I am trying to make a binder for an abstract class. The binder decides which implementation of the class to use.
public abstract class Pet
{
public string name { get; set; }
public string species { get; set; }
abstract public string talk { get; }
}
public class Dog : Pet
{
override public string talk { get { return "Bark!"; } }
}
public class Cat : Pet
{
override public string talk { get { return "Miaow."; } }
}
public class Livestock : Pet
{
override public string talk { get { return "Mooo. Mooo. Fear me."; } }
}
So I have a controller which takes a Pet, the binder decides (depending on the species string) if it is a Dog, Cat or Livestock.
public class PetBinder : IModelBinder
{
public object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext)
{
var values = (ValueProviderCollection)bindingContext.ValueProvider;
var name = (string)values.GetValue("name").ConvertTo(typeof(string));
var species = (string)values.GetValue("species").ConvertTo(typeof(string));
if (species == "dog")
{
return new Dog { name = name, species = "dog" };
}
else if (species == "cat")
{
return new Cat { name = name, species = "cat" };
}
else
{
return new Livestock { name = name, species = species };
}
}
}
public class HomeController : Controller
{
public JsonResult WorksFine(Pet pet)
{
return Json(pet);
}
public JsonResult DoesntWork(List<Pet> pets)
{
return Json(pets);
}
}
This works well, but as soon as the pet is in another structure (like List<Pet> or another object), I get a NullReferenceException (on the line var name = (string)values.GetValue("name").ConvertTo(typeof(string));
in the PetBinder). What am I doing wrong?
I added a Person class to test. It also gave me a NullReferenceException.
public class Person
{
public string name { get; set; }
public Pet pet { get; set; }
}
public class HomeController : Controller
{
public JsonResult PersonAction(Person p)
{
return Json(p);
}
}
ccurrens said the reason var name = (string)values.GetValue("name").ConvertTo(typeof(string));
returned null was because it couldn't get the values from a list.
I see they are named [n].name and [n].species when in a List<Pet>, but when in the Person object they are named pet.name and pet.species and when they are in a single Pet, they are just named name and species.
Solution
To get the parameter names with the right prefix ([n] or pet or anything else) for GetValue, I used the following code:
bool hasPrefix = bindingContext.ValueProvider.ContainsPrefix(bindingContext.ModelName);
string prefix = ((hasPrefix)&&(bindingContext.ModelName!="")) ? bindingContext.ModelName + "." : "";
If anyone is interested, I ended up inheriting from DefaultModelBinder using something similar to this answer. Here is the full code I used:
public class DefaultPetBinder : DefaultModelBinder
{
protected override object CreateModel(ControllerContext controllerContext,ModelBindingContext bindingContext,Type modelType)
{
//https://stackoverflow.com/questions/5460081/asp-net-mvc-3-defaultmodelbinder-inheritance-problem
bool hasPrefix = bindingContext.ValueProvider.ContainsPrefix(bindingContext.ModelName);
string prefix = ((hasPrefix)&&(bindingContext.ModelName!="")) ? bindingContext.ModelName + "." : "";
// get the parameter species
ValueProviderResult result;
result = bindingContext.ValueProvider.GetValue(prefix+"species");
if (result.AttemptedValue.Equals("cat"))
return base.CreateModel(controllerContext,bindingContext,typeof(Cat));
else if (result.AttemptedValue.Equals("dog"))
return base.CreateModel(controllerContext,bindingContext,typeof(Dog));
return base.CreateModel(controllerContext, bindingContext, typeof(Livestock)); // livestock
}
}
In the line you're getting your error, values could be null or what GetValue("name") returns could be null.
I'm assuming when you're calling the List<Pet> method, ValueProvider is returning the entire List instead of each individual Pet, so it can't get the value "name" since it doesn't exist in the List class.
I can't be more sure without seeing more code.
Related
I am trying to print out an object that implements TableEntity class, without those that should be Ignored regarding the persistence. The approach I generally use to print out objects is to use the StatePrinter.
public class MyEntity : TableEntity
{
public string MyProperty { get; set; }
[IgnoreProperty]
public string MyIgnoredProperty { get; set; }
public override string ToString()
{
Stateprinter printer = new Stateprinter();
return printer.PrintObject(this);
}
}
While this works pretty good for any kind of classes, with this MyEntity class it also prints the MyIgnoredProperty. Is there a clever way to also ignore the properties that have [IgnoredProperty] as attribute when printing out the object?
You can configure what fields/properties the Stateprinter cares about by configuring what "field harvester" to use.
