Is the following not a good practice?
public interface IMyImmutableData
{
int Data { get;}
}
public interface IMyMutableData
{
int Data { set;get;}//implements both get and set
}
public class MyData : IMyImmutableData, IMyMutableData
{
public int Data{get;set;} //implements both IMyImmutableData, IMyMutableData
}
void Main()
{
MyData myData = new MyData{Data=10};
Console.WriteLine(myData.Data);
}
The reason I ask is that resharper gives me the following warning: "possible ambiguity while accessing by this interface"
The reason I want to do the above is that when I create methods that use the MyData class, I would like to send it either as IMyMutable or IMyImmutable objects, so that users of the method know that they can expect the method to update or not update the passed in object.
I think you can ignore resharper's warning, as the ambiguity is intentional.
However, usually a wrapper class is used to provide readonly access to something, that way it can't be cast to anything that does provide more functionality.
public class MyReadonlyData : IMyReadonlyData {
private MyData instance;
public int Data {
get {
return instance.Data;
}
}
public MyReadonlyData( MyData mydata ) {
instance = mydata;
}
}
// no access to original object or setters, period.
You need to make one or both of the implementations explicit:
public int IMyImmutableData.Data { get; }
public int IMyMutableData.Data { get; set; }
When you mark one as explicit, it can only be accessed when specifically cast as that type:
MyData obj = new MyData();
obj.Data; // Doesnt exist
(obj as IMyImmutableData).Data // Exists, specifically cast as this interface
If you choose to not mark one as explicit, it will be the property chosen when cast as other appropriate types.
I think in this case your structure is fine. You don't want to explicitly implement the interfaces via separate properties, because then the Data you access via the immutable interface will actually be different than that for the mutable interface.
Also, your actual code is likely more complex, because in this case there is no ambiguity: you are accessing Data via the object itself, so interfaces need not be considered.
One solution with explicit interface implementation would be to use a common backing field, rather than auto-properties:
private int _data;
public int IMyImmutableData.Data
{
get
{
return this._data;
}
}
public int IMyMutableData.Data
{
get
{
return this._data;
}
set
{
this._data = value;
}
}
You could cast the variable and tell the compiler what exactly you mean: (resolve ambiguity)
MyData myData = new MyData{Data=10};
Console.WriteLine( ((IMyMutableData)(myData)).Data );
You need a combined interface with a "new" qualifier on the read-write interface to avoid the squawk. Further, your interfaces are poorly named. Better names would be something like "IReadableData" and "IWritableData", and "IReadWriteData". Note that while "IReadableData" does not provide any means of mutating the data, that by no stretch of the imagination implies that the data is immutable. If something is immutable it won't every be changed by anyone; that would clearly not be the case with an object of type MyData.
Related
What I am trying to do is find the most elegant way to create a "pointer-like" class for a specific object/class type that I have in a project.
What I mean is a little confusing without an example. Take this really simple class:
public class MyClass
{
private string _name;
public string GetName() { return _name; }
public void SetName(string name) { _name = name; }
}
I want to create a second class which is like a pointer to it like this:
public class MyClassPtr
{
private MyClass _obj;
public bool IsValid = false;
public MyClassPtr(MyClass obj) { _obj = obj; IsValid = true; }
public void InvalidatePtr()
{
IsValid = false;
obj = null;
}
// SOME MAGIC HERE?
}
The challenge: The key is that I want to elegantly have MyClassPtr provide an interface to all of the public methods/members in MyClass without writing wrappers and/or accessors around each method/member.
I know that I could do this:
public class MyClassPtr
{
public string GetName() { return _obj.GetName(); }
...
}
But that's what I want to avoid. Is there some fundamental abstraction that I don't know of that I can apply to MyClassPtr to allow it to easily re-expose the methods/members in MyClass directed through _obj? I do NOT want MyClassPtr to inherit MyClass. Should MyClassPtr be a type instead, and some trick with accessors to expose the methods/members of MyClass?
Edit: More context on why I am looking for such a design through an example. Here is the overall goal. Imagine a platform that parses through data about people and when it finds information about a person, it creates an instance of Person with that information. You could get a handle to that person like:
Person person1 = platform.GetPerson(based_on_data);
Now, imagine the platform had two instances of Person that it thought were different people, but all of a sudden information came in that strongly suggested those two instances actually refer to the same person. So, the platform wants to merge the instances together in to a new object, let's call it personX.
