id { get; private set; }
public string Name;
public Foo()
{
this.Id = 1; // This works!
}
}
Here, Name is a field and Id is a property with a private setter. Notice that we modify Id in the constructor and that works, because it is within the class Id belongs to. Moving outside the class however:
var foo = new Foo();
// Field (no get and set):
foo.Name = "test" // Works
string bar = foo.Name; // Works
// Property (get and *private* set)
int i = foo.Id; // Works, because get is public
foo.Id = 2; // Doesn't work, because set is private
So after the availability of automatic implementation, and initialization of a property, do I even need a field for my property?
This seems much cleaner:
class A {
public int X {
get;set;
} = 1;
}
Than this:
class A {
int x = 1;
public int X {
get {
return x;
}
set {
x = value;
}
}
}
In the first case, the compiler is already providing a backing field - it's just implicit (and it's given a name that you can't refer to in code). Note that there has to be a backing field in the generated code, as a property itself is really just a pair of methods with some metadata linking them - the presence of a property does not add any state to the object. State is only stored in fields.
It's even cleaner when written on one line - I'd usually see this as:
class A
{
public int X { get; set; } = 1;
}
You dont need a backing field in this situation, but if you want to manipulate the Property with e.g a OnPropertyChanged() then you need to have a backing field
public int X {
get {
return x;
}
set {
x = value;
OnPropertyChanged()
}
}
I am writing following code,
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
namespace ReadOnlyObject
{
class Program
{
private readonly int a = 20;
private readonly int b;
public int propa{get;private set;}
public int propb { get; private set; }
public Program(int tmp)
{
b = tmp;
}
static void Main(string[] args)
{
Program obj1 = new Program(30);
Console.WriteLine(obj1.propa); // Console.WriteLine(obj1.a);
Console.WriteLine(obj1.propb); // Console.WriteLine(obj1.b);
Console.Read();
}
}
}
After executing the above i got o/p as follows,
0
0
And when I change the code by replacing two commented statements printing direct member variables I got output as,
20
30
Why is so?
As far I know about properties they are associated with their definition in order the member variables are declared.
You have confusion about auto property, so:
private readonly int a = 20;
private readonly int b;
public int propa{get {return a; }}
public int propb { get {return b;} private set {b = value;} }
now this will print 20, 30
There are two ways to define properties in C#.
The first, the traditional way;
int myProperty;
public int MyProperty
{
get { return myProperty; }
set { myProperty = value; }
}
the second, the auto-property;
public int MyProperty {get;set;}
The first contains a backing variable that you reference in the property accessor. The second implicitly creates a backing variable, because the developers of the language understood that there are a lot of cases where you just need a property!
You can put scope on the auto-property, because you might want to prevent people from setting the value, but internally to the object you should be able to update the value of it.
"As far I know about properties they are associated with their
defination in order the member variables are declated."
Just to clarify all of what you were asking, unless I am reading this statement incorrectly, you're thinking that if you declare variable a and b and the property a and property b that they'll be associated. This is an incorrect assumption.
propa and a are not associated in your example. The compiler is making them auto-implemented properties. http://msdn.microsoft.com/en-us/library/bb384054.aspx If you want propa associated with a then you'd do:
public int propa{get { return a;} }
You're not initializing either property. You also can't set the value of b from a setter if it's marked readonly. You can implement your own 'readonly' type by just not letting the value get set more than once. (Although it doesn't stay true to the constraint that it needs to be initialized in the constructor)
Try this:
private readonly int a = 20;
public int A { get { return a; } }
private int b;
private bool bInitialized = false;
public int B
{
get { return b; }
private set
{
if (bInitialized) return;
bInitialized = true;
b = value;
}
}
The way your code is written propb and propa CANNOT be set outside the scope of the class. so remove the keyword private from the set keyword
if you wrote this.propb = b in your constructor, then I think it should work more like you are expecting.