Suppose I have an Interface with some properties:
public interface IDummy
{
string First {get;set;}
string Second {get;set;}
string Third {get;set;}
string Fourth {get;set;}
}
Now, I have a class which implements that interface:
public class DummyClass: IDummy
{
// ...
}
Is it possible, not to implement the interface properties explicitly and instead use DynamicObject? For example:
public class DummyClass: DynamicObject, IDummy
{
public override bool TryGetMember(GetMemberBinder binder, out object result)
{
// Get the value from a Config file or SQLite db or something
}
public override bool TrySetMember(SetMemberBinder binder, object value)
{
// Get the value to a Config file or SQLite db or something
}
}
I am just curious if this is possible or not?
Thanks.
No, this is not possible.
If you are implementing an interface, you need to implement all of its members. C# is still a statically typed language, after all.
When you say a type implements an interface, you are saying it conforms to its contract. Not implementing all of the members means that you are not complying with the contract.
The compiler would see your code and will not assume that you have implemented the contract correctly (in a dynamic fashion) - it will fail to compile.
No, basically. An interface is for static typing; to satisfy an interface your type must actually provide a regular (non-dynamic) implementation. You could not claim to implement it (IDummy), and detect the names, but that could relate to any interface that uses those same names, not just IDummy.
You could make a wrapper.
class DummyWrapper : IDummy
{
private readonly DynamicObject _wrapped;
public DummyWrapper(DynamicObject wrapped)
{
_wrapped = wrapped;
}
string First
{
get { return _wrapped.First; }
set { _wrapped.First = value; }
}
string Second
{
get { return _wrapped.Second; }
set { _wrapped.Second = value; }
}
string Third
{
get { return _wrapped.Third; }
set { _wrapped.Third = value; }
}
string Fourth
{
get { return _wrapped.Fourth; }
set { _wrapped.Fourth = value; }
}
}
You might also be interested in these questions:
Automatically creating a wrapper to implement an interface
Dynamically implementing an interface in .NET 4.0 (C#)
Related
i created an interface which will serve as a datatype for some set of objects, when defining those object the datatype of one of their properties are unknown, thus why i need an interface. but .net wouldnt let me do this;
public interface IClipbaseData
{
T ClipObjectData<T> { get; set;}
}
class ClipTextData : IClipbaseData
{
string _clipContent;
public string ClipObjectData
{
get
{
return _clipContent;
}
set
{
// validate the input
if (string.IsNullOrEmpty(value))
{
throw new ArgumentNullException("Error");
}
_clipContent = value;
}
}
}
i am getting an error on the interface, unexpected use of generic name. i know i could declare the generic on the interface but i dont want to because i am not aware of the generic datatype when using the interface as an object type.
but i am aware of the property datatype when creating classes to implement this interface
i can as well declare the property as object in the interface and dont use generics, but why am i unable to use generics
a typical scenario why i dont want to add the generics on the interface:
an object is defined of IClipbaseData, this object can be of any concrete class that implements IClipbaseData. the concrete class which it belongs to can only be determined at runtime, thus we dont know the datatype of the property ClipObjectData before runtime
Properties can't accept type parameters like that, you can define the type parameter on the interface itself, which will make your type work. This is one case where if you want to use a generic type parameter for a property, you must define it on the interface itself.
public interface IClipbaseData<T>
{
T ClipObjectData { get; set; }
}
class ClipTextData : IClipbaseData<string>
{
string _clipContent;
public string ClipObjectData
{
get
{
return _clipContent;
}
set
{
// validate the input
if (string.IsNullOrEmpty(value))
{
throw new ArgumentNullException("Error");
}
_clipContent = value;
}
}
}
Otherwise you would have to call properties like var someValue = clipTextDataObj.ClipObjectData<string>;, which really doesn't make a lot of sense to do it that way since the concrete type already defines what it returns.
If you do not want to define it on your interface, you'll have to create separate getter/setter methods:
public interface IClipbaseData
{
T GetClipObjectData<T>();
void SetClipObjectData<T>(T value);
}
But this would require you to implement it in a generic way:
class ClipTextData : IClipbaseData
{
public T GetClipObjectData<T>()
{
throw new NotImplementedException();
}
public void SetClipObjectData<T>(T value)
{
throw new NotImplementedException();
}
}
But you could add yet another abstraction here:
class StringClipTextData : ClipTextData
{
public string ClipObjectData
{
get { return base.GetClipObjectData<string>(); }
set { base.SetClipObjectData(value); }
}
}
I'm following the tutorial on this site which talks about using the Castle DictionaryAdapterFactory and an interface to access an applications app.setting keys without using strings throughout your code.
The way it works is you define an Interface that has the key names for your app.settings
public interface ISettings
{
string dog { get; }
string cat { get; }
}
Then use the DictionaryAdapterFactory to do the coding between the interface and your app.settings dictionary.
var factory = new DictionaryAdapterFactory();
var settings = factory.GetAdapter<ISettings>(ConfigurationManager.AppSettings);
Now you can access the values like this:
settings.dog
settings.cat
My question is, is it possible to have something more than a complicated than a simple getter. For example, can I tell DictionaryAdapterFactory to use a decryption method on the value of one of the keys and then return that instead of the key value?
