I have read multiple posts but haven't found an appropriate answer for my problem.
But there has to be one. So please be patient if this post really is an duplicate.
I try to initialize a Property of a generic type implementing a generic interface.
My interface looks basically like this:
public interface IMyInterface<TType> : IMyInterface
{
TType Value {get; set;}
// ... and some more ...
}
public interface IMyInterface
{
SetValue(string value);
}
I have multiple Classes implementing IMyInterface without knowing them
at the piece of code, where I need to set the value.
Is it possible to inizialize the "Value"-Property without using the Name of the property?
(Is there a "nice" way to do this?) - It isn't possible to use SetValue< TType >
void SetValue(object obj, string value)
{
// object obj implements IMyInterface<???>
PropertyInfo valueInfo = typeof(obj).GetType().GetProperty("Value")
valueInfo.SetValue(obj, Activator.CreateInstance(valueInfo.PropertyType))
((IMyInterface)obj).SetValue(value);
}
Thanks in advance.
EDIT: - removed -
EDIT 2:
This structure is given:
public Interface IGeneric<TType> : INonGeneric
{
TType Value2 {get;}
}
public Interface INonGeneric
{
object Value1 {get;}
}
using "Value1" from reflection is really easy:
INonGeneric myObject = (INonGeneric)givenObject;
doSomething(myObject.Value1)
if i need to access "Value2" it isn't that easy. As one can see in my first example I had to use the following construct which not seems to be the best way to access "Value2", because the properties name is hardcoded.
PropertyInfo valueInfo = givenObject.GetType().GetProperty("Value2");
object value = (object)valueInfo.GetValue(givenObject);
Is there any better solution?
If i got you right you have and instance that implements IGeneric, and you want to access the property Value2 (which has a generic return type).
The thing is, Generics are for compile time type safety. You cannot cast your object to IGeneric<...> if you don't know the type parameter. So why do you want to use Generics anyway if you don't know the type parameter?
There is a solution for this "problem", it is the same as IEnumerable and IEnumerable< T> uses. it can look something like this:
public interface INonGeneric
{
object Value {get; }
}
public interface IGeneric<T>
{
T Value { get; }
}
public class Magic : INonGeneric, IGeneric<string>
{
object INonGeneric.Value { get { return this.Value; } }
public string Value { get { return "test"; } }
}
You can now use cast the Object to INonGeneric if you don't use the type parameter, or use the Generic implementation if you know the type parameter at compile time.
But if you want to access a property of a generic type (you have no control over) without knowing the type parameter you will not get around either reflection or dynamic.
The dynamic solution can look like this:
dynamic generic = givenObject;
object value2 = generic.Value2;
Related
I was trying to create a class were one of the properties was generic without the class itself being generic. I discover that you can't do that; generic properties aren't allowed. A bit of digging here too me to the thread Making a generic property were I found a work around that works nicely for me.
Subsequently, I ended up with this class...
[Serializable]
public class ConfigurationItem
{
private readonly Type type;
public string Description { get; set; }
public IDictionary<string, ConfigurationItem> Items { get; private set; }
public string Name { get; set; }
public string Summary { get; set; }
public object Value { get; private set; }
public ConfigurationItem(string name, Type type = null, object value = null)
{
this.Name = name;
this.type = type ?? typeof(string);
this.Value = value;
Items = new Dictionary<string, ConfigurationItem>();
}
public string Export()
{
return JsonConvert.SerializeObject(this);
}
public T GetValue<T>()
{
return (T)Convert.ChangeType(Value, type);
}
}
Now the only issue I have is that if I want to get the value, properly cast, I have to supply the type when I call GetValue(). Instinctively, I can't help but feel, given that the class knows the type that should be returned, it should be possible for me to construct a GetValue() method that doesn't require any additional information from me.
I can't figure out how.
I did find the thread Getting a generic method to infer the type parameter from the runtime type which appears to suggest that it is possible, but I understand very little about Reflection and couldn't make any sense of what was being said.
Is there a way of constructing a GetType() method that doesn't require that I supply the generic type? And can you explain it in a way that my feeble brain can comprehend it?
EDIT
A number of people have actually pointed out that really I don't need to do this anyway, nonetheless as a learning exercise I followed up the suggestion from #ShoaibShakeel to look at the C# dynamic type and came up with these additional methods...
public dynamic GetValue()
{
return typeof(ConfigurationItem)
.GetMethod("GetReturnValue", System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Instance)
.MakeGenericMethod(type)
.Invoke(this, new object[] { });
}
private T GetReturnValue<T>()
{
return (T)Convert.ChangeType(Value, type);
}
Instinctively, I can't help but feel, given that the class knows the type that should be returned
While it is true that the Value object knows its own type, this information is only available at runtime though (once there is an actual object that has a type). So in order to have type information at compile time, you need to supply the necessary information. After all, it’s impossible to know statically what object is being assigned there.
