I'm trying to find the vocabulary to describe what I want and if it exists.
I have a table that shows a few data points from large objects. Loading the entire objects just for the table is very slow. Is there a way to only pass to the front the few properties I want without having to define a new object?
I found something called Sparse Fieldsets in JSON API, and I'm wondering if something like this exists for .NET under another name.
Update:
Talking to another coder, I realize it probably makes more sense to implement something like this between the backend and the database and make a specific call for this table. I still need to work out if I need to create a new object to support this. I think it'd still be faster if I just kept the same object but nulled out all of the connecting objects that I don't need for the table. But maybe that's considered bad practice? Also we're using Entity Framework for what it's worth.
Update 2:
I just created a new query without all of the .Include() and works well enough for what I need:
_dataContext.ApplePie
.Include(f => f.Apples).ThenInclude(f => f.Apple)
.Include(f => f.Sugars).ThenInclude(f => f.MolecularStructure)
.Include(f => f.Recipe)
Maybe you are looking for Anonymous Types?
For example, if you had a typed object with three properties, but you only wanted to operate on two:
var threePropThing = new ThreePropertyThing { Id = 1, Message = "test", ExtraProperty = "ex" };
var myAnonThing = new { Id = threePropThing.Id, Message = threePropThing.Message };
Best practice would be to not pass this anonymous object around. But, if you really needed to, you could return it as type object.
Typically, when passing data around in c#, you want to have it typed.
C# is a strongly-typed language and I would say that it is unusual for C# to support scenarios, when object definition (properties) are not known in advance, like in JSON API "fields" parameter case. Implementing this would imply using reflection to filter the properties, which is usually slow and error-prone.
When implementing C# web-services, people usually create one DTO response model per each request.
If your table has fixed set of fields, I would personally recommend to create a DTO class containing only the fields which are required for your table, and then create a method which returns this response for your specific request. While it doesn't align with "without having to define a new object" in the question, it makes the intention clear and makes it really easier to maintain the API in future.
You might want to use libraries like AutoMapper to save time and avoid duplicated code of copying the values from data model to DTO, if you have many such methods.
I need to get some Json to the client side from the server and it's somewhat troublesome as almost all tutorials assume an Ajax call to a separate action to get the Json.
This led me to think that there must be some reason why it is done this way. In Asp.Net MVC we can pass a Model along with the view to get the information but we can't seem to easily pass a Json object. Instead you are supposed to make a separate call to get this information.
What if the Json info is known when the page is generated, why not generate it at the same time?
I'm sorry if I wasn't clear enough. While it's nice to hear of ways to get Json to the client, the question is actually whether there is a specific reason the Ajax call method is much more popular, like security or anything like that.
Can you put something like this into your view? (rough pseudo code, assuming using a Razor view)
< script >
var myJSON = { Field: #model.Field, Field2: #model.Field2 };
< /script >
Because you do not need both at the same time... on the first call will be to get html (the view of the data - represented by a view model), and any ajax calls will be to get the possibly updated data (json serialized view model).
No reason why you can't. You could use the javacript serializer to create a JSON string that drop on the page. You could also create an action that return the json string that you called from a script tag.
What you want if you're using KnockOut, would be the Mapping plugin that turns an ordinary JS object, like that generated above, into an observable ready for KnockOut to use. See here from info. http://knockoutjs.com/documentation/plugins-mapping.html
You can use content-negotiation by setting accept header. This is considered a best practice (and according to some RESTful).
This needs to be supported and implemented at server as well. ASP NET MVC does not make it easy to support content-negotiation and you have to implement it yourself by if-else or using ActionFilter and implementing action selector.
I come from low level languages - C++ is the highest level I program in.
Recently I came across Reflection, and I just cannot fathom how it could be used without code smells.
The idea of inspecting a class/method/function during runtime, in my opinion, points to a flaw in design - I think most problems Reflection (tries to) solve could be used with either Polymorphism or proper use of inheritance.
Am I wrong? Do I misunderstand the concept and utility of Reflection?
