we have developed an api and used the tool Nswag to automatically generate the Swagger api documentation. We have some endpoints in our api, where we want to update some fields by using inheritance. For nearer explanation, we have one update method (like POST api/person/{id}) where the user provide a json in the body and by giving the discriminator, the program knows the type, can deserialize the json string and use the right update method, like UpdateAddress or something. When the user does not give this information, then the deserialized object in our client is null and results in errors.
Now there is a problem, that the generated Swagger documentation does not show the discriminator 'property'. It rightly visualizes the inheritance structure with the properties by using this approach:
[JsonConverter(typeof(JsonInheritanceConverter), "discriminator")]
[KnownType(typeof(PersonUpdateAddressCommand))]
public class PersonCommand : CommandBase
{
}
The user does not know, that he have to provide the discriminator property, until we say this to him, but the documentation should be self-explanatory in best case.
To solve this, I added a public string property in the CommandBase class with the name 'discriminator':
public abstract class CommandBase
{
public string discriminator { get; set; }
}
Now it would visualize the property in the documentation, but this seems a bit over the top, because this discriminator 'property' is already existing somewhere in the heap, so why define an extra property?
Is there a way, to show the discriminator in the generated swagger documentation without defining an extra property? Or is this the right approach to add a string property?
The underlying library NJsonschema does not support system.text.json yet hence it doesn't work unless you use json.net as your serializer
Related
Is it possible to customize the generate Swagger documentation for ASP.NET CORE (C#)? Specifically, it seems to be changing the order properties are displayed in my model (i.e. it puts derived class properties first).
class BaseObj
{
string Username {get;set;}
string Password {get;set;}
}
class Obj2 : BaseObj
{
string SomeotherProp {get;set;}
}
Swagger generates:
SomeotherProp
Username
Password
I want SomeotherProp to be at the bottom. I've tried using the Display(Order=1) attribute, but Swagger ignores that. I didn't see any hook in the configuration that I can custom sort.
After a lot of research and trial and error, I stumbled across the answer. Swagger doesn't actually reflect upon your types directly, rather it uses Json.Net to get the type schema. Json.Net respects the JsonProperty attribute. So, on my base type, I can set the JsonProperty=-2 on the properties to get them to show first. Note that you have to use -2 and not -1 since that is reserved. By using -2, you don't have to set JsonProperty on all the derived types.
This will work in my case, but I found another post where the guy defined a custom contract resolver and sorted the properties there... that'll be more generic and cleaner then JsonProperty. Need to figure out how to hook that into Asp.net core though.
But anyways, the point is, the funky ordering is coming from Json.net and not Swagger.
I have a class like below, auto generated by Entity Framework, based in our database:
public partial class TB_Cliente
{
public int IDCliente { get; set; }
public string Nome { get; set; }
// other properties
}
I'm using DataContractJsonSerializer and I need to change the properties' names in serialization. For instance, the property IDCliente must be serialized like ClientID.
I can't use [DataMember] in top of the property, because the class is auto generated, and any future changes will generate the class again and these changes will be lost.
I've had the same problem in the past, when I wanted to use data annotations. I've found the below solution, creating another file and using an interface, which works perfectly:
public interface ITB_Cliente
{
[Required]
string Nome { get; set; }
// other properties
}
[MetadataType(typeof(ITB_Cliente))]
public partial class TB_Cliente : ITB_Cliente
{
}
But this solution doesn't help me now, because (as far as I know) this attribute must be set directly in the class. I've tried to set it in the interface and it didn't work.
Is there a way to change the properties' names in the serialization, in my case? Any help will be greatly appreciated.
You probably want to use DTOs for serialization. I have not tried but AutoMapper can probably do the heavy lifting for you.
I have been trying to overcome a similar problem this week for JSON output from some legacy VB.Net classes that I would prefer not to change if I can avoid it. The serialisation is returning underlying private member names rather than the public property names, e.g. "mFirstName".
Also for autogenerated property names I am getting json like
{"k__BackingField":"Brian","k__BackingField":"Furlong"}
which is not good.
I considered a similar approach to Pawel's above (create DTOs and use Automapper which I have used extensively before).
I am also checking to see if I can make a customised json serialiser but haven't got very far yet.
The third way I have investigated is to create an "Aspect" using PostSharp which will decorate the business entity classes with the DataContract.
This would allow me to create the necessary [DataContract] and [DataMember] attributes on the public properties at compile time without having to modify the legacy code base. As I am using the legacy assemblies within a new WebAPI assembly it effectively extends the code for me.