I'm assuming that this is not possible since you can't define methods in an interface, but wanted to see if there was another way that I was missing.
You can use a wrapper class to wrap your interface with a class that implements custom methods.
You add [AppSettingWrapper] over your interface:
[AppSettingWrapper]
public interface ISettings
{
string dog { get; }
string cat { get; }
}
The AppSettingWrapper class is defined in the class below and lets you do what you want in the getter and setting.
public class AppSettingWrapperAttribute : DictionaryBehaviorAttribute, IDictionaryKeyBuilder, IPropertyDescriptorInitializer, IDictionaryPropertyGetter
{
public string GetKey(IDictionaryAdapter dictionaryAdapter, string key, PropertyDescriptor property)
{
return key;
}
public object GetPropertyValue(IDictionaryAdapter dictionaryAdapter, string key, object storedValue, PropertyDescriptor property, bool ifExists)
{
return storedValue;
}
public void Initialize(PropertyDescriptor propertyDescriptor, object[] behaviors)
{
propertyDescriptor.Fetch = true;
}
}
Most of this solution comes from https://gist.github.com/kkozmic/7858f4e666df223e7fc4.
following the example:
interface ISomeInterface
{
string SomeProperty { get; }
}
I have the implementation that compiles:
public class SomeClass : ISomeInterface
{
public string SomeProperty
{
get
{
throw new NotImplementedException();
}
set
{
throw new NotImplementedException();
}
}
}
which is a problem. How do I make the interface a contract that, won't allow set in it's implementation?
notice: I'm not looking for a solution how to avoid set IN the implementation but in the interface that will validate it from any new implementation, thanks.
Interfaces only specify what MUST be implemented, but do not restrict what other methods, or properties may also be implemented.
So the get is the only thing that you have specified.
Since you are silent on the set, any implementer of the interface is at liberty to add or not to add the set.
In short, with an interface specification you cannot do what you want to do.
If you want to make sure the set is never called then you could always cast the instance to the Interface
If you really need to make sure there is no set you could use an abstract class instead of an interface
abstract class SomeInterface
{
virtual string SomeProperty { get; }
}
Based on Mike answer, you could write something like that :
public interface ISomeInterface
{
string SomeProperty { get; }
}
public abstract class SomeInterfaceBase : ISomeInterface
{
public abstract string SomeProperty { get; }
}
So you can define your class like that:
public class SomeClass : SomeInterfaceBase
{
public override string SomeProperty { get; }
}
If you try to implement a setter, it will not compile.
Having a setter isn't an issue. The reason for this is because of how we treat interfaces.
Whether the concrete class has a setter or not does not matter, because we are supposed to treat the object as an ISomeInterface. In this case it only has a setter.
For instance lets take a factory method:
class Program
{
interface ISomeInterface
{
string SomeProperty { get; }
}
static ISomeInterface CreateSomeClass()
{
return new SomeClass();
}
class SomeClass : ISomeInterface
{
public string SomeProperty
{
get
{
throw new NotImplementedException();
}
set
{
throw new NotImplementedException();
}
}
}
static void Main(string[] args)
{
ISomeInterface someInterface = CreateSomeClass();
someInterface.SomeProperty = "test"; //Wont compile
}
}
The class's implementation of a setter is meaningless, because we are only interested in treating the object as ISomeInterface. Interfaces are additive. In other words, they define a contract of what needs to be defined, rather than what should not.
If I was to treat it in any other way, it would be something like this:
((SomeClass) someInterface).SomeProperty = "test"; //Code smell
which I would consider a code smell, because it's assuming that someInterface is SomeClass (treating interfaces as concrete classes)
I have a generic dictionary of objects where the key is of type Type:
public class DynamicObject : IDictionary<Type, object>
The idea is that this object is shared in a plugin-based architecture and so a type (which could reside in a plugin .dll) is used as a key in order to prevent clashes between plugins. This type is also used to store metadata about the field (such as a description of that field or the actual type of that field).
Currently plugins need to set the value of fields on this object using code similar to this:
DynamicObject someObject;
string someValue;
someObject[typeof(UsernameField)] = someValue;
The only problem with this is that this isn't type safe - even though the type UsernameField is aware of the exact type of the value it is expecting (e.g. int or in this case string), the value supplied here is just typed as an object. I'd like to use generics to make setting / getting of properties type safe but I'm not sure how. So far the best I've come up with is this:
// Field is a base class for all types used as keys on DynamicObject
[Description("Username of the user")]
public class UsernameField : Field
{
public static void Set(DynamicObject obj, string value)
{
obj[typeof(UsernameField)] = someValue;
}
}
// To set fields
UsernameField.Set(obj, someValue);
This is type safe, however it means that each of my field types (e.g. UsernameField) has a nearly identical static Set method.
How can I have type-safe access to values in this way without having lots of nearly identical methods on each of my field types?
As an aside, is using Type as a key like this a good idea or are there hidden pitfalls that I'm not yet aware of?
