I'm trying to write some code to help unit test WCF services. These services are accessed through a facade class that creates the proxy instance, then calls the proxy method and returns the result; for each proxy method. I'd like to be able to replace the current creation code with something that either creates the real service or a fake one.
I couldn't get that to work. I boiled it down to the following:
using System.ServiceModel;
namespace ExpressionTrees
{
public interface IMyContract
{
void Method();
}
public class MyClient : ClientBase<IMyContract>, IMyContract
{
public MyClient()
{
}
public MyClient(string endpointConfigurationName)
: base(endpointConfigurationName)
{
}
public void Method()
{
Channel.Method();
}
}
public class Test
{
public TClient MakeClient<TClient>()
where TClient : ClientBase<IMyContract>, IMyContract, new()
{
return new MyClient("config");
// Error:
// Cannot convert expression of type 'ExpressionTrees.ServiceClient' to return type 'TClient'
}
}
}
Why is it that, even though the MyClient class derives from ClientBase<IMyContract> and implements IMyContract, that I can't return a MyClient instance from a method meant to return a TClient? TClient specifies a type constraint I had thought meant the same thing.
My goal was to call code like this:
public void CallClient<TClient>()
where TClient : ClientBase<IMyContract>, IMyContract
{
TClient client = null;
bool success = false;
try
{
client = MakeClient<TClient>();
client.Method();
client.Close();
success = true;
}
finally
{
if (!success && client != null)
{
client.Abort();
}
}
}
But instead of always calling MakeClient<TClient>, I wanted to be able to have a unit test inject a mock object. Since the code above depends both on ClientBase<IMyContract>, IMyContract, it seems I was trying to "synthesize" a generic class that would satisfy that constraint.
In retrospect, this wouldn't make sense. As an example, the ClientBase<IMyContract> would expect to be instantiated in such a way that a Channel object would be constructed that it could then delegate the Close method to.
I've wound up punting on having the exact same code run both for the real and for the fake services. I'm now injecting an IMyService, and either calling IMyService.Method or client.Method depending on whether my injected property is null.
Basically your code boils down to:
public static T MakeFruit<T>() where T : Fruit
{
return new Apple();
}
This always returns an apple, even if you call MakeFruit<Banana>(). But MakeFruit<Banana>() is required to return a banana, not an apple.
The meaning of the generic type constraint is that the type argument provided by the caller must match the constraint. So in my example, you can say MakeFruit<Banana>() but not MakeFruit<Tiger>() because Tiger does not match the constraint that T must be convertible to Fruit. I think you believe that the constraint means something else; I'm not sure what that is.
Think about it like this. A formal parameter has a formal parameter type. The formal parameter type restricts the type of the expression that is used as an argument. So when you say:
void M(Fruit x)
you are saying "the argument passed for formal parameter x in M must be convertible to Fruit".
Generic type parameter constraints are exactly the same; they are restrictions on what type arguments may be passed for the generic type parameters. When you say "where T : Fruit", that is just the same as saying (Fruit x) in the formal parameter list. T has got to be a type that goes to Fruit, just as the argument for x has got to be an argument that goes to Fruit.
Why do you even want to have a generic method in the first place? I don't understand what exactly you're trying to model with this method or why you want it to be generic.
You are restricting the TClient in the MakeClient<TClient>() portion of the call, not the return type.
The return type has to match the type of the generic parameter, but picture this:
public class MyOtherClient : ClientBase<IMyContract>, IMyContract
{
public void Method()
{
Channel.Method();
}
}
That's also a valid return by calling MakeClient<MyOtherClient>, which MyClient is not convertable to, since it should return a type of MyOtherClient.
Note that changing the return to:
return new MyClient() as TClient;
would probably get it past the compiler, but be null in my scenario above at runtime.
This should resolve your issue.
static T Make<T>() where T : IConvertible
{
var s = "";
return (T)(Object)s;
}
Related
I have 2 interfaces defined like this
public interface IEnumerableDisposable<out T> : IEnumerable<T>, IDisposable
{
}
public interface IApiCollection
{
IEnumerableDisposable<TItem> GetItems<TItem>();
}
and a method that needs to return the first interface type like this
public IEnumerableDisposable<TItem> GetItems<TItem>()
where TItem can be either type Foo or type Bar
Question how do i cast the return type to IEnumerableDisposable ?
My understanding is that i need class
public class EnumandDisposeFoo : IEnumerableDisposable<Foo>
{
}
and instaciate this class inside my method. how would i then cast the type back to its generic form?
Any help would be greatly appreciated
var foo = new EnumandDisposeFoo(_pagedApi);
var fooitems = foo.GetEnumerator();
return foo; --> errors with
Cannot implicitly convert type 'ApiCollection.Implementation.EnumandDisposeFoo' to 'ApiCollection.IEnumerableDisposable<TItem>'. An explicit conversion exists (are you missing a cast?
