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Why is my implementation always calling the same derived class?

I have a base class that accepts a Type as a constructor argument, and two derived classes that inherit from that base class. I also have an interface of that base class, that I inject to use in other places.
When I call the base method "FormatValue", passing different types as parameters, I always get the same result (it call the method in one of the classes, ignoring my type parameter).
What am I doing wrong?
public interface IFormatService
{
string FormatValue(object value);
}
public abstract class FormatService : IFormatService
{
protected FormatService(Type type)
{ }
public abstract string FormatValue(object value);
}
public static class Program
{
private static void Main(string[] args)
{
var serviceProvider = new ServiceCollection()
.AddSingleton<IFormatService, CurrencyFormat>()
.AddSingleton<IFormatService, DateTimeFormat>()
.BuildServiceProvider();
var formatService = serviceProvider.GetService<IFormatService>();
Console.WriteLine(formatService.FormatValue(DateTime.Now));
Console.WriteLine(formatService.FormatValue(200));
Console.ReadLine();
}
}
public class CurrencyFormat : FormatService
{
public CurrencyFormat() : base(typeof(decimal))
{
}
public override string FormatValue(object value) => "CurrencyFormatter";
}
public class DateTimeFormat : FormatService
{
public DateTimeFormat() : base(typeof(DateTime))
{
}
public override string FormatValue(object value) => "DateTimeFormatter";
}
Current result:
DateTimeFormatter
DateTimeFormatter
Expected result:
DateTimeFormatter
CurrencyFormatter

Code pointed below overrides your previous CurrencyFormat registration, so it always resolves to DateTimeFormat.
var serviceProvider = new ServiceCollection()
.AddSingleton<IFormatService, CurrencyFormat>()
.AddSingleton<IFormatService, DateTimeFormat>() <---------
.BuildServiceProvider();

If you want to call different methods depending on the type of parameter, there are many ways to do it.
One way is by using the dynamic that choose the best overload at runtime:
public interface IFormatService
{
string FormatValue(object value);
}
public class FormatService : IFormatService
{
public string FormatValue(object value)
{
dynamic valueAsDynamic = value;
return FormatValueDynamic(valueAsDynamic);
}
string FormatValueDynamic(dynamic value) => FormatValuePrivate(value);
string FormatValuePrivate(DateTime value) => "DateTimeFormatter";
string FormatValuePrivate(decimal value) => "CurrencyFormatter";
string FormatValuePrivate(object value) => throw new NotSupportedException();
}
This way you can add all the methods you need.

Enforce an interface via parameter to an attribute [duplicate]

Is there a way to force the compiler to restrict the usage of a custom attribute to be used only on specific property types like int, short, string (all the primitive types)?
similar to the AttributeUsageAttribute's ValidOn-AttributeTargets enumeration.

No, you can't, basically. You can limit it to struct vs class vs interface, that is about it. Plus: you can't add attributes to types outside your code anyway (except for via TypeDescriptor, which isn't the same).

You can run this unit test to check it.
First, declare validation attribute PropertyType:
[AttributeUsage(AttributeTargets.Class)]
// [JetBrains.Annotations.BaseTypeRequired(typeof(Attribute))] uncomment if you use JetBrains.Annotations
public class PropertyTypeAttribute : Attribute
{
public Type[] Types { get; private set; }
public PropertyTypeAttribute(params Type[] types)
{
Types = types;
}
}
Create unit test:
[TestClass]
public class TestPropertyType
{
public static Type GetNullableUnderlying(Type nullableType)
{
return Nullable.GetUnderlyingType(nullableType) ?? nullableType;
}
[TestMethod]
public void Test_PropertyType()
{
var allTypes = AppDomain.CurrentDomain.GetAssemblies().SelectMany(a => a.GetTypes());
var allPropertyInfos = allTypes.SelectMany(a => a.GetProperties()).ToArray();
foreach (var propertyInfo in allPropertyInfos)
{
var propertyType = GetNullableUnderlying(propertyInfo.PropertyType);
foreach (var attribute in propertyInfo.GetCustomAttributes(true))
{
var attributes = attribute.GetType().GetCustomAttributes(true).OfType<PropertyTypeAttribute>();
foreach (var propertyTypeAttr in attributes)
if (!propertyTypeAttr.Types.Contains(propertyType))
throw new Exception(string.Format(
"Property '{0}.{1}' has invalid type: '{2}'. Allowed types for attribute '{3}': {4}",
propertyInfo.DeclaringType,
propertyInfo.Name,
propertyInfo.PropertyType,
attribute.GetType(),
string.Join(",", propertyTypeAttr.Types.Select(x => "'" + x.ToString() + "'"))));
}
}
}
}
Your attribute, for example allow only decimal property types:
[AttributeUsage(AttributeTargets.Property)]
[PropertyType(typeof(decimal))]
public class PriceAttribute : Attribute
{
}
Example model:
public class TestModel
{
[Price]
public decimal Price1 { get; set; } // ok
[Price]
public double Price2 { get; set; } // error
}