For guidance / hints please refer to the following links:
For background information http://pietschsoft.com/post/2008/02/NET-35-JSON-Serialization-using-the-DataContractJsonSerializer
For the question that gave the pointer: How to inject an attribute using a PostSharp attribute?
For a walkthrough on how to do something similar which is enough to get going on this: http://www.postsharp.net/blog/post/PostSharp-Principals-Day-12-e28093-Aspect-Providers-e28093-Part-1
I saw this in saample source code project.
[Input]
public int Length { get; set; }
It was defined in a class:
namespace PowerLanguage.Strategy
{
public class MovAvg_Cross_SE : SignalObject
{
....
What does the [input] mean?
That's an Attribute -- a way to declare information about your source code. What your particular attribute means depends on the namespace of the attribute. You can hover over it to get information on it or (if the declaration is part of your project) ctrl-click on it to see its source.
Its an attribute. The full class name is InputAttribute. Code can reflect over properties and discover attributes, which may modify the behavior or trigger other functionality. Another example of adding functionality is Data Annotations, which when used with something that will discover and run them, can be thought of as adding behavior. You can read more about attributes here (while older, the concept is the same).
So I've been studying the use of various Serializers in the .NET Framework and while trying to experiment on preventing certain objects in a class from being serialized I was thrusted back to some very basic programming questions that I "thought" I knew. Given this example:
public class Example
{
public string examName;
[XmlIgnore]
public int exampleNumber;
public Example()
{ }
[XmlIgnore]
public int ExampleNumberTwo { get; set; }
}
I can create an instance of this class and using the XMLSerializer can output the content of this class in XML format. The [XmlIgnore] attribute actually does what I'd expected; it prevents the serialization of the referenced items.
So venturing further I replaced the [XmlIgnore] declaration for "exampleNumber" with [NonSerializable] expecting the similar results but the output did not change. After searching through resources, it was stated that the [NonSerializable] attribute should only be used on fields and [XmlIgnore] attributes should be used on properties.
Yet another post stated that the [NonSerializable] attribute has no effect when using the XMLSerializer but will produce the expected results when using the SOAP or BinaryFormatter. So I'm lost on the concept at this point.
But this brought me to the basic question, what defines a field vs. a property? I know its a basic question and I've even viewed other discussions here but the degree of clarity I am looking for still wasn't really clear.
I can use the [XmlIgnore] attribute on the property (ExampleNumberTwo) or the variable (exampleNumber) so the statement that it can ONLY be used on Properties doesn't seem correct.
But then again, I have always referred to the objects in my example such as (examName) and (exampleNumber) as being member variables. So what exactly is the signature of a "Field"
Can anyone shed some light on this?
The MSDN documentation supports the idea that [NonSerialized] only gives the expected results with the binary and SOAP serializers:
When using the BinaryFormatter or SoapFormatter classes to serialize
an object, use the NonSerializedAttribute attribute to prevent a field
from being serialized. For example, you can use this attribute to
prevent the serialization of sensitive data.
The target objects for the NonSerializedAttribute attribute are public
and private fields of a serializable class. By default, classes are
not serializable unless they are marked with SerializableAttribute.
During the serialization process all the public and private fields of
a class are serialized by default. Fields marked with
NonSerializedAttribute are excluded during serialization. If you are
using the XmlSerializer class to serialize an object, use the
XmlIgnoreAttribute class to get the same functionality. Alternatively,
implement the ISerializable interface to explicitly control the
serialization process. Note that classes that implement ISerializable
must still be marked with SerializableAttribute.
In terms of "field" vs. "property", fields are straight data variables contained by a class. Properties are actually specially named methods on the class (get_PropName() and set_PropName()). In your code, the compiler allows you to use properties the same way you would use a field, and then inserts the appropriate get/set call for you.
Oftentimes, properties will be simple wrappers around a field:
private int myField;
public int MyProperty
{
get { return myField; }
set { myField = value; }
}
But they don't have to be:
public int TodaysDate
{
get { return DateTime.Today; }
}
In general, you want all your fields to be private, since they're supposed to be implementation details. Any simple data that you'd like to expose should be done via a property, since you can easily surround the data access with (changeable) logic.
In C#, the short answer is that properties have get and/or set methods, while fields do not. VB.NET makes it a little more evident by requiring the "Property" qualifier to be used to differentiate one.
With C#, you can just append " { get; set; }" to the end of a field's definition and it's now a property.