I am looking for a good explanation of when to utilize Reflection where other solutions will fail or be too cumbersome to implement as well as when NOT to use it.
Please enlighten this low-level lubber.
Reflection is most commonly used to circumvent the static type system, however it also has some interesting use cases:
Let's write an ORM!
If you're familiar with NHibernate or most other ORMs, you write classes which map to tables in your database, something like this:
// used to hook into the ORMs innards
public class ActiveRecordBase
{
public void Save();
}
public class User : ActiveRecordBase
{
public int ID { get; set; }
public string UserName { get; set; }
// ...
}
How do you think the Save() method is written? Well, in most ORMs, the Save method doesn't know what fields are in derived classes, but it can access them using reflection.
Its wholly possible to have the same functionality in a type-safe manner, simply by requiring a user to override a method to copy fields into a datarow object, but that would result in lots of boilerplate code and bloat.
Stubs!
Rhino Mocks is a mocking framework. You pass an interface type into a method, and behind the scenes the framework will dynamically construct and instantiate a mock object implementing the interface.
Sure, a programmer could write the boilerplate code for the mock object by hand, but why would she want to if the framework will do it for her?
Metadata!
We can decorate methods with attributes (metadata), which can serve a variety of purposes:
[FilePermission(Context.AllAccess)] // writes things to a file
[Logging(LogMethod.None)] // logger doesn't log this method
[MethodAccessSecurity(Role="Admin")] // user must be in "Admin" group to invoke method
[Validation(ValidationType.NotNull, "reportName")] // throws exception if reportName is null
public void RunDailyReports(string reportName) { ... }
You need to reflect over the method to inspect the attributes. Most AOP frameworks for .NET use attributes for policy injection.
Sure, you can write the same sort of code inline, but this style is more declarative.
Let's make a dependency framework!
Many IoC containers require some degree of reflection to run properly. For example:
public class FileValidator
{
public FileValidator(ILogger logger) { ... }
}
// client code
var validator = IoC.Resolve<FileValidator>();
Our IoC container will instantiate a file validator and pass an appropriate implementation of ILogger into the constructor. Which implementation? That depends on how its implemented.
Let's say that I gave the name of the assembly and class in a configuration file. The language needs to read name of the class as a string and use reflection to instantiate it.
Unless we know the implementation at compile time, there is no type-safe way to instantiate a class based on its name.
Late Binding / Duck Typing
There are all kinds of reasons why you'd want to read the properties of an object at runtime. I'd pick logging as the simplest use case -- let say you were writing a logger which accepts any object and spits out all of its properties to a file.
public static void Log(string msg, object state) { ... }
You could override the Log method for all possible static types, or you could just use reflection to read the properties instead.
Some languages like OCaml and Scala support statically-checked duck-typing (called structural typing), but sometimes you just don't have compile-time knowledge of an objects interface.
Or as Java programmers know, sometimes the type system will get your way and require you to write all kinds of boilerplate code. There's a well-known article which describes how many design patterns are simplified with dynamic typing.
Occasionally circumventing the type system allows you to refactor your code down much further than is possible with static types, resulting in a little bit cleaner code (preferably hidden behind a programmer friendly API :) ). Many modern static languages are adopting the golden rule "static typing where possible, dynamic typing where necessary", allowing users to switch between static and dynamic code.
Projects such as hibernate (O/R mapping) and StructureMap (dependency injection) would be impossible without Reflection. How would one solve these with polymorphism alone?
What makes these problems so difficult to solve any other way is that the libraries don't directly know anything about your class hierarchy - they can't. And yet they need to know the structure of your classes in order to - for example - map an arbitrary row of data from a database to a property in your class using only the name of the field and the name of your property.
Reflection is particularly useful for mapping problems. The idea of convention over code is becoming more and more popular and you need some type of Reflection to do it.
