I'm wondering if it's possible to cast an object to a Type... I've just started using Reflection, so maybe I'm doing it all wrong but here's what I would like to do:
...
Type type = ...;
Type interfaceType = someOtherType.GetInterface("IConverter`2");
return (Cast to interfaceType)Activator.CreateInstance(type);
Is the cast to the interface possible?
Update:
Compiler says that T and K can not be found. The myInterface Type instance knows the T and K class...
public IConverter<T, K> GetConverter(Type type)
{
if (dtoModelDictionary.ContainsKey(type))
{
Type foundType = dtoModelDictionary[type];
Type myInterface = foundType.GetInterface("IConverter`2");
return (IConverter<T, K>)Activator.CreateInstance(foundType);
}
else if (dalModelDictionary.ContainsKey(type))
{
Type foundType = dalModelDictionary[type];
return (IConverter<T, K>)Activator.CreateInstance(foundType);
}
else
{
throw new System.Exception();
}
}
Second update:
public SomeClass GetConverter(Type type)
{
if (dtoModelDictionary.ContainsKey(type))
{
Type foundType = dtoModelDictionary[type];
Type myInterface = foundType.GetInterface("IConverter`2");
IConverter<T, K> converter = (IConverter<T, K>)Activator.CreateInstance(foundType);
return converter.someMethod();
}
}
Answer to you update:
You cannot cast to a type where the generic arguments are not defined. T and K must be defined for the method that is using it.
Either declare it:
public IConverter<T, K> GetConverter<T, K>(Type type)
Or, if you face the problem often that this interface is used but you don't know any T or K types, use an interface without generics:
interface IConverter
{
// general members
}
interface IConverter<T, K> : IConverter
{
// typesave members
}
public IConverter GetConverter(Type type)
{
// ...
return (IConverter)Activator.CreateInstance(type);
}
Not really, no... at least not in this way. The problem is that your return value is going to have to be whatever your method's return value is typed as. Because everything must be typed at compile-time, there is limited or no real use case that I can see for this particular kind of type coersion - maybe you can say some more about what you are trying to accomplish?
Now if you are using generics, you do have a run-time typing story, you can return your Type parameter type:
public T MyMethod<T>(...)
...
return (T)Activator.CreateInstance(type);
You can only cast an object to something that it actually is. You can for example cast a String reference to IEnumerable, but you can't cast it to char[].
If what your method returns actually implements the interface, you can just cast it as usual.
Example:
return (IConverter<int,string>)Activator.CreateInstance(type);
Edit:
You need to make the method generic so that you can specify the data types when you call it:
public IConverter<T, K> GetConverter<T, K>(Type type) {
...
}
You can do this:
var type = typeof(IConverter<,>).MakeGenericType(new Type[] { typeof(T), typeof(K) });
Related
So I have a generic class, with Type as its generic parameter. In this class is a method, which has object parameter called value. Kinda like this:
public class Foo<Type> where Type : IComparable
{
public void Bar(object value)
{
DoSomething((Type)value);
}
}
As you can notice, I need to "DoSomething" with value (stored in object) that I first need to cast to Type. I even have my own overridden cast, which works on its own.
In this specific case, Type is generic, lets call it GenericType, and has this user-defined cast:
public static implicit operator GenericType<T>(T value)
{
return new GenericType<T>(value);
}
and value is an enum, lets say
public enum Number: short
{
Zero = 0,
One = 1,
Two = 2
}
The 'DoSomething((Type)value)' in this case is where Type is GenericType and value is Number.Zero. For some reason, this causes the cast to throw InvalidCastException: Specified cast is not valid. When I try it directly, i mean like..
GenericType<Number> z = (GenericType<Number>)Number.Zero;
..it works (I know, there is not explicit cast even needed). But for some reason, it does not work in the complex example I stated above. Can anyone help me understand and potentially fix that?