Where this really comes into play is in reflection. Fields and Properties are segregated from one another into different enumerable collections.
This answer to What are the differences between the XmlSerializer and BinaryFormatter will help you get started in the right direction.
My searches keep turning up only guides explaining how to use and apply attributes to a class. I want to learn how to create my own attribute classes and the mechanics of how they work.
How are attribute classes instantiated? Are they instantiated when the class they are applied to is instantiated? Is one instantiated for each class instantiated that it is applied to? E.g. if I apply the SerializableAttribute class to a MyData class, and I instantiate 5 MyData instances, will there be 5 instances of the SerializbleAttribute class created behind the scenes? Or is there just one instance shared between all of them?
How do attribute class instances access the class they are associated with? How does a SerializableAttribute class access the class it is applied to so that it can serialize it's data? Does it have some sort of SerializableAttribute.ThisIsTheInstanceIAmAppliedTo property? :) Or does it work in the reverse direction that whenever I serialize something, the Serialize function I pass the MyClass instance to will reflectively go through the Attributes and find the SerialiableAttribute instance?
I haven't use attributes in my day-to-day work before, but I have read about them.
Also I have done some tests, to back up what I'll say here. If I'm wrong in any place - feel free to tell me this :)
From what I know, attributes are not acting as regular classes. They aren't instantiated when you create an object that they are applied to, not one static instance, not 1 per each instance of the object.
Neither do they access the class that they are applied to..
Instead they act like properties (attributes? :P ) of the class. Not like the .NET class properties, more like in the "one property of glass is transparency" kind of property. You can check which attributes are applied to a class from reflection, and then act on it accordingly. They are essentially metadata that is attached to the class definition, not the objects of that type.
You can try to get the list of attributes on a class, method, property, etc etc.. When you get the list of these attributes - this is where they will be instantiated. Then you can act on the data within these attributes.
E.g. the Linq tables, properties have attributes on them that define which table/column they refer to. But these classes don't use these attributes. Instead, the DataContext will check the attributes of these objects when it will convert linq expression trees to SQL code.
Now for some real examples.. I've ran these in LinqPad, so don't worry about the strange Dump() method. I've replaced it with Console.WriteLine to make the code easier to understand for the people who don't know about it :)
void Main()
{
Console.WriteLine("before class constructor");
var test = new TestClass();
Console.WriteLine("after class constructor");
var attrs = Attribute.GetCustomAttributes(test.GetType()).Dump();
foreach(var attr in attrs)
if (attr is TestClassAttribute)
Console.WriteLine(attr.ToString());
}
public class TestClassAttribute : Attribute
{
public TestClassAttribute()
{
DefaultDescription = "hello";
Console.WriteLine("I am here. I'm the attribute constructor!");
}
public String CustomDescription {get;set;}
public String DefaultDescription{get;set;}
public override String ToString()
{
return String.Format("Custom: {0}; Default: {1}", CustomDescription, DefaultDescription);
}
}
[Serializable]
[TestClass(CustomDescription="custm")]
public class TestClass
{
public int Foo {get;set;}
}
The console result of this method is:
before class constructor
after class constructor
I am here. I'm the attribute constructor!
Custom: custm; Default: hello
And the Attribute.GetCustomAttributes(test.GetType()) returns this array:
(the table shows all available columns for all entries.. So no, the Serializable attribute does not have these properties :) )
Got any more questions? Feel free to ask!
UPD:
I've seen you ask a question: why use them?
As an example I'll tell you about the XML-RPC.NET library.
You create your XML-RPC service class, with methods that will represent the xml-rpc methods. The main thing right now is: in XmlRpc the method names can have some special characters, like dots. So, you can have a flexlabs.ProcessTask() xml rpc method.
You would define this class as follows:
[XmlRpcMethod("flexlabs.ProcessTask")]
public int ProcessTask_MyCustomName_BecauseILikeIt();
This allows me to name the method in the way I like it, while still using the public name as it has to be.
Attributes are essentially meta data that can be attached to various pieces of your code. This meta data can then be interogate and affect the behaviour of certain opperations.
Attributes can be applied to almost every aspect of your code. For example, attributes can be associated at the Assembly level, like the AssemblyVersion and AssemblyFileVersion attributes, which govern the version numbers associated with the assembly.
[assembly: AssemblyVersion("1.0.0.0")]
[assembly: AssemblyFileVersion("1.0.0.0")]
Then the Serializable attribute for example can be applied to a type declaration to flag the type as supporting serialization. In fact this attribute has special meaning within the CLR and is actually stored as a special directive directly on the type in the IL, this is optimized to be stored as a bit flag which can be processed much more efficiently, there are a few attributes on this nature, which are known as pseudo custom attributes.
