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Is it possible to define one attribute that applies other attributes in C#? [duplicate]

On a control I am using multiple attribute properties:
[Browsable(false)]
[Bindable(false)]
[EditorBrowsable(EditorBrowsableState.Never)]
[DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden)]
[Obsolete("", true)]
public new Boolean AllowDrop;
I am using those properties on a lot of the other control properties as well.
I am wondering if there is a way to reduce the amount of code to write each time.
It would be nice if I could combine multiple attributes like this:
[Hidden(true)]
public new Boolean AllowDrop;
Where the Hidden Property would include all the attributes above. So there is only 1 single line of code.
Maybe there is also a way to combine the attributes in a macro or something?
I am aware that there are other ways of hiding properties but I chose the way of using attributes.
Thanks

It depends to the framework which is using the attribute.
Combining attributes can be meaningful in order to the context which uses and interprets attributes. For example for those contexts which use .Net Type Description mechanisms you can customize the type description which .Net returns to consumers.
It's possible to provide custom metadata for types using the standard .Net mechanism for that purpose, registering a custom type descriptor for your object.
The idea will work this way, you create a custom type descriptor for your type. In the custom type descriptor, you return custom property descriptors for the properties of your type and in the property descriptor, you return a custom set of attributes for the property.
The approach requires more code, but it's really interesting and shares some good idea about how to provide custom metadata for your types:
IMetedataAttribute Interface
The usage is providing an standard way to create MetaDataAttributes. Each attribute which implements this interface will be used as metadata and instead of the attribute, those one which it returns in Process method will be used:
public interface IMetadatAttribute
{
Attribute[] Process();
}
Sample MetadataAttribute
It's a sample metadata attribute which returns some attribute instead when processing the attribute:
public class MySampleMetadataAttribute : Attribute, IMetadatAttribute
{
public Attribute[] Process()
{
var attributes = new Attribute[]{
new BrowsableAttribute(false),
new EditorBrowsableAttribute(EditorBrowsableState.Never),
new BindableAttribute(false),
new DesignerSerializationVisibilityAttribute(
DesignerSerializationVisibility.Hidden),
new ObsoleteAttribute("", true)
};
return attributes;
}
}
Property Descriptor
This class will be used by the custom type descriptor to provide a custom list of attributes for the property:
public class MyPropertyDescriptor : PropertyDescriptor
{
PropertyDescriptor original;
public MyPropertyDescriptor(PropertyDescriptor originalProperty)
: base(originalProperty) { original = originalProperty;}
public override AttributeCollection Attributes
{
get
{
var attributes = base.Attributes.Cast<Attribute>();
var result = new List<Attribute>();
foreach (var item in attributes)
{
if(item is IMetadatAttribute)
{
var attrs = ((IMetadatAttribute)item).Process();
if(attrs !=null )
{
foreach (var a in attrs)
result.Add(a);
}
}
else
result.Add(item);
}
return new AttributeCollection(result.ToArray());
}
}
// Implement other properties and methods simply using return original
// The implementation is trivial like this one:
// public override Type ComponentType
// {
// get { return original.ComponentType; }
// }
}
Type Descriptor
This is the type descriptor which provides a custom description for your type. In this example it uses custom property descriptors to provide custom attributes set for the properties of your class:
public class MyTypeDescriptor : CustomTypeDescriptor
{
ICustomTypeDescriptor original;
public MyTypeDescriptor(ICustomTypeDescriptor originalDescriptor)
: base(originalDescriptor)
{
original = originalDescriptor;
}
public override PropertyDescriptorCollection GetProperties()
{
return this.GetProperties(new Attribute[] { });
}
public override PropertyDescriptorCollection GetProperties(Attribute[] attributes)
{
var properties = base.GetProperties(attributes).Cast<PropertyDescriptor>()
.Select(p => new MyPropertyDescriptor(p))
.ToArray();
return new PropertyDescriptorCollection(properties);
}
}
Typedescriptor Provider
This class will be used in the attribute above your type to introduce the custom type descriptor which we created as the metadata engine for the type:
public class MyTypeDescriptionProvider : TypeDescriptionProvider
{
public MyTypeDescriptionProvider()
: base(TypeDescriptor.GetProvider(typeof(object))) { }
public override ICustomTypeDescriptor GetTypeDescriptor(Type objectType,
object instance)
{
ICustomTypeDescriptor baseDescriptor = base.GetTypeDescriptor(objectType, instance);
return new MyTypeDescriptor(baseDescriptor);
}
}
Sample Class
Here is my sample class which its Name property is decorated using MySampleMetadataAttribute and the class itself is registered to use our custom type descriptor provider:
[TypeDescriptionProvider(typeof(MyTypeDescriptionProvider))]
public class MySampleClass
{
public int Id { get; set; }
[MySampleMetadataAttribue]
[DisplayName("My Name")]
public string Name { get; set; }
}
To see the result it's enough to create an instance of the class and see the result in PropertyGrid:
var o = new MySampleClass();
this.propertyGrid1.SelectedObject = o;
Some notes on answer
Probably it's not as simple as you expected for such task. But it's working.
It's a lengthy answer, but contains a complete working example of how you can apply type descriptors to your types to provide custom metadata.
The approach will not work for engines which use reflection instead of type description. But it's completely working with for example PropertyGrid control which works with type description.