You could write code yourself to enforce correct use of your attribute class, but that's as much as you can do.

The code below will return an error if the attribute was placed on a property/field that is not List of string.
The line if (!(value is List<string> list)) may be a C#6 or 7 feature.
[AttributeUsage(AttributeTargets.Property |
AttributeTargets.Field, AllowMultiple = false)]
public sealed class RequiredStringListAttribute : ValidationAttribute
{
protected override ValidationResult IsValid(object value, ValidationContext context)
{
if (!(value is List<string> list))
return new ValidationResult($"The required attrribute must be of type List<string>");
bool valid = false;
foreach (var item in list)
{
if (!string.IsNullOrWhiteSpace(item))
valid = true;
}
return valid
? ValidationResult.Success
: new ValidationResult($"This field is required"); ;
}
}

The way I am doing this is following:
[AttributeUsage(AttributeTargets.Property)]
public class SomeValidationAttribute : ValidationAttribute
{
protected override ValidationResult IsValid(object value, ValidationContext validationContext)
{
if (value is not string stringToValidate)
{
throw new AttributeValueIsNotStringException(validationContext.DisplayName, validationContext.ObjectType.Name);
}
// validationContext.DisplayName is name of property, where validation attribut was used.
// validationContext.ObjectType.Name is name of class, in which the property is placed to instantly identify, where is the error.
//Some validation here.
return ValidationResult.Success;
}
}
And exception look like this:
public class AttributeValueIsNotStringException : Exception
{
public AttributeValueIsNotStringException(string propertyName, string className) : base(CreateMessage(propertyName, className))
{
}
private static string CreateMessage(string propertyName, string className)
{
return $"Validation attribute cannot be used for property: \"{propertyName}\" in class: \"{className}\" because it's type is not string. Use it only for string properties.";
}
}

InvalidCastException with custom generic TypeConverter for use within MCV environment, need fix

I need a TypeConverter that, idealy, translates a String to a type safe enum class (CountryIso) without having to write a converter for every type safe enum I'm about to make.
While I managed to get the following to work:
CountryIso cI = (CountryIso) "1";
I just can't get it to work with generics! The following sample doesn't work, but why?
TypeDescriptor.AddProvider(new ExplicitCastDescriptionProvider<CountryIso>(), typeof(CountryIso));
var descriptor = TypeDescriptor.GetConverter(typeof(CountryIso));
var result = descriptor.ConvertFrom("1");
I currently have a generic TypeConverter implementation:
public class ExplicitCastConverter<T>: TypeConverter
{
public override bool CanConvertFrom(ITypeDescriptorContext context, Type sourceType)
{
// Always true: the type determines if a cast is available or not
return true;
}
public override object ConvertFrom(ITypeDescriptorContext context, CultureInfo culture, object value)
{
String dummy = (String) value;
//CountryIso tst = (CountryIso) value; // Allowed, no problem casting
//CountryIso tst = (CountryIso) dummy; // Allowed, no problem casting
//var dum_001 = (T) ((String) value); // Does not compile
//var dumdum = (T) value; // Invalid case exception
//var hoot = (T) Convert.ChangeType(value, typeof (T)); // Invalid cast exception
return null;
}
}
The Provider is as follows:
//thanks: http://groups.google.com/group/wpf-disciples/browse_thread/thread/9f7bb40b7413fcd
public class ExplicitCastDescriptionProvider<T> : TypeDescriptionProvider //where T:TypeSafeEnum
{
public override ICustomTypeDescriptor GetTypeDescriptor(Type objectType, object instance)
{
return new ImplicitCastDescription<T>();
}
}
public class ImplicitCastDescription<T>: CustomTypeDescriptor //where T:TypeSafeEnum
{
public override TypeConverter GetConverter()
{
return new ExplicitCastConverter<T>();
}
}
I have a type safe enum implementation CountryIso (thanks StackOverflow!):
public sealed class CountryIso: TypeSafeEnum
{
private static readonly Dictionary<int, CountryIso> InstanceDict = new Dictionary<int, CountryIso>();
public static readonly CountryIso NL = new CountryIso(1, "NL", "Netherlands");
public static readonly CountryIso BE = new CountryIso(2, "BE", "Belgium");
private CountryIso(int value, String name, String description): base(value,name,description)
{
InstanceDict.Add(value, this);
}
public static Dictionary<int, CountryIso> Instances
{
get { return new Dictionary<int, CountryIso>(InstanceDict); }
}
public static explicit operator CountryIso(String i)
{
int index;
return Int32.TryParse(i,out index) ? InstanceDict[index] : null;
}
}
Which inherits from TypeSafeEnum:
public class TypeSafeEnum
{
protected TypeSafeEnum(int value, String name, String description)
{
Name = name;
Value = value;
Description = description;
}
public int Value{ get; private set; }
public String Name { get; private set; }
public String Description { get; private set; }
}