Here's a simple field harvester that only returns public properties without the 'IgnoreProperty' attribute.
class PersistencePropertiesHarvester : IFieldHarvester
{
public bool CanHandleType(Type type)
{
return typeof(TableEntity).IsAssignableFrom(type);
}
public List<SanitizedFieldInfo> GetFields(Type type)
{
var fields = new HarvestHelper().GetFieldsAndProperties(type);
return fields.Where(IsPerstistenceProperty).ToList();
}
private static bool IsPerstistenceProperty(SanitizedFieldInfo field)
{
return
// Only return properties ...
field.FieldInfo.MemberType == MemberTypes.Property
&&
// ... that has a public get method ...
(field.FieldInfo as PropertyInfo)?.GetGetMethod(false) != null
&&
// ... that does not have the IgnoreProperty attribute
field.FieldInfo.GetCustomAttribute<IgnoreProperty>() == null
;
}
}
Then you use it like this:
public class MyEntity : TableEntity
{
public string MyProperty { get; set; }
[IgnoreProperty]
public string MyIgnoredProperty { get; set; }
public override string ToString()
{
Stateprinter printer = new Stateprinter();
printer.Configuration.Add(new PersistencePropertiesHarvester());
return printer.PrintObject(this);
}
}
And the result of new MyEntity().ToString() is now
new MyEntity()
{
MyProperty = null
}
I have a ParentClass. Two classes are inherit from it, FirstChildClass and SecondChildClass. A class MultipleValueTypes contains a Dictionary and a method that adds values to it. My intention is to be able to pass values of different classes, which inherit from the same abstract class to the value parameter of the Dictionary. Therefore, I initialize the dictionary with the value List<ParentClass> so that I would be able to add objects made with the child classes to the Dictionary. I can do this, but I cannot access them, therefore in the abstract class I create a way to tell them apart, a virtual method that both the children classes override to return their own class type.
I test the values they return against the enum itself and based on whether the condition is fulfilled, the object would be casted as what it is instead of a List<ParentClass>. Is this the wrong approach? Is this impossible?
I think it should work, because in my thinking the FirstObject and SecondObject are still objects of their respective classes, so casting should work and I should be able to access the overridden method.
What doesn't work: I cannot access the method that returns what type of class it is, because it only gets methods from the List<ParentClass>.
What I've tried so far: searching for a way to access the method, but I did not find any.
What I still need help with: everything mentioned above.
public abstract class ParentClass
{
public string Name { get; set; }
public ParentClass(string Name)
{
this.Name = Name;
}
public enum ChildClasses
{
NoChildClass = 0,
FirstChildClass = 1,
SecondChildClass = 2
}
public virtual ChildClasses TypeOfClass()
{
return ChildClasses.NoChildClass;
}
}
public class FirstChildClass : ParentClass
{
private string _randomvalue;
public string RandomValue { get => _randomvalue; set => _randomvalue = value; }
public FirstChildClass(string Name) : base(Name)
{
}
public void ReturnMessage()
{
Console.WriteLine("This is the FirstChildClass");
}
public override ChildClasses TypeOfClass()
{
return ChildClasses.FirstChildClass;
}
}
public class SecondChildClass : ParentClass
{
private string _randomvalue;
public string RandomValue { get => _randomvalue; set => _randomvalue = value; }
public SecondChildClass(string Name) : base(Name)
{
}
public void ReturnMessage()
{
Console.WriteLine("This is the SecondChildClass");
}
public override ChildClasses TypeOfClass()
{
return ChildClasses.SecondChildClass;
}
}
class MultipleValueTypes
{
public Dictionary<string, List<ParentClass>> ADictionary = new Dictionary<string, List<ParentClass>>();
public void AddObject(string Name, ParentClass variable)
{
if (!ADictionary.ContainsKey(Name))
{
ADictionary.Add(Name, new List<ParentClass>());
}
ADictionary[Name].Add(variable);
}
}
class Program
{
static void Main(string[] args)
{
FirstChildClass FirstObject = new FirstChildClass("FirstObject");
SecondChildClass SecondObject = new SecondChildClass("SecondObject");
MultipleValueTypes TestDictionary = new MultipleValueTypes();
TestDictionary.AddObject("FirstObject", FirstObject);
TestDictionary.AddObject("SecondObject", SecondObject);
if(TestDictionary.ADictionary["FirstObject"].TypeOfClass() == ParentClass.ChildClasses.FirstChildClass) ///List<ParentClass>' does not contain a definition for 'TypeOfClass' and no accessible extension method 'TypeOfClass' accepting a first argument of type 'List<ParentClass>' could be found (are you missing a using directive or an assembly reference?)