In .NET 3.5+ you have an alternative, which is to use expression trees. These are strongly-typed, and many problems that were classically solved using Reflection have been re-implemented using lambdas and expression trees (see Fluent NHibernate, Ninject). But keep in mind that not every language supports these kinds of constructs; when they're not available, you're basically stuck with Reflection.
In a way (and I hope I'm not ruffling too many feathers with this), Reflection is very often used as a workaround/hack in Object-Oriented languages for features that come for free in Functional languages. As functional languages become more popular, and/or more OO languages start implementing more functional features (like C#), we will most likely start to see Reflection used less and less. But I suspect it will always still be around, for more conventional applications like plugins (as one of the other responders helpfully pointed out).
Actually, you are already using a reflective system everyday: your computer.
Sure, instead of classes, methods and objects, it has programs and files. Programs create and modify files just like methods create and modify objects. But then programs are files themselves, and some programs even inspect or create other programs!
So, why is it so OK for a Linux install to be reflexive that nobody even thinks about it, and scary for OO programs?
I've seen good usages with custom attributes. Such as a database framework.
[DatabaseColumn("UserID")]
[PrimaryKey]
public Int32 UserID { get; set; }
Reflection can then be used to get further information about these fields. I'm pretty sure LINQ To SQL does something similar...
Other examples include test frameworks...
[Test]
public void TestSomething()
{
Assert.AreEqual(5, 10);
}
Without reflection you often have to repeat yourself a lot.
Consider these scenarios:
Run a set of methods e.g. the testXXX() methods in a test case
Generate a list of properties in a gui builder
Make your classes scriptable
Implement a serialization scheme
You can't typically do these things in C/C++ without repeating the whole list of affected methods and properties somewhere else in the code.
In fact C/C++ programmers often use an Interface description language to expose interfaces at runtime (providing a form of reflection).
Judicious use of reflection and annotations combined with well defined coding conventions can avoids rampant code repetition and increase maintainability.
I think that reflection is one of these mechanisms that are powerful but can be easily abused. You're given the tools to become a "power user" for very specific purposes, but it is not meant to replace proper object oriented design (just as object oriented design is not a solution for everything) or to be used lightly.
Because of the way Java is structured, you are already paying the price of representing your class hierarchy in memory at runtime (compare to C++ where you don't pay any costs unless you use things like virtual methods). There is therefore no cost rationale for blocking it fully.
Reflection is useful for things like serialization - things like Hibernate or digester can use it to determine how to best store objects automatically. Similarly, the JavaBeans model is based on names of methods (a questionable decision, I admit), but you need to be able to inspect what properties are available to build things like visual editors. In more recent versions of Java, reflections is what makes annotations useful - you can write tools and do metaprogramming using these entities that exist in the source code but can be accessible at runtime.
It is possible to go through an entire career as a Java programmer and never have to use reflection because the problems that you deal with don't require it. On the other hand, for certain problems, it is quite necessary.
As mentioned above, reflection is mostly used to implement code that needs to deal with arbitrary objects. ORM mappers, for instance, need to instantiate objects from user-defined classes and fill them with values from database rows. The simplest way to achieve this is through reflection.
Actually, you are partially right, reflection is often a code smell. Most of the time you work with your classes and do not need reflection- if you know your types, you are probably sacrificing type safety, performance, readability and everything that's good in this world, needlessly. However, if you are writing libraries, frameworks or generic utilities, you will probably run into situations best handled with reflection.
This is in Java, which is what I'm familiar with. Other languages offer stuff that can be used to achieve the same goals, but in Java, reflection has clear applications for which it's the best (and sometimes, only) solution.
Unit testing software and frameworks like NUnit use reflection to get a list of tests to execute and executes them. They find all the test suites in a module/assembly/binary (in C# these are represented by classes) and all the tests in those suites (in C# these are methods in a class). NUnit also allows you to mark a test with an expected exception in case you're testing for exception contracts.
Without reflection, you'd need to specify somehow what test suites are available and what tests are available in each suite. Also, things like exceptions would need to be tested manually. C++ unit testing frameworks I've seen have used macros to do this, but some things are still manual and this design is restrictive.