Define an interface that all your plugins must implement:
public interface IPlugin {
string Name { get; }
string Author { get; }
string Description { get; }
void Init();
}
And then use a Dictionary<Type, IPlugIn>.
Typically the interface is declared in a separate dll (like "MyCompany.MyProject.PlugIns.Contracts.dll").
EDIT: Ok, I think that I know what you mean now.
The trick is to have a generic class between Field and UsernameField with a generic Set method. The fact that Field is not generic, makes all the field types assignable to it. This would not be the case, if it was declared as Field<T>.
public abstract class Field
{
}
public abstract class GenericField<T> : Field
{
public void Set(DynamicObject obj, T value)
{
obj[this.GetType()] = value;
}
}
public class UsernameField : GenericField<string>
{
#region Singleton Pattern
public static readonly UsernameField Instance = new UsernameField();
private UsernameField() { }
#endregion
}
Because we need to call GetType in the Set method, we cannot declare it as static. Therefore, I used the singleton pattern.
Now, we can set the field in a type safe way:
UsernameField.Instance.Set(obj, "Joe");
ADDITION 1:
Since now the fields are singletons, you could use the fields as key of the dictionary instead of their type.
public class DynamicObject : IDictionary<Field, object> { }
And Set would become:
public void Set(DynamicObject obj, T value)
{
obj[this] = value;
}
ADDITION 2:
You could also define DynamicObject like this:
public class DynamicObject : Dictionary<Field, object>
{
public void Set<T>(GenericField<T> field, T value)
{
this[field] = value;
}
}
Now you can set values like this:
obj.Set(UsernameField.Instance, "Sue");
This is type safe and seems more natural. The Set method in GenericField is obsolete now.
Fields shouldn't have to know about a DynamicObject to get and set the value. You could make DyanmicObject handle the getting and setting of values, but I think a better approach is to treat the DynamicObject as a collection of Field instances, and each field has its own value. Something like this:
interface IField
{
object Value { get; set; }
}
interface IField<T> : IField
{
new T Value { get; set; }
}
abstract class BaseField<T> : IField<T>
{
T _value;
public T Value
{
get { return _value; }
set
{
// could add stuff like OnValueChanging, IsValueValid, etc...
this._value = value;
}
}
object IField.Value
{
get { return Value; }
set { Value = (T)value; }
}
}
class DynamicObject : List<IField>
{
public TField GetField<TField>() where TField : IField
{
return this.OfType<TField>().Single();
}
}
And the usage would then be:
class UsernameField : BaseField<string> { }
[TestMethod]
public void test()
{
var parent = new DynamicObject();
var field = new UsernameField() { Value = "the username" };
parent.Add(field);
Assert.AreEqual("the username", parent.GetField<UsernameField>().Value);
}
I may just be missing the point of the question but if the goal is to reduce the amount of code duplication required to achieve type safety, then why not define accessor helpers based on the type of field rather than the field itself:
public abstract class StringField<T> : Field
{
public static void Set(DynamicObject obj, string value)
{
obj[typeof(T)] = someValue;
}
}
public class UsernameField : StringField<UsernameField> { }
// To set fields
UsernameField.Set(obj, someValue);
Edit:
Or you could use a slight variation of Oliver's solution without the Singleton:
public abstract class GenericField<T, U> : Field
{
public static void Set(DynamicObject obj, T value)
{
obj[typeof(U)] = value;
}
}
public class UsernameField : GenericField<string, UsernameField> { }
I have a nice interface, and I want to implement one member of it in a base class so the clients can derive from the base class and have less boiler-plate to write. However, even though declared abstract, the compiler complains that the class does not implement the interface?!? How is this supposed to be done?
Code in question:
public interface ITaskDefinition
{
ITask CreateTask(TaskId id);
string Name { get; }
bool HasName { get; }
}
public abstract class TaskDefinitionBase : ITaskDefinition
{
private string name_ = null;
public void SetName(string name)
{
name_ = name;
}
public string Name
{
get
{
return name_;
}
}
public bool HasName
{
get
{
return name_ != null;
}
}
}
and the error I get is:
ITaskDefinition.cs(15,27): error CS0535: 'NetWork.Task.TaskDefinitionBase' does not implement interface member 'NetWork.Task.ITaskDefinition.CreateTask(NetWork.Task.TaskId)'
You must add an abstract stub of the method:
public abstract ITask CreateTask(TaskId id);
Inheritors can then override it to implement the interface.
When an object implements an interface it must obey all the constraints applied by the interface. This means that you must implement all properties and methods declared in the interface.
In your case you're missing the implementation of CreateTask. Also, you should note that C# use properties as opposed to Java for example that uses getters/setters. For this reason you don't need a SetName method.
You code should look something like this:
public abstract class TaskDefinitionBase : ITaskDefinition
{
private string name_ = null;
public abstract ITask CreateTask(TaskId id);
public string Name
{
get
{
return name_;
}
set
{
name_ = value
}
}
public bool HasName
{
get
{
return name_ != null;
}
}
}
TaskDefinitionBase needs to include CreateTask - if you don't want to implement it, just mark the method as abstract to force any implementing classes to implement it.