Now, floating around in the platform someone had a copy of one of those two instances that got merged, which was person1. What I want to do is on-the-fly replace person1 with personX. Literally, I want person1==personX to be true, NOT just that they are two different objects with the same data. This is important since the platform could make a change to personX and unless the two objects are literally equal, a change to personX would not be automatically reflected in person1.
Since I can't on-the-fly replace person1 with personX I had that idea that I wouldn't give direct access to Person, instead I would give access to PersonPtr which the platform (on-the-fly) can change what Person it is pointing to. This would insurance that once person1ptr gets updated to point to personX, if a change is made in personX it will be seen in person1ptr
You could of course use something like
public class MyClassWrapper
{
MyClass _obj;
public MyClassWrapper(MyClass obj)
{
_obj = obj;
}
public void Invoke(Action<MyClass> action)
{
action(_obj);
}
public U Invoke<U>(Func<MyClass, U> func)
{
return func(_obj);
}
public void ChangeTo(MyClass obj)
{
_obj = obj;
}
}
Given your class looks like
public class MyClass
{
public string Name { get; set; }
}
Example:
var person1 = new MyClass { Name = "Instance1" };
var person2 = new MyClass { Name = "Instance2" };
var wrapper = new MyClassWrapper(person1);
wrapper.Invoke(x => x.Name += "original");
var x = wrapper.Invoke(x => x.Name); // Instance1original
wrapper.ChangeTo(person2);
var y = wrapper.Invoke(x => x.Name); // Instance2
but it has a major drawback: you can't access members directly, so you can't bind the data (to a DataTable or a Control).
It would be better to implement all members of your class also in your wrapper class. If you're afraid changes in your class will be forgotten to be implemented in your wrapper, just use an interface:
public interface IMyClass
{
string Name { get; set; }
}
public class MyClass : IMyClass
{
public string Name { get; set; }
}
public class MyClassWrapper: IMyClass
{
MyClass _obj;
public MyClassWrapper(MyClass obj)
{
_obj = obj;
}
public string Name
{
get { return _obj.Name; }
set { _obj.Name = value; }
}
}
Note that regardless which approach you use, you'll have to always keep a reference to the wrapper instance to actually change the underlying instance (using something like static aside).
Also, changing the underlying instance of such a wrapper without telling the component using it that it changed don't seem to be a good idea. Maybe your system is simple enough to get away with a wrapper; that's something you have to decide for yourself.
Maybe your wrapper should simply have an Invalid flag (and/or use an event to signal a change of the underlying object.). Once the underlying object is merged, it is set to true and each member access should throw an exception. This would force the component using the wrapper to deliberately react to changes and to reload the data from your service.
All in all, I think using such a wrapper will just clutter up your code and be error prone (just imagine adding multithreading to the mix). Think twice if you really need this wrapper.
Why not just simply ask your service for a new instance of your class everytime you use it (the service can simply use a cache)? Sure, you can't prevent that someone somewhere keeps a reference; but at least you'll keep your sanity.
Pretty simple question really, should I use my properties to initialize fields in the constructor or reference them directly?
Example:
public class Foo()
{
private string example;
public String Example
{
get/set etc..
}
public Foo(string exampleIn)
{
Example = exampleIn;
}
}
Or is it better practice to do this:
public class Foo()
{
private string example;
public String Example
{
get/set etc..
}
public Foo(string exampleIn)
{
example = exampleIn;
}
}
Either way, I don't think either would violate encapsulation so I am wondering if there is a preferred way to go?
There is really no right or wrong answer here (and because of that I am almost tempted to vote to close). But, I tend to agree with Jacob on this. I prefer the property getter and setter route especially now that we have automatic properties. Do keep in mind that you can have different access modifies on the getters and setters in case that influences your decision for any reason. I mean, if you are going to use the property in the constructor then try to be consistent and use it exclusively everywhere else in the class as well. That may mean that you do not want to expose the setter to the outside.
public class Foo()
{
private string example;
public String Example
{
get { return example; }
private set { example = value; }
}
public Foo(string exampleIn)
{
Example = exampleIn;
}
}
Before automatic properties, which were introduced in C# 3.0, your second example is more "proper" in my opinion. Now with automatic properties I think this is best:
public class Foo()
{
private string example;
public String Example
{
{ get; set; }
}
public Foo(string exampleIn)
{
Example = exampleIn;
}
}
It depends on whether the data value will further be processed inside the Setter. IF the value needs processing then it's better to use what #Jacob has said but if the value will not be further processed (which is the case in most scenarios), it's better to use the private member to avoid an extra method call to setter method. When CLR compiles the code, it create two methods for Get and Set property and using the Property to access/modify the value which defines the property will result in extra method call unnecessarily (if the value is not processed further).