Paul Graham has a great essay that may say it best:
Programs that write programs? When
would you ever want to do that? Not
very often, if you think in Cobol. All
the time, if you think in Lisp. It
would be convenient here if I could
give an example of a powerful macro,
and say there! how about that? But if
I did, it would just look like
gibberish to someone who didn't know
Lisp; there isn't room here to explain
everything you'd need to know to
understand what it meant. In Ansi
Common Lisp I tried to move things
along as fast as I could, and even so
I didn't get to macros until page 160.
concluding with . . .
During the years we worked on Viaweb I
read a lot of job descriptions. A new
competitor seemed to emerge out of the
woodwork every month or so. The first
thing I would do, after checking to
see if they had a live online demo,
was look at their job listings. After
a couple years of this I could tell
which companies to worry about and
which not to. The more of an IT flavor
the job descriptions had, the less
dangerous the company was. The safest
kind were the ones that wanted Oracle
experience. You never had to worry
about those. You were also safe if
they said they wanted C++ or Java
developers. If they wanted Perl or
Python programmers, that would be a
bit frightening-- that's starting to
sound like a company where the
technical side, at least, is run by
real hackers. If I had ever seen a job
posting looking for Lisp hackers, I
would have been really worried.
It is all about rapid development.
var myObject = // Something with quite a few properties.
var props = new Dictionary<string, object>();
foreach (var prop in myObject.GetType().GetProperties())
{
props.Add(prop.Name, prop.GetValue(myObject, null);
}
Plugins are a great example.
Tools are another example - inspector tools, build tools, etc.
I will give an example of a c# solution i was given when i started learning.
It contained classes marked with the [Exercise] attribute, each class contained methods which were not implemented (throwing NotImplementedException). The solution also had unit tests which all failed.
The goal was to implement all the methods and pass all the unit tests.
The solution also had a user interface which it would read all class marked with Excercise, and use reflection to generate a user interface.
We were later asked to implement our own methods, and later still to understand how the user interface 'magically' was changed to include all the new methods we implemented.
Extremely useful, but often not well understood.
The idea behind this was to be able to query any GUI objects properties, to provide them in a GUI to get customized and preconfigured. Now it's uses have been extended and proved to be feasible.
EDIT: spelling
It's very useful for dependency injection. You can explore loaded assemblies types implementing a given interface with a given attribute. Combined with proper configuration files, it proves to be a very powerful and clean way of adding new inherited classes without modifying the client code.
Also, if you are doing an editor that doesn't really care about the underlying model but rather on how the objects are structured directly, ala System.Forms.PropertyGrid)
Without reflection no plugin architecture will work!
Very simple example in Python. Suppose you have a class that have 3 methods:
class SomeClass(object):
def methodA(self):
# some code
def methodB(self):
# some code
def methodC(self):
# some code
Now, in some other class you want to decorate those methods with some additional behaviour (i.e. you want that class to mimic SomeClass, but with an additional functionality).
This is as simple as:
class SomeOtherClass(object):
def __getattr__(self, attr_name):
# do something nice and then call method that caller requested
getattr(self.someclass_instance, attr_name)()
With reflection, you can write a small amount of domain independent code that doesn't need to change often versus writing a lot more domain dependent code that needs to change more frequently (such as when properties are added/removed). With established conventions in your project, you can perform common functions based on the presence of certain properties, attributes, etc. Data transformation of objects between different domains is one example where reflection really comes in handy.