One option: use reflection
The problem lies with the static nature of the CountryIso members and (mostly) the casting operators. This prevents any blueprint definition enabling a generic typeconverter to know it can cast the CountryIso type safe enum. Also, you cannot cast 'down': a TypeSafeEnum can never become a CountryIso. Which is logical, but doesn't help.
[ Using reflection ]
Introduced a generic interface defining a casting method:
public interface ICast<out T>
{
T Cast(String obj);
}
Apply interface to CountryIso
public sealed class CountryIso: TypeSafeEnum , ICast<CountryIso>
Add interface as contstraint to the converter class
public class ExplicitCastConverter<T>: TypeConverter where T: ICast<T>
Add a (non-static) cast method to my CountryIso:
public new CountryIso Cast(String obj)
{
int index;
return Int32.TryParse(obj, out index) ? InstanceDict[index] : null;
}
Added a default static member to my type-safe-enum:
private static readonly CountryIso DefaultTypeSafeEnum = new CountryIso(
-1,
null,
null
);
Implement the ConvertFrom(..) in the Converter class:
T defaultMember = (T)typeof(T).GetField(
"DefaultTypeSafeEnum",
BindingFlags.NonPublic | BindingFlags.Static
).GetValue(null);
return defaultMember.Cast((String) value);
[ Type safe enum security ]
It's still possible to create & insert new instances of CountryIso via reflection (especially when using the InstanceDict for easy instance access!) Some sample code:
ConstructorInfo ci = typeof (T).GetConstructor(
BindingFlags.NonPublic|BindingFlags.Instance,
null,
CallingConventions.Standard,
new [] {typeof (int), typeof (String), typeof (String)},
new ParameterModifier[0]
);
CountryIso countryIso = (CountryIso) ci.Invoke(new object[]{30, "ZB", "Zanzibar"});
I now consider the usage of the private InstanceDict member a security hole (not a big one, since I'm not programming an API for the outside world, but still...)

Finding out type of a generic class

Lets say I have a generic class:
class Foo {
// protected Type t;
// public void SetT(string strval) {
// ((Foo<t>)this).Set(strval);
// }
}
class Foo<T> : Foo {
private T val;
public void Set(string strval) {
if (this is Foo<float>) {
this.val = float.Parse(strval);
} else if (this is Foo<int>) {
this.val = int.Parse(strval);
}
}
}
Now I create an object and put it in an ArrayList:
ArrayList a = new ArrayList();
a.Append(new Foo<float>);
And then I forget the type of Foo<>. Now, how do I Set? I tried the obvious candidates:
(Foo)a[0].Set("5.0");
(Foo<a[0].GetType()>)a[0].Set("5.0");
but those failed.
Is there a way I can call that Set method without explicitly knowing the type of Foo<>?
If not, can I somehow save type of Foo into Foo.t, and then uncomment and use Foo.SetT?
Ah, generics. Very nice tool if you know how to use them :-)
Regards,
dijxtra