I am trying to find a better way to handle some growing if constructs to handle classes of different types. These classes are, ultimately, wrappers around disparate value types (int, DateTime, etc) with some additional state information. So the primary difference between these classes is the type of data they contain. While they implement generic interfaces, they also need to be kept in homogeneous collections, so they also implement a non-generic interface. The class instances are handled according to the type of data they represent and their propogation continues or doesn't continue based on that.
While this is not necessarily a .NET or C# issue, my code is in C#.
Example classes:
interface ITimedValue {
TimeSpan TimeStamp { get; }
}
interface ITimedValue<T> : ITimedValue {
T Value { get; }
}
class NumericValue : ITimedValue<float> {
public TimeSpan TimeStamp { get; private set; }
public float Value { get; private set; }
}
class DateTimeValue : ITimedValue<DateTime> {
public TimeSpan TimeStamp { get; private set; }
public DateTime Value { get; private set; }
}
class NumericEvaluator {
public void Evaluate(IEnumerable<ITimedValue> values) ...
}
I have come up with two options:
Double Dispatch
I recently learned of the Visitor pattern and its use of double dispatch to handle just such a case. This appeals because it would allow undesired data to not propogate (if we only want to handle an int, we can handle that differently than a DateTime). Also, the behaviors of how the different types are handled would be confined to the single class that is handling the dispatch. But there is a fair bit of maintenance if/when a new value type has to be supported.
Union Class
A class that contains a property for each value type supported could be what each of these classes store. Any operation on a value would affect the appropriate component. This is less complex and less maintenance than the double-dispatch strategy, but it would mean that every piece of data would propogate all the way through unnecessarily as you can no longer discriminate along the lines of "I don't operate upon that data type". However, if/when new types need to be supported, they only need to go into this class (plus whatever additional classes that need to be created to support the new data type).
class UnionData {
public int NumericValue;
public DateTime DateTimeValue;
}
Are there better options? Is there something in either of these two options that I did not consider that I should?
method 1, using dynamic for double dispatch (credit goes to http://blogs.msdn.com/b/curth/archive/2008/11/15/c-dynamic-and-multiple-dispatch.aspx).
Basically you can have your Visitor pattern simplified like this:
class Evaluator {
public void Evaluate(IEnumerable<ITimedValue> values) {
foreach(var v in values)
{
Eval((dynamic)(v));
}
}
private void Eval(DateTimeValue d) {
Console.WriteLine(d.Value.ToString() + " is a datetime");
}
private void Eval(NumericValue f) {
Console.WriteLine(f.Value.ToString() + " is a float");
}
}
sample of usage:
var l = new List<ITimedValue>(){
new NumericValue(){Value= 5.1F},
new DateTimeValue() {Value= DateTime.Now}};
new Evaluator()
.Evaluate(l);
// output:
// 5,1 is a float
// 29/02/2012 19:15:16 is a datetime
method 2 would use Union types in c# as proposed by #Juliet here (alternative implementation here)
I tell you have I've solved a similar situation - is by storing the Ticks of a DateTime or TimeSpan as double in the collection and by using IComparable as a where constraint on the type parameter. The conversion to double / from double is performed by a helper class.
Please see this previous question.
Funnily enough this leads to other problems, such as boxing and unboxing. The application I am working on requires extremely high performance so I need to avoid boxing. If you can think of a great way to generically handle different datatypes (including DateTime) then I'm all ears!
