I have previously created methods that take a property as input using the following syntax:
public void MyMethod<T>(Expression<Func<T>> property) { }
public int MyProperty { get; set; }
public void SomeOtherMethod() {
MyMethod(() => MyProperty);
}
That way I can, for example, cast the property to a MemberExpression and get the fully qualified name of the property.
However, is it possible to do the same in a class constructor, or have a property that stores another property, like how the property variable does in MyMethod()?
I have tried the following, but that didn't work because the type parameter is not specified in either cases, but I wouldn't even know which type to use... especially because the type is magically inferred in the topmost example.
In constructor
public class MyClass<T> {
public MyClass(Expression<Func<T>> property) { }
}
public int MyProperty { get; set; }
var myClass = new MyClass(() => MyProperty);
As property
public class MyClass<T> {
public Expression<Func<T>> SomeProperty { get; set; }
}
public int MyProperty { get; set; }
var myClass = new MyClass() { SomeProperty = () => MyProperty };
So, how can I have a property expression in a constructor or property of its own without having the use the type definition?
Is it possible to [infer generic parameters] in a class constructor?
In C# generic inference is not supported on constructors, so you have to set it explicitly:
var myClass = new MyClass<int>(() => MyProperty);
The main reason it's not supported is because there are often many types that could be used as the generic constraint due to implicit type conversions, so the compiler would have to choose the "best" generic parameters based on the information that it has. The costs associated with designing, programming, testing, documenting, and integrating this logic outweigh the benefit, or at least don't provide a relative benefit that exceeds the other features that MS could spend time developing.
In this case you know the best generic parameter type because the MyProperty property returns an int. Technically you could use any type that is implicitly convertible from int (object, double, etc.) but int is the best fit.
Could I maybe use object as a catch-all?
Sure, but then your code isn't generic. You can just do
public class MyClass {
public MyClass(Expression<Func<object>> property) { }
}
You need to supply the type parameter of T
var myClass = new MyClass<int>(() => MyProperty);
var myClass = new MyClass<int>() { SomeProperty = () => MyProperty };
Otherwise it won't know what type of expression it should get in the constructor.
Related
I'm trying to convert some anonymous type back to its original strong type class.
I have some legacy code (which I cannot touch) which create an anonymous class:
public class Cat : FooId
{
public int Id { get; set; }
public string Name { get; set; }
}
var result = new
{
Id = Mapper.Map<TFooId>(someCat)
};
NOTE: I've tried to make this fake class and interface similar to my code.
This then gives me:
result.GetType().ToString() : <>f__AnonymousType1``1[MyProject.Cat]
From here, I'm not sure how to convert this back to a MyProject.Cat instance?
I've tried (and fails):
(MyProject.Cat)result
(dynamic)result
but both fail. The dynamic doesn't throw an error ... but I can't access any properties in it.
C# is a statically typed language, and those two types are not in any way related to one another. Unless you're able to modify the code which defines those types, the way you'd convert from one to the other would be to create a new instance of the target type and populate it from the source object.
For example:
var resultCat = new Cat { Id = result.Id };
Edit: From comments it looks like it may be possible that the Id property on the result object may be an instance of Cat or some other object? You're going to need to do some debugging to find out what your types are.
But the overall concept doesn't really change. If you have an instance of Cat in your results then you can use that instance. If you don't then in order to create one you'd need to create a new instance and populate it with the data you have. Even if two types are intuitively or semantically similar, they are different types.
It's true what David said with regard to the fact that C# is a statically-typed language and that the new instance should be populated from the source the way he suggested.
However, there are work-arounds (though less performant) for that, such as reflection.