{
TestDictionary.ADictionary["FirstObject"] = (FirstChildClass)TestDictionary.ADictionary["FirstObject"]; ///Cannot convert type 'System.Collections.Generic.List<Dictionary.ParentClass>' to 'Dictionary.FirstChildClass
}
}
}
You forgot to use indexer of the list value of the key of the dictionary here:
==> TestDictionary.ADictionary["FirstObject"][0]
Here is your code now refactored too:
class Program
{
static void Main(string[] args)
{
var FirstObject = new FirstChildClass("FirstObject");
var SecondObject = new SecondChildClass("SecondObject");
FirstObject.ReturnMessage();
SecondObject.ReturnMessage();
MultipleValueTypes TestDictionary = new MultipleValueTypes();
TestDictionary.AddObject("FirstObject", FirstObject);
TestDictionary.AddObject("SecondObject", SecondObject);
if ( TestDictionary.ADictionary["FirstObject"][0].TypeOfClass()
== ParentClass.ChildClasses.FirstChildClass )
{
TestDictionary.ADictionary["FirstObject"][0]
= (FirstChildClass)TestDictionary.ADictionary["FirstObject"][0];
}
Console.ReadKey();
}
}
public abstract class ParentClass
{
public string Name { get; set; }
public string RandomValue { get; set; }
public ParentClass(string Name)
{
this.Name = Name;
}
public virtual void ReturnMessage()
{
Console.WriteLine($"This is the {this.GetType().Name} instance");
}
public virtual ChildClasses TypeOfClass()
{
return ChildClasses.NoChildClass;
}
public enum ChildClasses
{
NoChildClass = 0,
FirstChildClass = 1,
SecondChildClass = 2
}
}
public class FirstChildClass : ParentClass
{
public FirstChildClass(string Name)
: base(Name)
{
}
public override ChildClasses TypeOfClass()
{
return ChildClasses.FirstChildClass;
}
}
public class SecondChildClass : ParentClass
{
public SecondChildClass(string Name)
: base(Name)
{
}
public override ChildClasses TypeOfClass()
{
return ChildClasses.SecondChildClass;
}
}
class MultipleValueTypes
{
public readonly Dictionary<string, List<ParentClass>> ADictionary
= new Dictionary<string, List<ParentClass>>();
public void AddObject(string Name, ParentClass variable)
{
if ( !ADictionary.ContainsKey(Name) )
{
ADictionary.Add(Name, new List<ParentClass>());
}
ADictionary[Name].Add(variable);
}
}
If the intention is to cast the whole list from List<ParentClass> to List<FirstChildClass> and List<SecondChildClass>, then Linq is your friend, just use the Cast function:
List<FirstChildClass> firstChildClasses = TestDictionary.ADictionary["FirstObject"]
.Cast<FirstChildClass>().ToList();
List<SecondChildClass> secondChildClasses = TestDictionary.ADictionary["SecondObject"]
.Cast<SecondChildClass>().ToList();
check that I have a customAttribute called ResponseAttributes. I have declared an interface and several concrete classes { Question 1, Question2, Question3 }
[AttributeUsage(AttributeTargets.Property)]
public class ResponseAttribute : Attribute { }
public interface IQuestion { }
public class Question1 : IQuestion
{
[Response]
public string Response { get; set; }
public Question1() { Response = "2+1"; }
}
public class Question2 : IQuestion
{
[Response]
public decimal Response { get; set; }
public Question2() { Response = 5; }
}
public class Question3 : IQuestion
{
[Response]
public string Response { get; set; }
public Question3() { Response = "2+1"; }
}
Now, what I'm trying to do is how to verify a class which contains that attribute is equals to another?
I mean:
List<IQuestion> questions = new List<IQuestion>()
{
new Question1(), new Question2()
};
Question3 question3 = new Question3();
foreach (var question in questions)
{
// how to verify this condition:
// if (customAttribute from question3 is equals to customAttribute fromquestion)
// of course the question3 is equals to 1
}
As you can, they are different types, that's the reason why I set as ResponseAttribute.
you could try using an interface, with Resposnse property (type object)
if you can not, you could use a class-level attribute that tell you the "Response property" than, you can use reflection on that property
Example:
public class ResponseAttribute : Attribute {
public string PropertyName { get; set }
}
[ResponseAttribute ("CustomResponse")}
public class Question1 {
public string CustomResponse;
}
via reflection
foreach(var question in questions) {
var responseAttr = (ResponseAttribute) question.GetType().GetCustomAttributes(typeof(ResponseAttribute));
var questionResponse= question.GetType().GetProperty(responseAttr.PropertyName,question,null);
}
try overriding equals method:
public override bool Equals(object obj)
{
if (obj is IQuestion)
return this.Response == ((IQuestion)obj).Response;
else
return base.Equals(obj);
}
hope this helps
I have made a custom binder for an abstract class. The binder decides which implementation to use.