Good question. The first thing that came to my mind was a reflective Strategy algorithm. The runtime can tell you, either statically or dynamically, the most derived type of the reference, regardless of the type of the variable you are using to hold the reference. However, unfortunately, it will not automatically choose an overload based on the derived type, only the variable type. So, we need to ask at runtime what the true type is, and based on that, manually select a particular overload. Using reflection, we can dynamically build a collection of methods identified as handling a particular sub-type, then interrogate the reference for its generic type and look up the implementation in the dictionary based on that.
public interface ITimedValueEvaluator
{
void Evaluate(ITimedValue value);
}
public interface ITimedValueEvaluator<T>:ITimedValueEvaluator
{
void Evaluate(ITimedValue<T> value);
}
//each implementation is responsible for implementing both interfaces' methods,
//much like implementing IEnumerable<> requires implementing IEnumerable
class NumericEvaluator: ITimedValueEvaluator<int> ...
class DateTimeEvaluator: ITimedValueEvaluator<DateTime> ...
public class Evaluator
{
private Dictionary<Type, ITimedValueEvaluator> Implementations;
public Evaluator()
{
//find all implementations of ITimedValueEvaluator, instantiate one of each
//and store in a Dictionary
Implementations = (from t in Assembly.GetCurrentAssembly().GetTypes()
where t.IsAssignableFrom(typeof(ITimedValueEvaluator<>)
and !t.IsInterface
select new KeyValuePair<Type, ITimedValueEvaluator>(t.GetGenericArguments()[0], (ITimedValueEvaluator)Activator.CreateInstance(t)))
.ToDictionary(kvp=>kvp.Key, kvp=>kvp.Value);
}
public void Evaluate(ITimedValue value)
{
//find the ITimedValue's true type's GTA, and look up the implementation
var genType = value.GetType().GetGenericArguments()[0];
//Since we're passing a reference to the base ITimedValue interface,
//we will call the Evaluate overload from the base ITimedValueEvaluator interface,
//and each implementation should cast value to the correct generic type.
Implementations[genType].Evaluate(value);
}
public void Evaluate(IEnumerable<ITimedValue> values)
{
foreach(var value in values) Evaluate(value);
}
}
Notice that the main Evaluator is the only one that can handle an IEnumerable; each ITimedValueEvaluator implementation should handle values one at a time. If this isn't feasible (say you need to consider all values of a particular type), then this gets really easy; just loop through every implementation in the Dictionary, passing it the full IEnumerable, and have those implementations filter the list to only objects of the particular closed generic type using the OfType() Linq method. This will require you to run all ITimedValueEvaluator implementations you find on the list, which is wasted effort if there are no items of a particular type in a list.
The beauty of this is its extensibility; to support a new generic closure of ITimedValue, just add a new implementation of ITimedValueEvaluator of the same type. The Evaluator class will find it, instantiate a copy, and use it. Like most reflective algorithms, it's slow, but the actual reflective part is a one-time deal.
Why not just implement the interface that you actually want, and allow the implementing type to define what the value is? For example:
class NumericValue : ITimedValue<float> {
public TimeSpan TimeStamp { get; private set; }
public float Value { get; private set; }
}
class DateTimeValue : ITimedValue<DateTime>, ITimedValue<float> {
public TimeSpan TimeStamp { get; private set; }
public DateTime Value { get; private set; }
public Float ITimedValue<Float>.Value { get { return 0; } }
}
class NumericEvaluator {
public void Evaluate(IEnumerable<ITimedValue<float>> values) ...
}
If you want the behavior of the DateTime implementation to vary based on the particular usage (say, alternate implementations of Evaluate functions), then they by definition need to be aware of ITimedValue<DateTime>. You can get to a good statically-typed solution by providing one or more Converter delegates, for example.
Finally, if you really only want to handle the NumericValue instances, just filter out anything that isn't a NumericValue instance:
class NumericEvaluator {
public void Evaluate(IEnumerable<ITimedValue> values) {
foreach (NumericValue value in values.OfType<NumericValue>()) {
....
}
}
}
I have an object that has properties of another object and one called DataValue, but the type that I want DataValue to return depends on information contained in the object in the other property. I'm not convinced my way is the best way to do this.
I have this business object called an AssetStructure.
An AssetStructure object contains a generic list of IAssetStructureField objects, which are a series of objects that basically hold information about the data that can be held in that field, a default value of a certain datatype and some displaying information properties. Each of the objects implementing the IAssetStructureField interface will hold different datatype. For example, one's DefaultValue's type maybe string and the other maybe a List<ofCustomType>.