Why not just let your class use the generic type?
public class Foo<T> where T : IComparable
{
public void Bar(T value)
{
DoSomething(value);
}
}
No casting needed... and please don't use reserved words to name stuff.
I can create a generic class that takes, as its template parameter, a C# type, and then within the generic class use the System.Type information corresponding to that C# type:
public class Generic<T>
{
public bool IsArray()
{
return typeof(T).IsArray();
}
public T Create()
{
return blah();
}
}
Generic<int> gi = new Generic<int>();
Debug.WriteLine("int isarray=" + gi.IsArray());
Generic<DateTime> gdt;
But now let's say what I have, is a System.Type. I can't use this to instantiate my generic class:
FieldInfo field = foo();
Generic<field.FieldType> g; // Not valid!
Is there some clever C# thing I can do, to convert a System.Type back to the original C# type? Or some other way, to create a generic that can (1) give me information about the System.Type, and (2) create objects of the associate C# type?
By the way, this is a very contrived example to explain the problem I'm trying to solve, don't worry too much about whether Generic makes sense or not!
The only thing you can do is use reflection. This because while the int of Generic<int> is known at compile-time, the field.FieldType is known only at runtime.
Reflection example:
Type type = typeof(Generic<>).MakeGenericType(field.FieldType);
// Object of type Generic<field.FieldType>
object gen = Activator.CreateInstance(type);
But even here, from a Type (field.FieldType) you obtain another Type (type)
There are normally three ways of using this:
Full reflection: you use the object of type Generic<type> only through reflection. You create it through Activator.CreateInstance and from there you begin using Type.GetMethod() and Invoke()
Type type = typeof(Generic<>).MakeGenericType(field.FieldType);
// Object of type Generic<field.FieldType>
object gen = Activator.CreateInstance(type);
MethodInfo isArray = type.GetMethod("IsArray");
bool result = (bool)isArray.Invoke(gen, null);
Interfaces/base classes: you have a non-generic base class or interface that is common between all the Generic<T>. You use your object only though that interface/base class.
public class Generic<T> : IComparable where T : new()
{
public bool IsArray()
{
return typeof(T).IsArray;
}
public T Create()
{
return new T();
}
public int CompareTo(object obj)
{
return 0;
}
}
Type type = typeof(Generic<>).MakeGenericType(field.FieldType);
IComparable cmp = (IComparable)Activator.CreateInstance(type);
int res = cmp.CompareTo(cmp);
A generic method where you put all the handling of the Generic<T>. That is the only method that is used through reflection.
public static void WorkWithT<T>() where T : new()
{
Generic<T> g = new Generic<T>();
T obj = g.Create();
Console.WriteLine(g.IsArray());
}
var method = typeof(Program).GetMethod("WorkWithT").MakeGenericMethod(field.FieldType);
// Single reflection use. Inside WorkWithT no reflection is used.
method.Invoke(null, null);
I haven't slept in a while so this is probably easier than I think it is.
I have a generic class that's more or less this:
public class Reference<T> where T : APIResource //<- APIResource is abstract btw
{
private T _value = null;
public T value
{
get { return _value; }
}
}
Elsewhere, in a custom serialize method, someone is passing in an object that is actually an instance of Reference<(something)>. I simply want to skip to the "value" property that every Reference<> object has, so I want to go:
string serialize(object o)
{
return base.serialize( ((Reference<>) o).value );
}
Of course, life isn't that simple because as the compiler puts it:
using the generic type "Reference<T>" requires 1 type arguments
How can I do what I want to do?
You can create a covariant generic interface with the property:
interface IReference<out T> where T : ApiResource {
T Value { get; }
}
You can then cast IReference<Anything> to IReference<object> or IReference<ApiResource>.
