Say you have a property in a model that looks like this:
[DataType(DataType.DateTime)]
[DisplayFormat(DataFormatString = "{0:yyyy-MM-dd}", ApplyFormatInEditMode = true)]
public DateTime CreationDate { get; set; }
Could you do something like this:
[ComboAttribute] // Does the same thing as the two attributes above
public DateTime CreationDate { get; set; }
I'm exploring ways to ensure all my DateTime properties have the same set of 4 to 5 attributes. If I have to add or change an attribute, currently I have to use Ctrl + F and make sure I get every instance, which is just asking for trouble.
Is this possible? Is there a better way to get the error-proofing I'm looking for?
Attributes are data structures attached a type or member definition. At runtime, components can use reflection APIs to find out the attributes which have been applied to a type or member and adjust their behaviour accordingly.
It's important to understand that attributes themselves don't do anything. There always has to be a piece of code looking for that specific attribute. If you define a new attribute, the code will not find it unless it is looking for attributes matching a specific convention or inheriting from a special base type. You need to understand the code looking for the attributes in order to understand whether you can create your own.
For your specific task of creating an aggregation of attributes, the only thing that can work is a pre-processor (such as PostSharp) that can take your custom attribute and literally re-write the code as though you had put the two attributes there.
If you do go down the PostSharp route, there's an example of generating attributes using custom attributes right here: How to inject an attribute using a PostSharp attribute?
You have two issues here: one, that your attribute will be looked up by whatever framework is expecting it (e.g. MVC is expecting some attributes on properties). Two, that your attribute will perform the logic of all attributes combined.
For the first problem, your attribute will have to inherit from known base attributes, like ValidationAttribute, or implement a known interface, like IActionFilter, depending on what attributes you want to replace.
For the second problem, your attribute can instantiate an instance of each attribute it is replacing, and delegate the functionality it inherited/implemented to the appropriate attribute instance.
Related
I keep seeing the following syntax when looking at C# code (using .NET 4.0 framework):
[XmlIgnore, Bindable(false)]
public virtual FieldBase Field {get;set;}
What is the purpose of the square brackets above the property header?
These are attributes, they can be applied to elements of your code-base and in doing so applies metadata to that thing - like descriptive declarations. These things can have multiple attributes. There are a bunch of 'built-in' attributes exposes by the .NET framework, you can however define your own.
Types that are attributes are actually defined with a fully qualified name of SuchAThingAttribute, whereas in being applied you need only specify the name minus Attribute which becomes SuchAThing. And they must derive from System.Attribute (at least to be compliant).
An attribute can have 'settings', that is, you can specify (when writing your own) which types of elements the attribute is applicable to, and whether an element can have more than one of this type of attribute or not, and so on.
The metadata of the attribute can later be got at using Reflection and GetCustomAttribute-like methods. Links here and here show examples of doing so.
These are attributes.
Please take a look at Attributes (C# and Visual Basic).
These are attributes assigned to the variable. A classic use case is for enforcing model properties that they are applied to.
When used in conjunction with Code First principles, the validation rules specified in these attributes in a model are enforced before the application saves changes in the database and avoids bad data going into your database.
public class Person
{
//We cannot have a digit as part of a Person's name, unless ofcourse
//you are Elon Musks child
[RegularExpression(#"[^0-9]*")]
public string Name{ get; set; }
}
What you are looking at is an attribute. The square bracket is the syntax required to specifiy the application of an attribute to a given member, in this case a property accessor, but attributes can also be applied to classes etc.
Is it possible in C# in an attribute constructor to get the class that has the attribute assigned to it without having to pass that class name in.
[MyAttr]
public class A{}
[AttributeUsage(AttributeTargets.Class, AllowMultiple = false)]
[Serializable]
public class MyAttrAttribute: Attribute
{
public MyAttrAttribute()
{
//get info here about the class A etc
}
}
Attribute instances are completely independent of the types/fields/properties that they decorate; there is absolutely no way of accessing the context from an attribute. However, attributes also aren't created until you explicitly query them with reflection.
If you want to invoke some logic, then it must be done explicitly through code - so you might consider adding a method on your attribute that accepts the context object:
public void Invoke(object instance) {...}
for example, then use GetCustomAttribute to obtain it, cast it, and call .Invoke()
No. But why would you want to do this? What are you trying to achieve?
When you retrieve an attribute at run time, you do so from the type object representing the class. So even though the information is not stored in the attribute object, it is readily available.
This would have been a convenient feature, nice question. But fundamentally attributes are meant only as meta data to inspectors not to be inspectors - they are constant data.
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
According to a book I'm reading, the AllowMultiple public property of AttributeUsage specifies:
...whether the target can have multiple instances of the attribute applied to it.
Why would I want/not want to use this?