It works well, but when I add a property which does not exist in the abstract class to a child class, it is always null.
Here is the code for the abstract class Pet and derived classes Dog and Cat.
public abstract class Pet
{
public string name { get; set; }
public string species { get; set; }
abstract public string talk { get; }
}
public class Dog : Pet
{
override public string talk { get { return "Bark!"; } }
}
public class Cat : Pet
{
override public string talk { get { return "Miaow."; } }
public string parasite { get;set; }
}
public class DefaultPetBinder : DefaultModelBinder
{
protected override object CreateModel(ControllerContext controllerContext,ModelBindingContext bindingContext,Type modelType)
{
bool hasPrefix = bindingContext.ValueProvider.ContainsPrefix(bindingContext.ModelName);
string prefix = ((hasPrefix)&&(bindingContext.ModelName!="")) ? bindingContext.ModelName + "." : "";
// get the parameter species
ValueProviderResult result;
result = bindingContext.ValueProvider.GetValue(prefix+"species");
if (result.AttemptedValue.Equals("cat")){
//var model = base.CreateModel(controllerContext, bindingContext, typeof(Cat));
return base.CreateModel(controllerContext,bindingContext,typeof(Cat));
}
else (result.AttemptedValue.Equals("dog"))
{
return base.CreateModel(controllerContext,bindingContext,typeof(Dog));
}
}
}
The controller just takes a Pet parameter and returns it as JSON.
If I send
{name:"Odie", species:"dog"}
I get back
{"talk":"Bark!","name":"Odie","species":"dog"}
For the Cat, there is a parasite property which does not exist in the abstract class Pet. If I send
{"parasite":"cockroaches","name":"Oggy","species":"cat"}
I get back
{"talk":"Miaow.","parasite":null,"name":"Oggy","species":"cat"}
I have tried this with other more complex classes, this is just a simple example.
I have looked in the debugger, the parasite value is in the value provider, the model the binder returns contains a field for the parasite.
Can anyone see where the problem is?
Try like this:
protected override object CreateModel(ControllerContext controllerContext,ModelBindingContext bindingContext,Type modelType)
{
bool hasPrefix = bindingContext.ValueProvider.ContainsPrefix(bindingContext.ModelName);
string prefix = ((hasPrefix)&&(bindingContext.ModelName!="")) ? bindingContext.ModelName + "." : "";
// get the parameter species
ValueProviderResult result;
result = bindingContext.ValueProvider.GetValue(prefix+"species");
if (result.AttemptedValue.Equals("cat"))
{
var model = Activator.CreateInstance(typeof(Cat));
bindingContext.ModelMetadata = ModelMetadataProviders.Current.GetMetadataForType(() => model, typeof(Cat));
return model;
}
else if (result.AttemptedValue.Equals("dog"))
{
var model = Activator.CreateInstance(typeof(Dog));
bindingContext.ModelMetadata = ModelMetadataProviders.Current.GetMetadataForType(() => model, typeof(Dog));
return model;
}
throw new Exception(string.Format("Unknown type \"{0}\"", result.AttemptedValue));
}
I am trying to run this code, but a is always null. I made sure that there's a Name, why does it always return null?
public Animal FindAnimal(string Name)
{
Animal a = Animals.Find(
delegate(Animal bk)
{
return bk.AnimalName.ToLower() == Name.ToLower();
}
);
return a;
}
Your code looks fine to me. Here are some small optimizations though:
public Animal FindAnimal(string name)
{
return Animals.Find((a) => String.Equals(a.AnimalName, name, StringComparison.CurrentCultureIgnoreCase));
}
EDIT
Test of code:
void Main()
{
var foundAnimal = FindAnimal("Monkey"); //Not null
}
List<Animal> Animals = new List<Animal>(){ new Animal() { AnimalName = "Monkey" }};
public Animal FindAnimal(string name)
{
return Animals.Find((a) => String.Equals(a.AnimalName, name, StringComparison.CurrentCultureIgnoreCase));
}
public class Animal
{
public string AnimalName { get; set; }
}