Does your code perhaps look similar?
I tried to derive it from the provided error message...
public IEnumerableDisposable<TItem> GetItems<TItem>()
{
var foo = new EnumandDisposeFoo(_pagedApi);
// var fooitems = foo.GetEnumerator(); // not used and is in fact of type `IEnumerator<Foo>`
return foo;
}
If yes, EnumandDisposeFoo implements IEnumerableDisposable<T> while specifying the element type T as Foo. At this point it is no longer generic. Trying to cast (implicitly) to the generic IEnumerableDisposable<TItem> will fail unless you provide an override for that specific implicit cast.
I doubt you want that.
Either you change EnumandDisposeFoo to a generic implementation:
public class EnumandDisposeFoo<TItem> : IEnumerableDisposable<TItem> { }
or you limit your method to return the (non-generic) IEnumerableDisposable<Foo>.
The compiler doesn't know which kind of type you want to return, thus no error at compile time.
Calling your method with GetItems<Foo>() will probably succeed.
But calling your method with any other type than Foo for TItem will try an implicit cast at run-time which might or might not fail.
I have a CreateMessage class that is meant to handle incoming messages from a TCPClient, get the user type and return how the message should be formatted to its calling method.
In the method GetUserType, I want to pass UserBaseType as a parameter, which is a generic abstract class that takes a Type of UserType. However, it gives me the error:
Using the generic type 'UserTypeBase' requires one type argument.
I'm still trying to wrap my head around using generics and constraints, so I don't know if I'm going about this the wrong way. I've done a bit of digging to try to find a solution myself, but haven't found anything that more or less tailors to what I'm trying to do.
internal class CreateMessage
{
internal static string user;
internal static string message;
internal CreateMessage(string data)
{
user = Lists.users[data.Substring(1, 3)];
message = data.Substring(5, data.Length - 5);
}
private UserType GetUserType(UserTypeBase type)
{
return type.CreateType();
}
internal string Message()
{
UserType Type = null;
if (user.Contains("[M]"))
Type = GetUserType(UserMod);
else if (user.Contains("[B]"))
Type = GetUserType(UserVIP);
else
Type = GetUserType(UserRegular);
return Type.Message();
}
}
UserBaseType.cs
internal abstract class UserTypeBase<T> where T: UserType
{
public abstract string User { get; }
public abstract string Message { get; }
public abstract T CreateType();
}
You're going to want to make the method that takes the parameter generic as well. You're going to also want to mirror the type constraints as they appear on the parametric type to avoid compilation errors, as well as be explicit in what is and is not acceptable for the method.
private T GetUserType<T>(UserTypeBase<T> type) where T : UserType
{
return type.CreateType();
}
You can then call it with the type provided explicitly or implicitly, depending on the situation.
var someType = new UserTypeDerived<UserType>();
var resultImplicit = GetUserType(someType);
var resultExplicit = GetUserType<UserType>(someType);
Since it's a parameter that is being used generically, the compiler can implicitly determine what the expected value of T is based on the type of the parameter supplied.
I need to test if a value is an instance of a generic base class, without knowing the generic type parameter. Using the MSDN example as the base of my example, this is what I'd like to accomplish:
using System;
public class Class1<T> { }
public class DerivedC1 : Class1<int> { }
class IsSubclassTest
{
public static void Main()
{
Console.WriteLine(
"DerivedC1 subclass of Class1: {0}",
typeof(DerivedC1).IsSubclassOf(typeof(Class1<>)) // <- Here.
);
}
}
While this is syntactically correct, it always yields false. If I remove the generic type parameter, it works as expected (returns true).
How can I test if a class type is a subclass of a generic base class, without knowing its generic type parameter as such?
The problem is that DrevidedC1 is not a sublcass of Class1<T>, it's a subclass of Class1<int>. Make sure you understand this subtle diference; Class1<T> is a open type (T can be anything, it hasn't been set) while DerivedC1 extends a closed type Class1<int> (it's not open in T anymore, T is set to int and only int). So when you do the following:
typeof(DerivedC1).IsSubclassOf(typeof(Class1<>))
The answer is evidently false.
What you need to do is check if the generic type definition of DerivedC1's base type (think of it as the corresponding open generic type of Class1<int>) equals Class1<T> which it clearly does.
The correct code is therefore:
typeof(DerivedC1).BaseType.GetGenericTypeDefinition() == typeof(Class1<>));
Or better yet, as Matías Fidemraizer states in his answer:
typeof(DerivedC1).BaseType.GetGenericTypeDefinition().IsAssignableFrom(typeof(Class1<>)));
There's special methods on Type for this sort of thing. As far as I can see, you'll need to walk up your base-types and check each in turn until you either (a) hit a match or (b) get to the top of the inheritance hierarchy (i.e. System.Object).