I have my Asset object containing a generic list of objects called AssetDataField. The AssetDataField has properties of one containing the AssetStructureField and one called DataValue, the Asset's data for that StructureField.
My problem is datatype of AssetDataField DataValue property, it will need to be different depending on the details of the AssetStructureField object. This StructureField may hold data representing all the user groups with access to the Asset (datatype List<UserGroups>), and another might just be a description field (datatype string), so I need the DataValue coming out of the AssetDataField to be of the same type.
What I'm thinking of doing now, and that I feel can probably be done much better, is having the AssetDataField.DataValue return an object, and then cast it to the typeof the AssetDataField.StructureField.DefaultValue.
object fieldValue;
object fieldDefaultValue;
Asset certainAsset = new Asset(32423);
foreach (AssetDataField dataField in certainAsset.DataFields)
{
fieldDefaultValue = datafield.StructureField.DefaultValue;
fieldValue = datafield.DataValue as typeof(fieldDefaultValue);
// then do stuff depending on what typeof(fieldValue) is. This is where I
// see things getting particularly ugly. Not only just because that
// this class here will need to know the possible types that may be
// returned, so it can deal.
if (typeof(fieldValue) == whatever)
{
// deal;
}
else if (typeof(fieldValue) == whatever2)
{
// deal differently;
}
}
Does anyone have any suggestions? I am not a opposed, at all, to a complete redo. I'm really sorry this is so long-winded, I just wanted to try and explain the situation well. I tried to put together a UML diagram to help out, but my ArgoUML was acting up. Thanks for any insights at all that you can provide.
It seems like you should make AssetDataField a possibly abstract base class, and derive other classes from it to perform the work. For example:
class Program
{
static void Main(string[] args)
{
Asset certainAsset = new Asset(32423);
foreach (AssetDataField dataField in certainAsset.DataFields)
{
dataField.Deal();
}
}
}
class Asset
{
public List<AssetDataField> DataFields = new List<AssetDataField>();
public Asset(int id)
{
// Load asset #id
if (id == 32423)
{
DataFields.Add(new AssetDataFieldString());
DataFields.Add(new AssetDataFieldFloat());
}
}
}
abstract class AssetDataField
{
public AssetDataField()
{
FieldValue = DefaultValue;
}
public abstract object DefaultValue { get; }
public abstract object FieldValue { get; set; }
public abstract void Deal();
}
abstract class AssetDataFieldType<T> : AssetDataField
{
protected T internalValue;
public override object FieldValue
{
get
{
return TypedValue;
}
set
{
TypedValue = (T)System.Convert.ChangeType(value, typeof(T));
}
}
public virtual T TypedValue
{
get
{
return internalValue;
}
set
{
internalValue = value;
}
}
}
class AssetDataFieldString : AssetDataFieldType<string>
{
public override object DefaultValue
{
get { return "Default String"; }
}
// Optionally override TypedValue
public override void Deal()
{
Console.WriteLine(TypedValue.PadLeft(20));
}
}
class AssetDataFieldFloat : AssetDataFieldType<float>
{
public override object DefaultValue
{
get { return 0; }
}
// Optionally override TypedValue
public override void Deal()
{
Console.WriteLine(TypedValue.ToString("#0.000"));
}
}
Note: this smells like the result of querying an EAV based system. In the same way that meta data is the backbone of this sort of system the code referencing it should strive to know what it is accessing (and thus the types) at compile time. That said if you want to simply display the data this sort of thing is required no matter what.
C# is statically typed so you cannot put 'different things' into the same 'slot' (variable, array location) unless the slot is the right 'shape' to take all of them(1). The only slot currently available in c# for this is object. This will work but will box any value types(2).
In c# 4.0 you can use dynamic, which under the hood will be an object but at least will let you invoke any methods on it you want even if the compiler doesn't think it's legal via object.
If all the types in question share a common interface then you can avoid object and get some useful semantics (say if double Sum(double d) was a meaningful operation for any instance you were dealing with then this could yield useful results. However it sounds like you do not control the types present (and thus stand no chance of getting them to conform to useful interfaces).
If the set of possible types is tractable the technique described below can work but it is still cumbersome.