One way is to make your generic Foo class implement an interface:
interface IFoo {
void Set(string strval);
}
class Foo<T> : IFoo {
private T val;
public void Set(string strval) {
...
}
}
Then you can cast to IFoo and call Set():
((IFoo)a[0]).Set("5.0");

There's absolutely no reason to be using generics here. Generics are intended to be used when the type of operations you will be performing are generic. In other words, they are independent of the type(s) on which they are performed. You are doing the opposite: the operation will be different depending on the types.
Given that, you should remove the generic parameter, make Set() and Foo abstract, and derive appropriate classes to handle the different types:
abstract class Foo
{
public abstract void Set(string value);
}
class FooDouble : Foo
{
double val;
public override void Set(string value)
{
this.val = double.Parse(value);
}
}
// Etc.
Then, you should be storing your Foos in a List<T>:
List<Foo> fooList = new List<Foo>();
fooList.Add(new FooDouble());
Later, you can say this:
fooList[0].Set("5.0");
And it will just work! No need to remember!

You want to override the implementation of Set in the derived classes.
class Foo {
public virtual void Set(string val);
}
class Foo<T> : Foo {
public override void Set(string val);
}

In addition to what Jimmy pointed out for your base class, you could use a generic collection instead of an ArrayList and make use of a type converter:
public interface IFoo
{
void Set(string value);
}
public class Foo<T> : IFoo
{
private T val;
public void Set(string value)
{
var typeConverter = TypeDescriptor.GetConverter(typeof(T));
if(typeConverter.CanConvertFrom(typeof(string)))
{
val = (T)typeConverter.ConvertFromString(value);
}
else
{
throw new InvalidOperationException();
}
}
}
The above will work with either your ArrayList:
ArrayList a = new ArrayList();
a.Append(new Foo<float>());
((IFoo)a[0]).Set("123.4");
Or with a typed collection:
List<IFoo> list = new List<IFoo>();
list.Add(new Foo<float>());
list[0].Set("123.4");
As an added bonus, you don't need to have an if statement in your Set method and try to account for all possible types.

If you want to know the type parameter that was used in you generic, use the GetGenericArguments method.
class Foo<T> {
int input_as_int;
float input_as_float;
public void Set(string strval) {
if (this.GetType().GetGenericArguments().First() == typeof(float)) {
this.input_as_float = float.Parse(strval);
} else if (this.GetType().GetGenericArguments().First() == typeof(int)) {
this.input_as_int = int.Parse(strval);
}
// Else .. throw an exception? return default value? return 0? what makes sense to your application
}
or alternately if you could by pass the Interface entirely and pass the input string in the constructor.
public class Foo<T>
{
public Foo (string input)
{
var typeConverter = TypeDescriptor.GetConverter(typeof(T));
if (typeConverter.CanConvertFrom(typeof(string)))
{
Value = (T)typeConverter.ConvertFromString(input);
}
else
{
throw new InvalidOperationException();
}
}
public T Value { get; set;
}
}
then you can just use it like so.
var test = new List<int> Foo ("3");

using System;
using System.Collections;
using System.Collections.Generic;
class Foo {
}
class Foo<T> : Foo {
private T val;
public void Set(string strval) {
var _type = typeof(T);
val = (T)(_type.InvokeMember("Parse", System.Reflection.BindingFlags.InvokeMethod, null, null, new Object[] { strval }));
}
override public string ToString(){
return String.Format("{0}", val);
}
}
class Sample {
static void Main(string[] args){
ArrayList a = new ArrayList();
a.Add(new Foo<float>());
a.Add(new Foo<int>());
dynamic ax = a[0];
ax.Set("5.5");
ax = a[1];
ax.Set("55");
//EDIT
//But I may have to set the float value to Foo <int> If you forgot
// ((Foo<float>)a[0]).Set("5.5");
// ((Foo<int>)a[1]).Set("55");
Console.WriteLine("{0},{1}", a[0], a[1]);
}
}