Or a more simple example within a domain, where you want to transform data from the database to data objects without needing to modify the transformation code when properties change, so long as conventions are maintained (in this case matching property names and a specific attribute):
///--------------------------------------------------------------------------------
/// <summary>Transform data from the input data reader into the output object. Each
/// element to be transformed must have the DataElement attribute associated with
/// it.</summary>
///
/// <param name="inputReader">The database reader with the input data.</param>
/// <param name="outputObject">The output object to be populated with the input data.</param>
/// <param name="filterElements">Data elements to filter out of the transformation.</param>
///--------------------------------------------------------------------------------
public static void TransformDataFromDbReader(DbDataReader inputReader, IDataObject outputObject, NameObjectCollection filterElements)
{
try
{
// add all public properties with the DataElement attribute to the output object
foreach (PropertyInfo loopInfo in outputObject.GetType().GetProperties())
{
foreach (object loopAttribute in loopInfo.GetCustomAttributes(true))
{
if (loopAttribute is DataElementAttribute)
{
// get name of property to transform
string transformName = DataHelper.GetString(((DataElementAttribute)loopAttribute).ElementName).Trim().ToLower();
if (transformName == String.Empty)
{
transformName = loopInfo.Name.Trim().ToLower();
}
// do transform if not in filter field list
if (filterElements == null || DataHelper.GetString(filterElements[transformName]) == String.Empty)
{
for (int i = 0; i < inputReader.FieldCount; i++)
{
if (inputReader.GetName(i).Trim().ToLower() == transformName)
{
// set value, based on system type
loopInfo.SetValue(outputObject, DataHelper.GetValueFromSystemType(inputReader[i], loopInfo.PropertyType.UnderlyingSystemType.FullName, false), null);
}
}
}
}
}
}
// add all fields with the DataElement attribute to the output object
foreach (FieldInfo loopInfo in outputObject.GetType().GetFields(BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.GetField | BindingFlags.Instance))
{
foreach (object loopAttribute in loopInfo.GetCustomAttributes(true))
{
if (loopAttribute is DataElementAttribute)
{
// get name of field to transform
string transformName = DataHelper.GetString(((DataElementAttribute)loopAttribute).ElementName).Trim().ToLower();
if (transformName == String.Empty)
{
transformName = loopInfo.Name.Trim().ToLower();
}
// do transform if not in filter field list
if (filterElements == null || DataHelper.GetString(filterElements[transformName]) == String.Empty)
{
for (int i = 0; i < inputReader.FieldCount; i++)
{
if (inputReader.GetName(i).Trim().ToLower() == transformName)
{
// set value, based on system type
loopInfo.SetValue(outputObject, DataHelper.GetValueFromSystemType(inputReader[i], loopInfo.FieldType.UnderlyingSystemType.FullName, false));
}
}
}
}
}
}
}
catch (Exception ex)
{
bool reThrow = ExceptionHandler.HandleException(ex);
if (reThrow) throw;
}
}
One usage not yet mentioned: while reflection is generally thought of as "slow", it's possible to use Reflection to improve the efficiency of code which uses interfaces like IEquatable<T> when they exist, and uses other means of checking equality when they do not. In the absence of reflection, code that wanted to test whether two objects were equal would have to either use Object.Equals(Object) or else check at run-time whether an object implemented IEquatable<T> and, if so, cast the object to that interface. In either case, if the type of thing being compared was a value type, at least one boxing operation would be required. Using Reflection makes it possible to have a class EqualityComparer<T> automatically construct a type-specific implementation of IEqualityComparer<T> for any particular type T, with that implementation using IEquatable<T> if it is defined, or using Object.Equals(Object) if it is not. The first time one uses EqualityComparer<T>.Default for any particular type T, the system will have to go through more work than would be required to test, once, whether a particular type implements IEquatable<T>. On the other hand, once that work is done, no more run-time type checking will be required since the system will have produced a custom-built implementation of EqualityComparer<T> for the type in question.
Everywhere I read about the new DLR in .net 4, they say that a good use for it is reflection, and the code snippet always shown is something like
dynamic d = GetSomeObject();
d.DoSomething();
d.SomeMember = 1;
What does GetSomeObject() look like? I can't find anywhere that explains that.
I understand that it can be anything, but in the context of reflection what is it? Is it the assembly? an instance of a type?
