Develop Reference

Conditional Attributes c#

In my program, I have two attributes: VisibleToA and VisibleToB. These are used to restrict visibility of certain functions through the API.
The problem occurs when I need to have a function visible to A or B. Attributes are "anded" together by default such that I create a method:
[VisibleToA(), VisibleToB()]
public void ConditionallyVisibleMethod() { }
It is effectively equivalent to VisibleToAandB. This is a problem because if they are both true, then it makes it impossible for A to see it, since A is not B, and makes it impossible for B to see it, since B is not A.
Because of this, I want to make it visible to A OR B, only needing to satisfy a single condition in order to view it.
So, simple fix, I created an OrConditionAttribute that takes in an array of Attributes. Loop through the condition on each one, if true, return true. Then I found out attributes can only have parameters of primitive types due to how they are written in the metadata.
They have to be attributes, due to how the structure of the program works, so I need to use one in one way or another. I thought about passing in strings representing the attributes' class names, then inside the OrConditionAttribute create an instance of the class from the string name, but I wasn't sure how to pass in parameters as well since VisibleToA and VisibleToB both require parameters.
Is there a way to implement this functionality?

To answer your question directly, unfortunately, there is no way to (declaratively) conditionally apply two attributes to your class/method/property, they are either applied, or not, so in your case you have both VisibleToA and VisibleToB. Values provided to attribute constructors must also be compile-time constants, which is probably also part of your problem.
In terms of your design, it sounds to me like you've got some amount of business logic in your attribute classes themselves, which might be causing your problem. It's generally good design practice to restrict the amount of actual logic contained in attributes, and instead have some other mechanism to interpret the metadata that you've applied to your classes/methods/properties.
If you can change your design, I would suggest something like the following:
public enum VisibilitySpecifier
{
A,
B,
}
public class AndVisibleToAttribute : Attribute
{
public VisibilitySpecifier[] Visibility { get; set; }
public AndVisibleToAttribute(params VisibilitySpecifier[] visibility)
{
Visibility = visibility;
}
}
public class OrVisibleToAttribute : Attribute
{
public VisibilitySpecifier[] Visibility { get; set; }
public OrVisibleToAttribute(params VisibilitySpecifier[] visibility)
{
Visibility = visibility;
}
}
Then, simply process the values in the Visibility property according to the logic defined by the particular attribute chosen.

Avoiding Magic strings Reflection vs Constant

What would be a better approach to avoid the magic, based on the following 2 examples?
Known relationship
we know the Class which is meant
public string Notes
{
get { return notes; }
set { SetPropertyValue("Notes", ref notes, value); }
}
Unknow relationship
we do not know the class (because multiple classes could have a property with this name) or even if the property exist here
public void SomeStuffeChanged()
{
PropertyEditor propertyEditor = ((DetailView)View).FindItem("Anniversary") as PropertyEditor;
// do some stuff
}
I could create a define class/es which will contain all properties as const so i would avoid redundance of my properties, but it's also work to maintain this define.
When i would go for reflection i would do just MemberName.GetMemberName<Contact>( x=> x.Anniversary) and any changed would be automatically applied, but when i have the second case where i just assume there could be this property. I can't use the show reflection because even if the code is still valid the propertyname could have changed in a other class. Also reflection add's some overhead.
Update
The Question is strongly related with DevExpress because they use "MagicStrings" all over the place and you can't change the baseclass

How the meta tags of a method work? [duplicate]

What are attributes in .NET, what are they good for, and how do I create my own attributes?