As such, the following (recursive) extension method:
public static class TypeExtensions
{
public static bool IsDerivedFromGenericParent(this Type type, Type parentType)
{
if(!parentType.IsGenericType)
{
throw new ArgumentException("type must be generic", "parentType");
}
if(type == null || type == typeof(object))
{
return false;
}
if(type.IsGenericType && type.GetGenericTypeDefinition() == parentType)
{
return true;
}
return type.BaseType.IsDerivedFromGenericParent(parentType)
|| type.GetInterfaces().Any(t=>t.IsDerivedFromGenericParent(parentType));
}
}
will allow you to do the following
typeof(DerivedC1).IsDerivedFromGenericParent(typeof(Class1<>))
...and will also work if you test something derived from DerivedC1.
Changing typeof(DerivedC1).IsSubclassOf(typeof(Class1<>)) to typeof(Class1<>).IsAssignableFrom(typeof(DerivedC1).BaseType.GetGenericTypeDefinition()) should be enough in your case.
Type.IsAssignableFrom is more powerful than using Type.IsSubClassOf because it just checks if some type is assignable to other type. This includes, the same type, interface types and other cases.
I have an interface for specifying GetData method which returns one instance of the class by its ID
public interface ILabelData<T> {
T GetData(object id);
}
and also have many different classes those who implements the interface and having members of course:
public class BTAC : ILabelData<BTAC> {
// members...
// and interface impl:
public BTAC GetData(object id) {
return null;
}
}
public class KTAC : ILabelData<KTAC> {
// members...
// and interface impl:
public KTAC GetData(object id) {
return null;
}
}
Within a calling method I would like to instantiate a BTAC/KTAC/... class and call their GetData method. (The purpose of it is becasue after I have the required instance I want to get its members and attributes. Getting the members and attributes is not part of my question.)
ILabelData<object> o = Activator.CreateInstance(type, new object[] { myID });
^^^^^^^^^^^^^^^^^^^^
object data = o.GetData(myID);
the problem is compiler error Cannot implicitly convert type 'object' to 'ILabelData< object >'
After instantiating the proper class of course I do need for the members and attributes, too, so it isn't enough to getting back ILabelData typed object.
How can I get a such kind of object? Some factory or whatever?
You're mixing up the factory and the entity. That's unnecessary - it really doesn't make sense to have GetData be an instance method on BTAC.
Activator.CreateInstance returns object - you need to explicitly cast it to the type you want.
Neither of your types implements ILabelData<object>.
The thing is, with the code as is, there's no point in having ILabelData generic at all! If you're going to work with object anyway, remove the generic parameter from ILabelData and just do this:
var o = (ILabelData)Activator.CreateInstance(type, new object[] { myID });
var data = o.GetData(myID);
Having the interface (and GetData method) generic only makes sense if you actually use it that way.
If you usually use it directly, and you only need it like this in a few special cases (why?) you could also make the interface covariant:
public interface ILabelData<out T>
{
T GetData(object id);
}
This will allow you to implement ILabelData<BTAC> in BTAC, giving you the "user friendly" BTAC GetData(object) method, while still allowing you to do the explicit cast to ILabelData<object>. Even then, I'd avoid this approach. It smells.
Working Code here.
var instances = (from t in System.Reflection.Assembly.GetCallingAssembly().GetTypes()
where t.GetInterfaces().Any(x => x.IsGenericType && x.GetGenericTypeDefinition() == typeof(ILabelData<>))
&& t.GetConstructor(Type.EmptyTypes) != null
select Activator.CreateInstance(t)).ToList();
foreach (dynamic item in instances)
{
var res = item.GetData(1);
}
I was hoping to do something like this, but it appears to be illegal in C#:
public Collection MethodThatFetchesSomething<T>()
where T : SomeBaseClass
{
return T.StaticMethodOnSomeBaseClassThatReturnsCollection();
}
I get a compile-time error:
'T' is a 'type parameter', which is not valid in the given context.
Given a generic type parameter, how can I call a static method on the generic class? The static method has to be available, given the constraint.
In this case you should just call the static method on the constrainted type directly. C# (and the CLR) do not support virtual static methods. So:
T.StaticMethodOnSomeBaseClassThatReturnsCollection
...can be no different than:
SomeBaseClass.StaticMethodOnSomeBaseClassThatReturnsCollection
Going through the generic type parameter is an unneeded indirection and hence not supported.