The return type of GetSomeObject() will be an instance of some type. For example, here's what it might look like:
public Customer GetSomeObject() {
return new Customer("John", "Doe", 12345);
}
And then the code would say:
dynamic customer = GetSomeObject();
string s = customer.FirstName;
// now the "s" variable would have "John" in it
The GetSomeObject() can return anything. It might return a Customer object or a Product. And it doesn't matter! The idea is that when the variable is declared as being dynamic that when you call a method or a property, as you have shown, the compiler will generate code that uses Reflection to try and call the method or property. If they exist then the calls will succeed. If not then you'll get an error at runtime.
In the general case this example is just simplifying the usage of Reflection by having the compiler generate the code for you.
Having said that, if the Customer or Product object implement IDynamicObject themselves then they can do far more advanced stuff.
What you are describing is the duck-typing aspect of dynamic (there are other facets). The answer is that it could be anything:
a true dynamic object (IDynamicObject)
any regular object, via reflection
A useful example (for reading properties, at least) might be an anonymous type; it could also be a COM object, for example - or (in Silverlight) an object in the html DOM. Or it could be your vendor's Customer object that doesn't implement any common interface, but is remarkably like your own InternalCustomer object. Heck, it could be an IronPyton object.
Well, GetSomeObject() could, for instance, return an instance of type _ComObject. That's one of the primary reasons of having it dynamic, I think.
I think that it's more interesting, as far as dynamic, DLR and reflection concerns, to see what happend in line 2 for instance.
using dynmic you go like this
dynamic d = GetSomeObject();
d.DoSomething();
while with reflection it's a bit more noisy
var d = GetSomeObject();
var mi = d.GetType().GetMethod("DoSomething");
mi.Invoke(d,mi);
As I see it, the first one is more elegant and we are talking about an argument less method, things can go really crazy when you are interoping with COM or APIs with long signature methods. I been there ;)
I have one JSON that is coming in a string format. I need to store it in a key-pair value or something like that. I am using asp.net 2.0 and can not use 3rd party DLL like Newtonsoft.Json.dll. I guess last option will be to use regular expression.
Can anybody please help me in this?
If you go to http://www.json.org/ and look towards the bottom of the page there are dozens of json libraries most of them open source, I believe they list 8 for C#. If you can not reference one of these libraries, I think your best bet would be to find one with a permissive license and simply add the code to your project.
Another idea is to look at the diagrams, grammer, and syntax at http://www.json.org/ and just write your own parser, but regex is NOT the way to do it. If you dont know how to write a parser you could look at one of the open source json libraries or start with something less complicated like a good CSV parser, here is a paper that looks pretty good: http://www.boyet.com/Articles/CsvParser.html
It is possible to serialize JSON using JScript in C# into key/value pairs. You need to add a few references to your project. They're part of the .NET framework, you just need to add the references to your project. You'll need:
Microsoft.JSript
Microsoft.Vsa
First, the usings at the top of your class:
using Microsoft.JScript;
using Microsoft.JScript.Vsa;
Then the Engine that will execute the script needs to be initialized somewhere in your 'Page' code-behind:
VsaEngine Engine = VsaEngine.CreateEngine();
Then you just create this method and call it by passing in your JSON object:
object EvalJScript(string JScript)
{
object result = null;
try
{
result = Microsoft.JScript.Eval.JScriptEvaluate(JScript, Engine);
}
catch (Exception ex)
{
return ex.Message;
}
return result;
}
The type of object returned (if JSON is passed in) is a 'JSObject'. You can access its values as key/value pairs. Read the MSDN documentation for more details on this object.
Here's an example of using the code:
string json = "({Name:\"Dan\",Occupation:\"Developer\"})";
JSObject o = EvalJScript(json) as JSObject;
string name = o["Name"] as string; // Value of 'name' will be 'Dan'
Could you use JScript.NET?
If so, should be easy enough with eval() - then just loop through the objects returned and translate into KeyValuePair's or whatever
You will need to use jscript.net as the code behind language, but other pages of your site should be fine to stay as c# if thats what you prefer.
As mentioned in previous comment, you will need to be aware of the security aspects and risks - only use eval if you trust the JSON you're parsing!