Consider you have a console app where you have defined ObjectExtensions as follows:
public static class ObjectExtensions
{
public static TOut Map<TOut>(this object #in)
where TOut : new()
{
TOut #out = new TOut();
if (#in?.GetType() is Type tin)
{
Type tout = typeof(TOut);
foreach ((PropertyInfo pout, PropertyInfo pin) in tout.GetProperties().Join(tin.GetProperties(), pi => pi.Name, pi => pi.Name, (pout, pin) => (pout, pin)))
{
pout.SetValue(#out, pin.GetValue(#in));
}
}
return #out;
}
}
And Class1 as follows:
public class Class1
{
public string A { get; set; } = "A";
public string B { get; set; } = "B";
public string C { get; set; } = "C";
public override string ToString()
{
return $"{{A={A}, B={B}, C={C}}}";
}
}
You will be able to map your anonymous type back to its original strongly-typed class like this:
Console.WriteLine(new { A = "Anonymous A", B = "Anonymous B", C = "Anonymous C" }.Map<Class1>());
Therefore the bloc above should show the following output:
{A=Anonymous A, B=Anonymous B, C=Anonymous C}
In this case, of course, I have assumed that Class1 (Cat in your example) must have a public parameterless constructor. That may not always be the case. There are more sophisticated scenarios of course that might involve other techniques for creating the object such as cloning or dependency injection. Just saying that the idea of yours is possible.
This question already has answers here:
Generic list of generic objects
(3 answers)
Closed 4 years ago.
I have an object that I want to put into a List or Collection. Is there a way to do that without having the T specified?
I want to do something like this: List<CommonProperty<T>>
Here's the object for reference:
internal class CommonProperty<T>
{
public string Name { get; set; }
public PropType Type { get; set; }
public List<T> PossibleValues { get; set; }
private T _value;
public T Value
{
get { return _value; }
set
{
if (!_value.Equals(value))
{
_value = value;
}
}
}
}
No, you can't use open generic types like that.
You could have a List<CommonProperty<T>> within a context where T is already a type parameter:
public class Foo<T>
{
static void Bar()
{
// This is fine, but is not what you're looking for - it uses
// the type parameter T as the type argument
List<CommonProperty<T>> list = new List<CommonProperty<T>>();
}
}
Typically the solution here is to have a non-generic base class or interface which the generic class or interface derives from:
// Consider making it abstract
internal class CommonProperty
{
public string Name { get; set; }
public PropType Type { get; set; }
}
internal class CommonProperty<T> : CommonProperty
{
public List<T> PossibleValues { get; set; }
private T _value;
public T Value
{
get => _value;
set
{
// TODO: Is this really necessary?
if (!_value.Equals(value))
{
_value = value;
}
}
}
}
You can then create a List<CommonProperty>... although be aware that it's entirely possible that you could end up with an element which wasn't a CommonProperty<T> at all that way.
From the List<CommonProperty> you'd be able to retrieve the names and types of all the properties - but the values wouldn't be available without casting to the specific type. You could have an abstract object Value { get; set; } property in the base class, which was then overridden in the derived class, potentially - but it's not clear whether that's necessary or helpful in your use case.
I think the closest you can get is to define an interface to match an un-typed (specific) CommonProperty, using Object instead of the T. Then have your CommonProperty<T> implement that interface. Now you can use the interface with your list.
But this isn't great. You'll lose a lot of nice type checking, and have to do more casting. If this is the primary way you'll use these objects, there's not much point to having a generic class at all anymore.
It is not possible to put a mixture of generics instantiated with different type arguments into the same collection. Such collection would not be useful anyway, because the caller would be expected to supply T for each item at compile time.
Suppose you could do what you want, i.e.
// imagine that you could do this
List<CommonProperty<T>> mixedList = GetAllProperties();
You would be forced to supply T once you start using items from that list, i.e.
foreach (CommonProperty<T> prop in mixedList) {
... // ^^^
... // Here you would need to specify T, but you don't know which one
}
In other words, such list would be unusable.
A list of properties of a specific type, on the other hand, would be useful:
List<CommonProperty<string>> stringPropList = GetPropertiesOfType<string>();
foreach (CommonProperty<string> prop in stringPropList ) {
...
}
Therefore, one solution to your problem would be building GetPropertiesOfType<T> method returning List<CommonProperty<T>> bound to type T from the caller.
An alternative approach would be to make CommonProperty non-generic, and let the caller check the type at runtime:
internal class CommonProperty {
public string Name { get; set; }
public PropType Type { get; set; }
public List<T> GetPossibleValues<T> { get; set; }
private object _value;
public T GetValue<T>() {
return (T)_value;
}
public void SetValue<T>(T val) {
_value = val;
}
}
I'm trying to implement a class to access items of different types, in a similar way to database rows.