Still other attributes can be applied to methods, properties, fields, enums, return values etc. You can get an idea of the possible targets an attribute can be applied to by looking at this link
http://msdn.microsoft.com/en-us/library/system.attributetargets(VS.90).aspx
Further to this, you can define your own custom attributes which can then be applied to the applicable targets that your attributes are intended for. Then at runtime your code could reflect on the values contained in the custom attributes and take appropriate actions.
For a rather naive example, and this is just for the sake of example :)
You might want to write a persistence engine that will automatically map Classes to tables in your database and map the properties of the Class to table columns. You could start with defining two custom attributes
TableMappingAttribute
ColumnMappingAttribute
Which you can then apply to your classes, as an example we have a Person class
[TableMapping("People")]
public class Person
{
[ColumnMapping("fname")]
public string FirstName {get; set;}
[ColumnMapping("lname")]
public string LastName {get; set;}
}
When this compiles, other than the fact that the compiler emits the additional meta data defined by the custom attributes, little else is impacted. However you can now write a PersistanceManager that can dynamically inspect the attributes of an instance of the Person class and insert the data into the People table, mapping the data in the FirstName property to the fname column and the LastName property to the lname column.
As to your question regarding the instances of the attributes, the instance of the attribute is not created for each instance of your Class. All instances of People will share the same instance of the TableMappingAttribute and ColumnMappingAttributes. In fact, the attribute instances are only created when you actually query for the attributes the first time.
Yes they're instantiated with the parameters you give it.
The attribute does not "access" the class. The attribute is attached to the class' / property's attribute list in the reflection data.
[Serializable]
public class MyFancyClass
{ ... }
// Somewhere Else:
public void function()
{
Type t = typeof(MyFancyClass);
var attributes = t.GetCustomAttributes(true);
if (attributes.Count(p => p is SerializableAttribute) > 0)
{
// This class is serializable, let's do something with it!
}
}
Think of attributes are post-its that are attached to the classes or method definitions (embedded in the assembly metadata).
You can then have a processor/runner/inspector module that accepts these types by reflecting, looks for these post-its and handles them differently. This is called declarative programming. You declare some behavior instead of writing code for them in the type.
Serializable attribute on a type declares that it is built to be serialized. The XmlSerializer can then accept an object of this class and do the needful. You mark the methods that need to be serialized/hidden with the right post-its.
another example would the NUnit. The NUnit runner looks at the [TestFixture] attributes all classes defined in the target assembly to identify test classes. It then looks for methods marked with [Test] attribute to identify the tests, which it then runs and displays the results.
You may want to run through this tutorial at MSDN which has most of your questions answered along with an example at the end. Although they could have extracted a method called
Audit(Type anyType); instead of duplicating that code. The example 'prints information' by inspecting attributes.. but you could do anything in the same vein.
If you take an eye out this downloadable open source code LINQ to Active Directory (CodePlex), you might find interesting the mechanism of the Attributes.cs file where Bart De Smet has written all of his attributes classes definitions. I have learned attributes there.
In short, you may specialize the Attribute class and code some specialized properties for your needs.
public class MyOwnAttributeClass : Attribute {
public MyOwnAttributeClass() {
}
public MyOwnAttributeClass(string myName) {
MyName = myName;
}
public string MyName { get; set; }
}
and then, you may use it wherever MyOwnAttributeClass gets useful. It might either be over a class definition or a property definition.
[MyOwnAttributeClass("MyCustomerName")]
public class Customer {
[MyOwnAttributeClass("MyCustomerNameProperty")]
public string CustomerName { get; set; }
}
Then, you can get it through reflection like so:
Attribute[] attributes = typeof(Customer).GetCustomAttribute(typeof(MyOwnAttributeClass));
Consider that the attribute you put between square brackets is always the constructor of your attribute. So, if you want to have a parameterized attribute, you need to code your constructor as such.
This code is provided as is, and may not compile. Its purpose is to give you an idea on how it works.
Indeed, you generally want to have a different attribute class for a class than for a property.
Hope this helps!
Not much time to give you a fuller answer, but you can find the Attributes that have been applied to a value using Reflection. As for creating them, you inherit from the Attribute Class and work from there - and the values that you supply with an attribute are passed to the Attribute class's constructor.
It's been a while, as you might be able to tell...
Martin