Metadata. Data about your objects/methods/properties.
For example I might declare an Attribute called: DisplayOrder so I can easily control in what order properties should appear in the UI. I could then append it to a class and write some GUI components that extract the attributes and order the UI elements appropriately.
public class DisplayWrapper
{
private UnderlyingClass underlyingObject;
public DisplayWrapper(UnderlyingClass u)
{
underlyingObject = u;
}
[DisplayOrder(1)]
public int SomeInt
{
get
{
return underlyingObject .SomeInt;
}
}
[DisplayOrder(2)]
public DateTime SomeDate
{
get
{
return underlyingObject .SomeDate;
}
}
}
Thereby ensuring that SomeInt is always displayed before SomeDate when working with my custom GUI components.
However, you'll see them most commonly used outside of the direct coding environment. For example the Windows Designer uses them extensively so it knows how to deal with custom made objects. Using the BrowsableAttribute like so:
[Browsable(false)]
public SomeCustomType DontShowThisInTheDesigner
{
get{/*do something*/}
}
Tells the designer not to list this in the available properties in the Properties window at design time for example.
You could also use them for code-generation, pre-compile operations (such as Post-Sharp) or run-time operations such as Reflection.Emit.
For example, you could write a bit of code for profiling that transparently wrapped every single call your code makes and times it. You could "opt-out" of the timing via an attribute that you place on particular methods.
public void SomeProfilingMethod(MethodInfo targetMethod, object target, params object[] args)
{
bool time = true;
foreach (Attribute a in target.GetCustomAttributes())
{
if (a.GetType() is NoTimingAttribute)
{
time = false;
break;
}
}
if (time)
{
StopWatch stopWatch = new StopWatch();
stopWatch.Start();
targetMethod.Invoke(target, args);
stopWatch.Stop();
HandleTimingOutput(targetMethod, stopWatch.Duration);
}
else
{
targetMethod.Invoke(target, args);
}
}
Declaring them is easy, just make a class that inherits from Attribute.
public class DisplayOrderAttribute : Attribute
{
private int order;
public DisplayOrderAttribute(int order)
{
this.order = order;
}
public int Order
{
get { return order; }
}
}
And remember that when you use the attribute you can omit the suffix "attribute" the compiler will add that for you.
NOTE: Attributes don't do anything by themselves - there needs to be some other code that uses them. Sometimes that code has been written for you but sometimes you have to write it yourself. For example, the C# compiler cares about some and certain frameworks frameworks use some (e.g. NUnit looks for [TestFixture] on a class and [Test] on a test method when loading an assembly).
So when creating your own custom attribute be aware that it will not impact the behaviour of your code at all. You'll need to write the other part that checks attributes (via reflection) and act on them.

Many people have answered but no one has mentioned this so far...
Attributes are used heavily with reflection. Reflection is already pretty slow.
It is very worthwhile marking your custom attributes as being sealed classes to improve their runtime performance.
It is also a good idea to consider where it would be appropriate to use place such an attribute, and to attribute your attribute (!) to indicate this via AttributeUsage. The list of available attribute usages might surprise you:
Assembly
Module
Class
Struct
Enum
Constructor
Method
Property
Field
Event
Interface
Parameter
Delegate
ReturnValue
GenericParameter
All
It's also cool that the AttributeUsage attribute is part of the AttributeUsage attribute's signature. Whoa for circular dependencies!
[AttributeUsageAttribute(AttributeTargets.Class, Inherited = true)]
public sealed class AttributeUsageAttribute : Attribute

Attributes are a kind of meta data for tagging classes. This is often used in WinForms for example to hide controls from the toolbar, but can be implemented in your own application to enable instances of different classes to behave in specific ways.
Start by creating an attribute:
[AttributeUsage(AttributeTargets.Class, AllowMultiple=false, Inherited=true)]
public class SortOrderAttribute : Attribute
{
public int SortOrder { get; set; }
public SortOrderAttribute(int sortOrder)
{
this.SortOrder = sortOrder;
}
}
All attribute classes must have the suffix "Attribute" to be valid.
After this is done, create a class that uses the attribute.
[SortOrder(23)]
public class MyClass
{
public MyClass()
{
}
}
Now you can check a specific class' SortOrderAttribute (if it has one) by doing the following:
public class MyInvestigatorClass
{
public void InvestigateTheAttribute()
{
// Get the type object for the class that is using
// the attribute.
Type type = typeof(MyClass);
// Get all custom attributes for the type.
object[] attributes = type.GetCustomAttributes(
typeof(SortOrderAttribute), true);
// Now let's make sure that we got at least one attribute.
if (attributes != null && attributes.Length > 0)
{
// Get the first attribute in the list of custom attributes
// that is of the type "SortOrderAttribute". This should only
// be one since we said "AllowMultiple=false".
SortOrderAttribute attribute =
attributes[0] as SortOrderAttribute;
// Now we can get the sort order for the class "MyClass".
int sortOrder = attribute.SortOrder;
}
}
}
If you want to read more about this you can always check out MSDN which has a pretty good description.
I hope this helped you out!