To elaborate on a previous answer, I think reflection is closer to what you want here. I could give 1001 reasons why you should or should not do something, I'll just answer your question as asked. I think you should call the GetMethod method on the type of the generic parameter and go from there. For example, for a function:
public void doSomething<T>() where T : someParent
{
List<T> items=(List<T>)typeof(T).GetMethod("fetchAll").Invoke(null,new object[]{});
//do something with items
}
Where T is any class that has the static method fetchAll().
Yes, I'm aware this is horrifically slow and may crash if someParent doesn't force all of its child classes to implement fetchAll but it answers the question as asked.
You can do what I call a surrogate singleton, I've been using it as a sort of "static inheritance" for a while
interface IFoo<T> where T : IFoo<T>, new()
{
ICollection<T> ReturnsCollection();
}
static class Foo<T> where T : IFoo<T>, new()
{
private static readonly T value = new();
public static ICollection<T> ReturnsCollection() => value.ReturnsCollection();
}
// Use case
public ICollection<T> DoSomething<T>() where T : IFoo<T>, new()
{
return Foo<T>.ReturnsCollection();
}
The only way of calling such a method would be via reflection, However, it sounds like it might be possible to wrap that functionality in an interface and use an instance-based IoC / factory / etc pattern.
It sounds like you're trying to use generics to work around the fact that there are no "virtual static methods" in C#.
Unfortunately, that's not gonna work.
I just wanted to throw it out there that sometimes delegates solve these problems, depending on context.
If you need to call the static method as some kind of a factory or initialization method, then you could declare a delegate and pass the static method to the relevant generic factory or whatever it is that needs this "generic class with this static method".
For example:
class Factory<TProduct> where TProduct : new()
{
public delegate void ProductInitializationMethod(TProduct newProduct);
private ProductInitializationMethod m_ProductInitializationMethod;
public Factory(ProductInitializationMethod p_ProductInitializationMethod)
{
m_ProductInitializationMethod = p_ProductInitializationMethod;
}
public TProduct CreateProduct()
{
var prod = new TProduct();
m_ProductInitializationMethod(prod);
return prod;
}
}
class ProductA
{
public static void InitializeProduct(ProductA newProduct)
{
// .. Do something with a new ProductA
}
}
class ProductB
{
public static void InitializeProduct(ProductB newProduct)
{
// .. Do something with a new ProductA
}
}
class GenericAndDelegateTest
{
public static void Main()
{
var factoryA = new Factory<ProductA>(ProductA.InitializeProduct);
var factoryB = new Factory<ProductB>(ProductB.InitializeProduct);
ProductA prodA = factoryA.CreateProduct();
ProductB prodB = factoryB.CreateProduct();
}
}
Unfortunately you can't enforce that the class has the right method, but you can at least compile-time-enforce that the resulting factory method has everything it expects (i.e an initialization method with exactly the right signature). This is better than a run time reflection exception.
This approach also has some benefits, i.e you can reuse init methods, have them be instance methods, etc.
You should be able to do this using reflection, as is described here
Due to link being dead, I found the relevant details in the wayback machine:
Assume you have a class with a static generic method:
class ClassWithGenericStaticMethod
{
public static void PrintName<T>(string prefix) where T : class
{
Console.WriteLine(prefix + " " + typeof(T).FullName);
}
}
How can you invoke this method using relection?
It turns out to be very easy… This is how you Invoke a Static Generic
Method using Reflection:
// Grabbing the type that has the static generic method
Type typeofClassWithGenericStaticMethod = typeof(ClassWithGenericStaticMethod);
// Grabbing the specific static method
MethodInfo methodInfo = typeofClassWithGenericStaticMethod.GetMethod("PrintName", System.Reflection.BindingFlags.Static | BindingFlags.Public);
// Binding the method info to generic arguments
Type[] genericArguments = new Type[] { typeof(Program) };
MethodInfo genericMethodInfo = methodInfo.MakeGenericMethod(genericArguments);
// Simply invoking the method and passing parameters
// The null parameter is the object to call the method from. Since the method is
// static, pass null.
object returnValue = genericMethodInfo.Invoke(null, new object[] { "hello" });
As of now, you can't. You need a way of telling the compiler that T has that method, and presently, there's no way to do that. (Many are pushing Microsoft to expand what can be specified in a generic constraint, so maybe this will be possible in the future).
Here, i post an example that work, it's a workaround
public interface eInterface {
void MethodOnSomeBaseClassThatReturnsCollection();
}
public T:SomeBaseClass, eInterface {
public void MethodOnSomeBaseClassThatReturnsCollection()
{ StaticMethodOnSomeBaseClassThatReturnsCollection() }
}
public Collection MethodThatFetchesSomething<T>() where T : SomeBaseClass, eInterface
{
return ((eInterface)(new T()).StaticMethodOnSomeBaseClassThatReturnsCollection();
}