However, I have two different ideas in mind, and I don't know which one to choose:
Design 1
public enum ObjectTypeA
{
Undefined,
Integer,
Float
}
public class MyObjectA
{
private object val;
public ObjectTypeA Type
{
get;
private set;
}
public int Integer
{
get
{
if (Type != ObjectTypeA.Integer) throw new Exception();
return (int)val;
}
set
{
Type = ObjectTypeA.Integer;
val = value;
}
}
public float Float
{
get
{
if (Type != ObjectTypeA.Float) throw new Exception();
return (float)val;
}
set
{
Type = ObjectTypeA.Float;
val = value;
}
}
}
Less compile-time checks possible.
Can't use the is operator, GetType(), etc. (reinvents the type system).
Boxing and unboxing for value types.
Can be inherited by other classes (e.g. I can create a "named object" using inheritance).
Design 2
public abstract class MyObjectB
{
}
public class MyIntegerB : MyObjectB
{
public int Value
{
get;
set;
}
public MyIntegerB(int _value)
{
Value = _value;
}
}
public class MyFloatB : MyObjectB
{
public float Value
{
get;
set;
}
public MyFloatB(float _value)
{
Value = _value;
}
}
Shorter and simpler implementation.
Very verbose (casting) to use.
Performance is not critical, but it's still important, since most of the objects that are going to be stored are integers or floats, so boxing overhead matters.
The classes will just contain the values, not methods that depend on the type, etc. so it doesn't matter if the solution uses inheritance.
IMPORTANT: One of the requirements is that there may be two types that use the same underlying type (e.g. two classes derived from MyObjectB may use int as the Value), so using object or generics may not be possible.
Any suggestion about which design to use, or another different design?
EDIT:
The reason I don't like the second one is because it's very verbose to use:
MyObjectB objB = new MyIntegerB(12);
Console.WriteLine(((MyIntegerB)objB).Value);
And because I can't inherit it to create something like a "named object", so I have to attach MyObjectB to the class, and the usage is even more verbose.
I don't see why you wouldn't use generics here. More strongly: I don't see why you need this at all: It seems like Nullable<T> would cover all of your use cases very nicely. If not, implementing this generically is trivial:
public class ValueWrapper<T>
{
public T Value
{
get;
private set;
}
public Type WrappedType
{
get { return typeof(T); }
}
}
public MySpecialInt : ValueWrapper<int>
{
/* etc */
}
why not use generics?
public abstract class MyObjectB<T>
{
public T Value
{
get;
set;
}
public MyObjectB(T _value)
{
Value = _value;
}
}
you only need one class at this point. just instantiate it differently:
var myObj = new MyObjectB<Int>(1);
or
var myObj = new MyObjectB<Float>(0.012);
I know you mentioned not wanting to deal with boxing and unboxing, but I still think a Generic class would be your best bet here.
public class MyObject<T>
{
public MyObject(T t) {
Value = t;
}
public T Value { get; set; }
}
Edit:
One of the requirements is that there
may be two types that use the same
underlying type (e.g. two classes
derived from MyObjectB may use int as
the Value), so using object or
generics may not be possible.
That would only apply if you're extending the class. There's no problem if you wrap the class instead, i.e. create a MyObject<int> and access its Value property, rather than subclassing it.
Having said that, if you want to subclass a generic class, the subclass would also need to be a generic class.
Have you considered generics?
public class MyObjectA<T> {
public T Value {
get; set;
}
}
I've written a similar class that could hold either a single instance of ClassX or an array of ClassX. The trick was that it could change during runtime, so a generic wouldn't suffice, but I still wanted it strong-typed in all cases. It sounds like that's similar to what you're trying to accomplish here.
I chose the first option, and here's why: Wherever possible, I encapsulate complexity within a class to make the class easier to use. Classes should encapsulate away complexity from the caller, making calls to it more concise. If using MyObjectB makes your code more verbose, than I don't think that's the right answer.
if you need heterogeneous collections then this would do.
public enum ObjectTypeA
{
Undefined,
Integer,
Float
}
public class MyObjectA
{
public MyObjectA(object value) : this(value, InfereType(value))
{ }
public MyObjectA(object value, ObjectTypeA type)
{
Value = value;
Type = type;
}
public object Value { get; private set; }
public ObjectTypeA Type
{
get;
private set;
}
public T ValueAs<T>()
{
return (T)Value;
}
}
then use it like
List<MyObjectA> list = GetAllValues();
foreach (var item in list)
{
switch (item.WrappedType)
{
case MyObjecttypeA.Float:
float f = item.ValueAs<float>();
// do something with float
}
}
This question already has answers here:
Making a generic property
(3 answers)
Closed 9 years ago.