An attribute is a class that contains some bit of functionality that you can apply to objects in your code. To create one, create a class that inherits from System.Attribute.
As for what they're good for... there are almost limitless uses for them.
http://www.codeproject.com/KB/cs/dotnetattributes.aspx

Attributes are like metadata applied to classes, methods or assemblies.
They are good for any number of things (debugger visualization, marking things as obsolete, marking things as serializable, the list is endless).
Creating your own custom ones is easy as pie. Start here:
http://msdn.microsoft.com/en-us/library/sw480ze8(VS.71).aspx

In the project I'm currently working on, there is a set of UI objects of various flavours and an editor to assembly these objects to create pages for use in the main application, a bit like the form designer in DevStudio. These objects exist in their own assembly and each object is a class derived from UserControl and has a custom attribute. This attribute is defined like this:
[AttributeUsage (AttributeTargets::Class)]
public ref class ControlDescriptionAttribute : Attribute
{
public:
ControlDescriptionAttribute (String ^name, String ^description) :
_name (name),
_description (description)
{
}
property String ^Name
{
String ^get () { return _name; }
}
property String ^Description
{
String ^get () { return _description; }
}
private:
String
^ _name,
^ _description;
};
and I apply it to a class like this:
[ControlDescription ("Pie Chart", "Displays a pie chart")]
public ref class PieControl sealed : UserControl
{
// stuff
};
which is what the previous posters have said.
To use the attribute, the editor has a Generic::List <Type> containing the control types. There is a list box which the user can drag from and drop onto the page to create an instance of the control. To populate the list box, I get the ControlDescriptionAttribute for the control and fill out an entry in the list:
// done for each control type
array <Object ^>
// get all the custom attributes
^attributes = controltype->GetCustomAttributes (true);
Type
// this is the one we're interested in
^attributetype = ECMMainPageDisplay::ControlDescriptionAttribute::typeid;
// iterate over the custom attributes
for each (Object ^attribute in attributes)
{
if (attributetype->IsInstanceOfType (attribute))
{
ECMMainPageDisplay::ControlDescriptionAttribute
^description = safe_cast <ECMMainPageDisplay::ControlDescriptionAttribute ^> (attribute);
// get the name and description and create an entry in the list
ListViewItem
^item = gcnew ListViewItem (description->Name);
item->Tag = controltype->Name;
item->SubItems->Add (description->Description);
mcontrols->Items->Add (item);
break;
}
}
Note: the above is C++/CLI but it's not difficult to convert to C#
(yeah, I know, C++/CLI is an abomination but it's what I have to work with :-( )
You can put attributes on most things and there are whole range of predefined attributes. The editor mentioned above also looks for custom attributes on properties that describe the property and how to edit it.
Once you get the whole idea, you'll wonder how you ever lived without them.

As said, Attributes are relatively easy to create. The other part of the work is creating code that uses it. In most cases you will use reflection at runtime to alter behavior based on the presence of an attribute or its properties. There are also scenarios where you will inspect attributes on compiled code to do some sort of static analysis. For example, parameters might be marked as non-null and the analysis tool can use this as a hint.
Using the attributes and knowing the appropriate scenarios for their use is the bulk of the work.

Attributes are, essentially, bits of data you want to attach to your types (classes, methods, events, enums, etc.)
The idea is that at run time some other type/framework/tool will query your type for the information in the attribute and act upon it.
So, for example, Visual Studio can query the attributes on a 3rd party control to figure out which properties of the control should appear in the Properties pane at design time.
Attributes can also be used in Aspect Oriented Programming to inject/manipulate objects at run time based on the attributes that decorate them and add validation, logging, etc. to the objects without affecting the business logic of the object.

You can use custom attributes as a simple way to define tag values in sub classes without having to write the same code over and over again for each subclass. I came across a nice concise example by John Waters of how to define and use custom attributes in your own code.
There is a tutorial at http://msdn.microsoft.com/en-us/library/aa288454(VS.71).aspx

To get started creating an attribute, open a C# source file, type attribute and hit [TAB]. It will expand to a template for a new attribute.

Attributes are also commonly used for Aspect Oriented Programming. For an example of this check out the PostSharp project.

How do attribute classes work?

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

Why would one want to use AttributeUsage AllowMultiple when creating attributes?

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.