SLaks answer is perfect. I just want to extend it a little bit:
There are sometimes situations, where you can't substitute class with interface. Only in that cases you may want to use dynamic feature, so that you can call value property:
string serialize(object o)
{
if(typeof(Reference<>) == o.GetType().GetGenericTypeDefinition())
return base.serialize( ((dynamic)o).value );
//in your case you will throw InvalidCastException
throw new ArgumentException("not a Reference<>", "o");
}
This is just another options and I suggest to use it very carefully.
Dont forget to check whether its generic type or not ---> o.GetType().IsGenericType, before
using o.GetType().GetGenericTypeDefinition() else it throws exception..
Given the class:
public class Foo
{
public string Name { get; set; }
}
Is it possible to have a Foo instance created from a string through Convert.ChangeType:
Type type = typeof(Foo);
object value = "one";
value = System.Convert.ChangeType(value, type);
This is how a 3rd party API is attempting to rebuild objects. Someone mentioned this is possible with implicit operators, but from my understanding that will let me do the following, not create the object:
Foo foo = new Foo() { Name = "one" };
string fooAsString = foo; // implicit conversion -- no cast needed
Is there a way to create the object this way? Also, I do have the ability to change the Convert.ChangeType if there is another way to do this.
Update:
The reason I am asking is because it throws and exception:
Invalid cast from 'System.String' to
'JibbaJabba+Foo'.
and adding the operator did not resolve the issue.
According to the MSDN documentation:
For the conversion to succeed, value
must implement the IConvertible
interface, because the method simply
wraps a call to an appropriate
IConvertible method. The method
requires that conversion of value to
conversionType be supported.
Looking at the IConvertible interface, it has a ToType method. You could try that, maybe? (Disclaimer: I haven't. It's just a thought.)
Edit: In your case, it seems that you want to convert from a string to a Foo. Since the string type (obviously) does not define a conversion to Foo in its IConvertible implementation, I believe you're out of luck.
Update: I don't want to suggest that this is how you should always approach this sort of problem, but...
I took a look at the code for Convert.ChangeType in Reflector. It's long; I won't reproduce it here. But basically it's doing as the documentation says: it only works if:
The value parameter is a non-null instance of a type that implements IConvertible, or:
The type of the value parameter and the conversionType parameter are the same (so: Convert.ChangeType(myFoo, typeof(Foo)) would also work, though it'd be pretty useless).
Then, it cycles through all the types supported by IConvertible (which obviously does not include any user-defined types) and ultimately uses ToType as a fallback.
So, we need to look at the string type's implementation of ToType.
Sadly, it is one unfortunate line:
return Convert.DefaultToType(this, type, provider);
What does DefaultToType do? Exactly the same thing as ChangeType (minus the ToType fallback, obviously to avoid infinite recursion).
So this just simply isn't going to work.
If you're absolutely tied to this 3rd party library that's using Convert.ChangeType behind the scenes, I would recommend contacting the library's developer and asking them to extend their API in some way that will allow you to accomplish what you're trying to accomplish. Some possiblities might be:
Accepting an optional Converter<string, T> or Func<string, T> delegate parameter, as suggested by Ben Voigt in a comment.
Accepting a TypeConverter parameter
Accepting a parameter of some type that implements an interface like IParser<T>
Anyway, best of luck.
Direct cast from string won't work as Dan Tao already pointed out. Could you maybe implement your own wrapper for the string and use that? Something like
class MyString: IConvertible {
public string Value { get; set; }
...
object IConvertible.ToType(Type conversionType, IFormatProvider provider) {
if (conversionType == typeof(Foo))
return new Foo { Name = Value };
throw new InvalidCastException();
}
}
...
MyString myString = new MyString{Value="one"};
Foo myFoo = (Foo)Convert.ChangeType(myString, typeof(Foo));
Don't know if it is a useful idea but anyway..
An implicit operator won't work here? If Foo is a class you can modify then I've used something like this in the past, which also allows you to compare Foo instances to a string.
public class Foo
{
public string Name { get; set; }
public static implicit operator Foo(string value)
{
return new Foo { Name = value };
}
}
...