But assuming that GetValue() was able to return the typed object automatically, what would you want to do with it?
var x = configurationItem.GetValue();
So what type should x be of, and what would you want to do with it afterwards? Do you only want to print it or something? Then object is enough. Do you want to calculate something with it? Then you would already require that it’s an int, or a float or something, so you would need an actual static type—which is why you would have to supply that type information.
Just because the return type is object that does not mean that the object itself loses the type information. A string assigned to an object property will still be a string, even if you retrieve it much later. And if you expect a string and want to do something with it, then you can surely just cast it to a string.
No.
The compiler cannot infer the type from the variable it is assigned to since it is ambiguous, especially in large inheritance trees.
Consider the line
object o = configurationItem.GetValue<int>();
int may be a valid conversion for your value but object isn't, since objectdoes not implement the IConvertible interface (required by Convert.ChangeType().
I was able to get my class to return a value, cast correctly, using reflection and dynamics using the following additions to my original class.
public dynamic GetValue()
{
return typeof(ConfigurationItem)
.GetMethod("GetReturnValue", System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Instance)
.MakeGenericMethod(type)
.Invoke(this, new object[] { });
}
private T GetReturnValue<T>()
{
return (T)Convert.ChangeType(Value, type);
}
I'd like to use a certain operation for multiple variable types (both native and objects) so I'm using the generic return type as follows.
private Generic Field<Generic>(String field)
{
if (BagOfJunk.Properties.Contains(field))
return (Generic)BagOfJunk[field];
return default(Generic);
}
This works well (and BagOfJunk is just a property of this from which I'm pulling out Object typed values). Now, during run-time, when a field isn't contained in the bag, I get the default value to be null. Hence, in the code, I need to perform a check as follows.
NumericType protoNumber = Field<NumericType>("beep");
int number = protoNumber != null ? protoNumber.Value : -1;
DateType protoDate = Field<DateType>("boop");
DateTime date = protoDate != null ? protoDate.Value : null;
I'd like to make the code more compact, so I tried to design a method that does the above four lines in one swoop, for a generic type. The result is below but, of course, it doesn't compile, because the type GenericIn isn't specific enough to have a property Value.
private GenericOut Field<GenericIn, GenericOut>(String field)
{
GenericIn input = Field<GenericIn>(field);
if (input != null)
return (GenericOut)input.Value;
return default(GenericOut);
}
How can I ensure the computer that my GenericIn isn't general - by promising that whatever stuff I'll shove into it, it'll always have the property Value in it?
Edit
It should be emphasized that the type of Value needs to be generic ( equivalent to GenericOut). I noticed that I didn't stress that strongly enough. So the interface that can be used need to declare a property of general type like the following.
interface ObjectWithValue { public Generic Value { get; } }
You can use an interface and apply a where constraint on the type to implement that interface, like below:
interface IHasPropertyValue<TValue> {
TValue Value { get; }
}
class MyType {
public TValue Method<T, TValue>(T obj) where T : IHasPropertyValue<TValue> {
return obj.Value;
}
}
EDIT: Modified the code above to make it more specific to the comment asked below.
put that property in an interface (or a class) and use the generic constraint "where":
public interface IMyInterface
{
public object Value { get; set; }
}
public class C<T> where T:IMyInterface
To build upon the answers so far, you need to create an interface that will be implemented by your GenericIn that will both guarantee that it has a property Value and that the property is of type GenericOut.
interface IHasValue<TOut>
{
TOut Value { get; }
}
private TOut Field<TIn, TOut>(string field) where TIn : IHasValue<TOut>
{
var input = Field<TIn>(field);
return input == null ? default(TOut) : input.Value;
}
**Preface I'm very new to generics. I have a class I created called Geno. My Peno class simply contains a string Name property. I'm wondering why I cannot call T.Name from within a method on this class. How would I access the properties on T? Any good resources on how this works?
public class Geno<T> where T : Peno
{
}
public string GetName()
{
return T.Name;
}
Is your Name property an instance property or a static property? If it's a static property, you don't get polymorphism anyway - you could just call Peno.Name.
If it's an instance property, you need an instance on which to call it. For example:
public class Geno<T> where T : Peno
{
private readonly T value;
public Geno(T value)
{
this.value = value;
}
public string GetName()
{
return value.Name;
}
}
If this doesn't help, please give a more complete example of what you're trying to do.
T is the type Peno. So what you try to do is Peno.Name, which is probably not what you want. Something like this would work:
public string GetName(T t)
{
return t.Name;
}
Static properties does not support inheritence. Hence you can just call Peno.Name rather than going for T.
If in your case you want to access a normal property, you need to create an instance of T which will let you call its property Name.