Attributes are meta-data. Typically, you'll want to decorate a member or type with an Attribute in order to track some information about it.
For example, the DescriptionAttribute is used by the PropertyGrid to label a description of a property:
[Description("This is my property")]
public int MyProperty { get; set; }
Most of the time, having more than one description would not make sense.
However, it is possible that a specific attribute makes sense to use more than once. In that case, you'd want to set the Attribute to allow multiple instances of itself tagged to the same attribute.
(Not that I'd do this, but...) Say you made a custom attribute to track major changes to a class. You might want to list this for every major change:
[Changes(Version=1.1, Change="Added Foo Feature")]
[Changes(Version=2.0, Change="Added Bar Feature")]
public class MyClass
{
// ...
This example might be a little contrived but hopefully it gets the point across.
[Convertable(typeof(Int32)), Convertable(typeof(Double))]
public class Test
{
}
This depends what the attributes are.
For example, you could make an attribute that marks a class as depending on something, and you could allow multiple dependencies.
For a concrete example, look at SuppressMessage, which suppresses a code analysis warning. A member can have multiple warnings that you might want to suppress.
Another example is WebResource; an assembly can contain multiple resources.
No contrived example here, I used it in real production code. I wrote some code to parse a file containing pairs of data like (code=value). I put a custom attribute on a function to indicate it should be called for a given code.
[CanParseCode("G1")]
[CanParseCode("G2")]
private void ParseGXCodes(string code, string value)
{
...
}
This particular format is a somewhat old and domain specific with hundreds of different codes. My goal was to write a framework to make it easier to write file processors that could extract only the codes it needs and ignore the rest. Allowing the same attribute multiple times made it easy to express the intent of the code by simply declaring attributes on the function(s) that process each code.
Real World Application of Attribute AllowMultiple=true usefulness
[ManagesType(typeof(SPEC_SEC_OC), true)]
[ManagesType(typeof(SPEC_SEC_04_OC), true)]
public class LibSpecSelectionView : CustomView
{
public LibSpecSelectionView(SPEC_SEC_OC)
{}
public LibSpecSelectionView(SPEC_SEC_O4_OC)
{}
....
}
public static class ViewManager
{
... static Dictionary of views built via reflection
public void LaunchView(this CollectionBaseClass cbc)
{
... Find class registered to handle cbc type in dictionary and call ctor
}
}
SPEC_SEC_OC myOC = DataClient.Instance.GetSPEC_SEC_OC();
myOC.LaunchView()
I flipped AllowMultiple=true earlier today to allow for the ManagesType attribute to be used more than once. We have several hundred Custom Collection Classes. Most of these custom collections have a view that inherits from CustomView designed to handle creation of a UI view for a specific type of custom collection and presenting it to the user. The ManagesType attribute is used via reflection to build a dictionary of EVERY View in our app that inherits from CustomView to "register" what object type it was designed to handle. The LibSpecSelectionView "broke that pattern" by displaying two different collections at the same time (creates two tabs and shows one custom collection in one tab and the other in the second tab) So the same view is capable of handling two different custom collections.
The dictionary of which views are capable of handling which collection types is then leveraged through an extension method to allow any of our custom collections to launch the registered view (or a default one if there is not a "registered" view) through a one line call to the view manager.
I've been reading a text about an extension to C# and at one point it says that "An attribute decoration X may only be applied to fields of type Y."
I haven't been able to find a definition for attribute decoration, and I'm not making much sense out of this by exchanging the two.
It's probably referring to the Attribute class. For example, you can mark a type as serializable via the SerializableAttribute. When you apply an attribute, you can leave off the "Attribute" suffix.
[Serializable]
public class SomeClass {
}
Attributes provide a means to add meta-data about the code.
Attributes are used to add metadata to .NET (C#) code in a structured manner. What a lot of people don't realise, though, is that there are actually two types of attribute.
The simplest is custom attributes, where you define an attribute that specific classes look for to alter the way they work. A common example is the System.Xml.Serialization attributes which are read by the XmlSerializer to alter its output, e.g. a class could be marked up something like the following to specify its namespace and that the field should be an attribute:
[XmlType(Namespace = "http://mycompany.com/")]
public class MyClass
{
[XmlAttribute]
public string MyField;
}
Custom attributes like this have no meaning to the compiler or the runtime, they are just added to the class as part of its metadata, and can be retrieved by a call to Type.GetCustomAttributes.
The other main group of attributes is pseudo-custom attributes, which actually have meaning to either the compiler or the runtime. The example in the post by Haacked with SerializableAttribute is actually an example of a pseudo-custom attribute. It is actually stored as part of the type definition and cannot be retrieved using Type.GetCustomAttributes. You cannot create your own pseudo-custom attributes.
So it's likely what you're dealing with here is a custom attribute which is being looked for by a specific tool.