Duck Typing DynamicObject derivate

I wrote a class that allows a derivate to specify which of its properties can be lazy loaded. The code is:
public abstract class SelfHydratingEntity<T> : DynamicObject where T : class {
private readonly Dictionary<string, LoadableBackingField> fields;
public SelfHydratingEntity(T original) {
this.Original = original;
this.fields = this.GetBackingFields().ToDictionary(f => f.Name);
}
public T Original { get; private set; }
protected virtual IEnumerable<LoadableBackingField> GetBackingFields() {
yield break;
}
public override bool TryGetMember(GetMemberBinder binder, out object result) {
LoadableBackingField field;
if (this.fields.TryGetValue(binder.Name, out field)) {
result = field.GetValue();
return true;
} else {
var getter = PropertyAccessor.GetGetter(this.Original.GetType(), binder.Name);
result = getter(this.Original);
return true;
}
}
public override bool TrySetMember(SetMemberBinder binder, object value) {
LoadableBackingField field;
if (this.fields.TryGetValue(binder.Name, out field)) {
field.SetValue(value);
return true;
} else {
var setter = PropertyAccessor.GetSetter(this.Original.GetType(), binder.Name);
setter(this.Original, value);
return true;
}
}
}
And a derivate class:
public class SelfHydratingPerson : SelfHydratingEntity<IPerson> {
private readonly IDataRepository dataRepository;
public SelfHydratingDerivate(IDataRepository dataRepository, IPerson person)
: base(person) {
this.dataRepository = dataRepository
}
protected override IEnumerable<LoadableBackingField> GetBackingFields() {
yield return new LoadableBackingField("Address", () => this.dataRepository.Addresses.Get(this.Original.AddressID));
}
}
This works perfectly fine for getting and settings property values, but I get a either a RuntimeBinderException when I implicitly cast or an InvalidCastException with an explicitly cast SelfHydratingEntity back to T.
I know that you can override the DynamicObject.TryConvert method, but I'm wondering what exactly to put in this method. I've read a lot about duck typing today, and have tried out several libraries, but none of them work for this particular scenario. All of the libraries I've tried today generate a wrapper class using Reflection.Emit that makes calls to "get_" and "set_" methods and naturally use reflection to find these methods on the wrapped instance. SelfHydratingEntity of course doesn't have the "get_" and "set_" methods defined.
So, I'm wondering if this kind of thing is even possible. Is there any way to cast an instance of SelfHydratingEntity to T? I'm looking for something like this:
var original = GetOriginalPerson();
dynamic person = new SelfHydratingPerson(new DataRepository(), original);
string name = person.Name; // Gets property value on original
var address = person.Address; // Gets property value using LoadableBackingField registration
var iPerson = (IPerson)person;
- or -
var iPerson = DuckType.As<IPerson>(person);

Have you seen this Duck Typing project. It looks pretty good. I have just found a great example from Mauricio. It uses the Windsor Castle dynamic proxy to intercept method calls
Using the code from Mauricio the following code works like a dream
class Program
{
static void Main(string[] args)
{
dynamic person = new { Name = "Peter" };
var p = DuckType.As<IPerson>(person);
Console.WriteLine(p.Name);
}
}
public interface IPerson
{
string Name { get; set; }
}
public static class DuckType
{
private static readonly ProxyGenerator generator = new ProxyGenerator();
public static T As<T>(object o)
{
return generator.CreateInterfaceProxyWithoutTarget<T>(new DuckTypingInterceptor(o));
}
}
public class DuckTypingInterceptor : IInterceptor
{
private readonly object target;
public DuckTypingInterceptor(object target)
{
this.target = target;
}
public void Intercept(IInvocation invocation)
{
var methods = target.GetType().GetMethods()
.Where(m => m.Name == invocation.Method.Name)
.Where(m => m.GetParameters().Length == invocation.Arguments.Length)
.ToList();
if (methods.Count > 1)
throw new ApplicationException(string.Format("Ambiguous method match for '{0}'", invocation.Method.Name));
if (methods.Count == 0)
throw new ApplicationException(string.Format("No method '{0}' found", invocation.Method.Name));
var method = methods[0];
if (invocation.GenericArguments != null && invocation.GenericArguments.Length > 0)
method = method.MakeGenericMethod(invocation.GenericArguments);
invocation.ReturnValue = method.Invoke(target, invocation.Arguments);
}
}

impromptu-interface
https://github.com/ekonbenefits/impromptu-interface
Can static cast interfaces onto objects derived from DynamicObject.