I have a class:
public class class1
{
public string Property1 {get;set;}
public int Property2 {get;set;}
}
Which will be instantiated:
var c = new class1();
c.Property1 = "blah";
c.Property2 = 666;
So bear with me (I am new to generics), I need another class with a property of a generic type so that Property1 or Property2 can be used to set Property3:
public class Class2
{
public GenericType Property3 {get;set;}
}
I want to be able to:
var c2 = new class2();
c2.Property3 = c1.Property2 // Any property of any type.
#bytenik I think the originator is asking that class3 be defined to contain a generic property. That way when he / she has a property from class1 or class2 which in this case is a string / int that class3's property could handle either case.
public class Class3<T>
{
public T Property3 {get;set;}
}
I think the intent is the poster wants to do this:
Class3.Property3 = Class2.Property2
I think the poster will need to cast it to type T for this to work though.
Look at the link that was posted for an example: Making a generic property
Here is what you can do:
namespace GenericSO
{
public class Class1
{
public int property1 { get;set;}
}
public class Class2<T>
{
public T property2 { get; set; }
}
class Program
{
static void Main(string[] args)
{
Class1 c1 = new Class1();
c1.property1 = 20;
Class2<int> c2 = new Class2<int>();
c2.property2 = c1.property1;
}
}
}
Notice how your template property2 gets the value of property1.
You have to tell it what kind of generic.
public class class1<T>
{
public T Property3 {get;set;}
}
Regarding to edited version of the question:
If you need a property, which can be set with any type, the most reasonable solution here is to simply use property of type Object. For C# compiler there is no way to find out instance of which exactly type you've previously pushed into property setter.
I think you may have misunderstood generics. Another word that could be used is "template" but that is avoided because it is used for more advanced things in C++.
The following will create a generic class of a currently undefined type T.
public class Class2<T>
{
public T Property3 { get; set; }
}
To use this you need to specify the missing type:
var x = new Class2<int>();
This will create an object that has a property Property3 that is of type int.
... or ...
var y = new Class2<string>();
This will create an object that has a property Property3 that is of type string.
From your question I believe you actually want a type where you can assign any type to it at runtime, but this is not what generics provide.
I have a class that stores a serialized value and a type. I want to have a property/method returning the value already casted:
public String Value { get; set; }
public Type TheType { get; set; }
public typeof(TheType) CastedValue { get { return Convert.ChangeType(Value, typeof(_Type)); }
Is this possible in C#?
It's possible if the class containing the property is generic, and you declare the property using the generic parameter:
class Foo<TValue> {
public string Value { get; set; }
public TValue TypedValue {
get {
return (TValue)Convert.ChangeType(Value, typeof(TValue));
}
}
}
An alternative would be to use a generic method instead:
class Foo {
public string Value { get; set; }
public Type TheType { get; set; }
public T CastValue<T>() {
return (T)Convert.ChangeType(Value, typeof(T));
}
}
You can also use the System.ComponentModel.TypeConverter classes to convert, since they allow a class to define it's own converter.
Edit: note that when calling the generic method, you must specify the generic type parameter, since the compiler has no way to infer it:
Foo foo = new Foo();
foo.Value = "100";
foo.Type = typeof(int);
int c = foo.CastValue<int>();
You have to know the type at compile time. If you don't know the type at compile time then you must be storing it in an object, in which case you can add the following property to the Foo class:
public object ConvertedValue {
get {
return Convert.ChangeType(Value, Type);
}
}
Properties, events, constructors etc can't be generic - only methods and types can be generic. Most of the time that's not a problem, but I agree that sometimes it's a pain. Brannon's answer gives two reasonable workarounds.
I don't believe the example you've given here is possible. The type of CastedValue has to be defined at compile time, which means it can't depend on a runtime value (the value of the TheType property).
EDIT: Brannon's solution has some good ideas for how to handle this using a generic function rather than a property.