// boxes if needed
public interface IGeneralValue
{
object Value { get; }
Type GetValueType();
}
public class Value<T> : IGeneralValue
{
public T Value { get; set;}
object IGeneralValue.Value
{
get { return (object)this.Value; }
}
public Type GetValueType()
{
return typeof(T);
}
}
Then you can stay statically typed where possible but if not something similar to your previous code will work.
Asset certainAsset = new Asset(32423);
foreach (IGeneralValue dataField in certainAsset.DataFields)
{
object fieldValue = datafield.Value;
Type fieldType = dataField.GetValueType();
if (typeof(double).Equals(fieldType))
{
double d = ((double)fieldValue);
}
else if (typeof(string).Equals(fieldType))
{
string d = ((string)fieldValue);
}
else if (typeof(whatever).Equals(fieldType))
{
// deal with whatever
}
else
{
// the safe option
throw new NotSupportedException(fieldType +" is not supported!");
}
}
Without unsafe code or unions (only structs) at least.
This has implications not just on performance, you cannot unbox an int as a double for example, despite that conversion working on unboxed instances.
I've been looking around, and so far haven't managed to find a good way to do this. It's a common problem, I'm sure.
Suppose I have the following:
class SomeClass : IComparable
{
private int myVal;
public int MyVal
{
get { return myVal; }
set { myVal = value; }
}
public int CompareTo(object other) { /* implementation here */ }
}
class SortedCollection<T>
{
private T[] data;
public T Top { get { return data[0]; } }
/* rest of implementation here */
}
The idea being, I'm going to implement a binary heap, and rather than only support Insert() and DeleteMin() operations, I want to support "peeking" at the highest (or lowest, as the case may be) priority value on the stack. Never did like Heisenberg, and that whole "you can't look at things without changing them" Uncertainty Principle. Rubbish!
The problem, clearly, is that the above provides no means to prevent calling code from modifying MyVal (assuming SortedCollection) via the Top property, which operation has the distinct possibility of putting my heap in the wrong order. Is there any way to prevent modifications from being applied to the internal elements of the heap via the Top property? Or do I just use the code with a warning: "Only stable if you don't modify any instances between the time they're inserted and dequeue'd. YMMV."
To answer your question: No, there's no way to implement the kind of behavior you want - as long as T is of reference type (and possibly even with some value-types)
You can't really do much about it. As long as you provide a getter, calling code can modify the internal contents of your data depending on the accessibility of said data (i.e. on properties, fields, and methods).
class SomeClass : IComparable
{
private int myVal;
public int MyVal
{
get { return myVal; }
set { myVal = value; }
}
public int CompareTo(object other) { /* implementation here */ }
}
class SortedCollection<T>
{
private T[] data;
public T Top { get { return data[0]; } }
/* rest of implementation here */
}
//..
// calling code
SortedCollection<SomeClass> col;
col.Top.MyVal = 500; // you can't really prevent this
NOTE What I mean is you can't really prevent it in the case of classes that you don't control. In the example, like others have stated you can make MyVal's set private or omit it; but since SortedColleciton is a generic class, you can't do anything about other people's structures..
You can have a readonly property (that is, a property with only a getter):
private int myVal;
public int MyVal { get { return myVal; } }
But be careful: this may not always work how you expect. Consider:
private List<int> myVals;
public List<int> MyVals { get { return myVals; } }
In this case, you can't change which List the class uses, but you can still call that List's .Add(), .Remove(), etc methods.
Your properties don't have to have the same accessibility for get/set. This covers you for anything that returns a value type (typically structs that only contain value types) or immutable reference types.
public int MyVal
{
get { return myVal; }
private set { myVal = value; }
}
For mutable reference types, you have other options, such as returning Clone()s or using ReadOnlyCollection<T> to keep the caller from changing them:
private List<int> data;
public IList<int> Data
{
get { return new ReadOnlyCollection<int>(this.data); }
}
Only implement getters for your properties and modify the collection by having add/remove methods
I understand your problem now. I think this should work:
class SortedCollection<T> where T: ICloneable
{
private T[] data;
public T Top
{
get
{
T ret = (T)data[0].Clone();
return ret;
}
}
/* rest of implementation here */
}
The ICloneable constraint ensures that the type parameter implements the ICloneable interface. (if this is acceptable)