T is a Generic Type Parameter, not a parameter being passed into the class that is using T.
A Good example of this is List<T> you say this is a List, which means you are creating a list that contains object of the type of String.
hey. Is it possible to have a method that allows the user to pass in a parameter of a certain type and have the method instantiate a new object of that type? I would like to do something like this: (I don't know if generics is the way to go, but gave it a shot)
public void LoadData<T>(T, string id, string value) where T : new()
{
this.Item.Add(new T() { ID=id, Val = value});
}
The above doesn't work, but the idea is that the user passes the object type they want to instantiate and the method will fill in details based on those parameters.
I could just pass an Enum parameter and do a Switch and create new objects based on that, but is there a better way?
thanks
The only way to do this would be to add an interface that also specifies the parameters you want to set:
public interface ISettable
{
string ID { get; set; }
string Val { get; set; }
}
public void LoadData<T>(string id, string value) where T : ISettable, new()
{
this.Item.Add(new T { ID = id, Val = value });
}
Unfortunately I can't test to verify at the moment.
If the ID and Val properties come from a common base class or interface, you can constrain T to inherit that type.
For example:
public void LoadData<T>(string id, string value) where T : IMyInterface, new()
You can then use all the members of IMyInterface on T instances.
If they're just unrelated properties in different types that happen to have the same name, you'll have to use reflection.
Also, you need to remove T, from your parameter list.
A more dynamically typed language could do that with ease. I'm sure its possible with C#, it'll just take more effort. Theres probably some reflection library that will help you here.
Why don't you just create the object when you are calling the method? What you are trying to do seems way overly complex to me.
I have several templated objects that all implement the same interface:
I.E.
MyObject<datatype1> obj1;
MyObject<datatype2> obj2;
MyObject<datatype3> obj3;
I want to store these objects in a List... I think I would do that like this:
private List<MyObject<object>> _myList;
I then want to create a function that takes 1 parameter, being a datatype, to see if an object using that datatype exists in my list.... sorta clueless how to go about this. In Pseudo code it would be:
public bool Exist(DataType T)
{
return (does _myList contain a MyObject<T>?);
}
Some Clarification....
My interface is IMyObject<T>, my objects are MyObject<T>. I have a new class MyObjectManager which I need to have a List of MyObject<T> stored within. I need a function to check if a MyObject<T> exists in that list. The type T are datatypes which were auto-generated using T4.... POCO classes from my Entity Data Model.
You can make a generic function:
public bool Exists<T>() where T : class {
return _myList.OfType<MyObject<T>>().Any();
}
Note that this requires that you know T at compile-time.
If all you have is a System.Type object at runtime, you'll need to use reflection:
public bool Exists(Type t) {
var objectOfT = typeof(MyObject<>).MakeGenericType(t);
return _myList.Any(o => o.GetType() == objectOfT);
}
Note, however, that a List<MyObject<object>> cannot hold a MyObject<SomeType>.
You need to change the list to a List<object>, or make MyObject implement or inherit a non-generic type and make the list contain that type.
How about an extension method?
public static bool HasAny(this IEnumerable source, Type type) {
foreach (object item in source)
if (item != null && item.GetType().Equals(type))
return true;
return false;
}
Usage:
bool hasDataType1 = myList.HasAny(typeof(MyObject<datatype1>));
Note that if you don't want to have to type out typeof(...) -- i.e., if you basically want your Exist method to only care about objects of type MyObject<T>, I'd go with something like SLaks's answer:
public static bool Exist<T>(this IEnumerable source) {
return source.OfType<MyObject<T>>().Any();
}
Also, SLaks is right that you really can't have a List<MyObject<object>> that's full of anything other than objects of type MyObject<object> or some derived class (and MyObject<datatype1>, etc. do not derive from MyObject<object> -- generics don't work that way).
Another way I might suggest to work around the whole "you can't get the type of a generic class using a System.Type object without using reflection" issue would be this: Make your MyObject<T> implement a non-generic interface, like this:
public interface IMyObject {
Type DataType { get; }
}
public class MyObject<T> : IMyObject<T>, IMyObject {
public Type DataType {
get { return typeof(T); }
}
}
Then your list could be a List<IMyObject> (the non-generic interface) and your Exist method could look like this:
public static bool Exist<T>(this IEnumerable source, Type type) {
return source.OfType<IMyObject>().Any(x => x.DataType.Equals(type));
}
Since they all implement the same interface, instead of casting them to object and calling GetType (which can be expensive) why not add a property to your interface called class name (or something)? Then you can use the linq in order to grab that property. And don't forget using System.Linq
using System.Linq;
public bool Exist(List<IMyInterface> objects, IMyInterface typeToCheck)
{
return objects.Any(t => t.ObjectName == typeToCheck.ObjectName);
}