Foo foo = "fooTest";
Console.WriteLine("Foo name: {0}", foo.Name);
...
Edit: If you must use ChangeType then as far as I know you're out of luck. If you can modify the API to use a TypeConverter, you can use something like the following.
...
Type type = typeof(Foo);
object value = "one";
var converter = TypeDescriptor.GetConverter(type);
if (converter.CanConvertFrom(value.GetType()))
{
object newObject = converter.ConvertFrom(value);
Console.WriteLine("Foo value: {0}", newObject.ToString());
}
...
public class FooConverter : TypeConverter
{
public override bool CanConvertFrom(ITypeDescriptorContext context, Type sourceType)
{
return sourceType == typeof(string);
}
public override object ConvertFrom(ITypeDescriptorContext context, System.Globalization.CultureInfo culture, object value)
{
var name = value as string;
if (name != null)
return new Foo { Name = name };
else
return base.ConvertFrom(context, culture, value);
}
}
[TypeConverter(typeof(FooConverter))]
public class Foo
{
public string Name { get; set; }
public override string ToString()
{
return Name;
}
}
object value1 = "one";
Foo value2 = (Foo)
System.Convert.ChangeType(value, typeof(Foo));
Edit:
Ok then, if you are trying to create type of Foo from string, you could use reflection:
String controlToCreate = "Foo";
Type typeofControl = Type.GetType(controlToCreate,true);
i'm implementing a generic interface (iqueryprovider, specifically). at some point, i'm forced to return a generic result, that i need to get from some internal interface:
public TResult Execute<TResult>(...) {
return something.Foo<TResult>();
}
where something.Foo is
public T Foo<T>() where T: MyBaseClass, new() {
...
}
this of course blows up because the externally defined TResult doesn't have the same type restrictions as the internally-defined T. the question: is there a way to make TResult palatable to Foo? can i somehow explicitly test for those two conditions and coerce the type variable?
You could try something like this:
public TResult Execute<TResult>(...)
{
if (typeof(TResult) is MyBaseClass)
{
Type mytype = typeof(TResult);
MethodInfo method = typeof({TypewhereFoo<>IsDeclared}).GetMethod("Foo");
MethodInfo generic = method.MakeGenericMethod(myType);
return (TResult)generic.Invoke(this, null);
}
else
{
// Throw here
}
}
Nope. If TResult has no constraints on it, then it can be any old thing. If your helper method cannot take any old thing, then you'll need to get a better helper method. The interface requires you to provide more services than your helper can provide, therefore, you're going to have to do the work to provide that service.
You will have to add the type restrictions to your generic method:
public TResult Execute<TResult>(...) where TResult: MyBaseClass, new() {
return something.Foo<TResult>();
}
Ouch... you have a problem. There is no way to call something.Foo() since you do not have a type that is compatible. You could 'hack' around this by creating a 'wrapper' type that is compatible to call the Foo() and then 'unwrap' :
class MyNastyFix<T> : MyBaseClass
{
public T Unwrap()
{
//assert that T has the correct base type
if (!typeof(T).IsSubclassOf(typeof(MyBaseClass)))
throw new ArgumentException();
//must use reflection to construct
T obj = (T)typeof(T).InvokeMember(null, BindingFlags.CreateInstance, null, null, null);
//cast to a type of MyBaseClass so we can copy our values
MyBaseClass c = (MyBaseClass)(object)obj;
c.SomeValue = this.SomeValue;
return obj;
}
}
public static TResult Execute<TResult>()
{
return something.Foo<MyNastyFix<TResult>>().Unwrap();
}
Update: The reflection answer might be a better approach if that works.
Change Foo to check the constraints at run-time:
public T Foo<T>() {
if (!typeof(T).IsAssignableFrom(typeof(MyBaseClass))
|| !typeof(T).GetConstructor(...))
throw new System.NotImplementedException();
...
}
The generic constraints are checked at compile-time so they can't be based on run-time conditions.