The best way for me to do this, is by using Metalama (modern rewrite of PostSharp for new .NET releases).
It is absolutely the best framework for doing AOP in .NET from the same guys that did PostSharp. It is still in preview, but Metalama 1.0 will be released in a week or 2, and in next year, it will probably get most of features found in PostSharp... And it has a nice community on Slack and the authors of this Metalama framework are super supportive, they helped me with each question I had, and I had a lot of them already :D
And so this library is perfect for creating custom aspects, but could easily be used for this merging of attributes :) It will be even better then the approach above, because once you see transformed file (using Metalama Diff Preview - you gotta install Metalama extension to VS), then you will actually see all those original attributes there, in a transformed file :)
And so this is how easily I will merge 3 attributes into 1 with Metalama:
Metalama easy merging of attributes 1
(This would be for aspect attributes created by Metalama)
Or, for other attributes (from other libraries), that don't need to do the aspect work, it would by like this: (And this is probably what you want to use, not the first example...):

Automapper: Access member information in custom converter

I wrote my custom converter to map between classes, but although the types are all the same for all attributes, some of my attributes need a special treatment.
So I would like to access the name of the property, what is currently mapped. There used to be a property "memberName" in the ResolutionContext, but it is not here anymore.
I could also decorate every attribute or pass information about every attribute in the Map-function itself.
What is the recommended way in this case?
public class IntToTermsetConverter : ITypeConverter<List<GenericHistory<Term>>, List<GenericHistory<int?>>>
{
ITermsetService termsetService;
public IntToTermsetConverter()
{
this.termsetService = new TermsetService(new TermsetRepository(new UnitOfWork()));
}
public List<GenericHistory<int?>> Convert(List<GenericHistory<Term>> genHis, List<GenericHistory<int?>> genTarget, ResolutionContext context)
{
//Need some information about the currently mapped attribute - for example the name or something else I could pass
if (genHis != null)
return termsetService.FromTermToIndex(genHis, (string)context.Options.Items["termsetName"]);
else
return null;
}
}

Implement interfaces based on class properties without reflection

This page on the PostSharp website has the following teaser:
One of the common situations that you will encounter is the need to implement a specific interface on a large number of classes. This may be INotifyPropertyChanged, IDispose, IEquatable or some custom interface that you have created.
I'd like to write a custom aspect that implements a general version of IEquatable based on the properties of the class it's applied to (preferably at compile-time instead of by using reflection at runtime). It would be good to just be able to add an attribute to a simple class rather than having to implement a custom method each time. Is that possible? I'd hope so, since it's specifically called out in this introduction, but I haven't been able to track down any example code.
I've seen this example from the PostSharp website that includes an example of introducing the IIdentifiable interface. But it just returns a GUID that's independent of the class that the new interface is added to.
Is there a way to construct a custom attribute that implements IEquatable based on the properties of the type that it's applied to (i.e. making two instances equal if all of their properties are equal)?
I've found a solution using T4 templates but would like to know if the same can be achieved using PostSharp.
Edit:
To be clear, I'd like to be able to write something like this:
[AutoEquatable]
public class Thing
{
int Id { get; set; }
string Description { get; get; }
}
and have it automatically converted to this:
public class Thing
{
int Id { get; set; }
string Description { get; get; }
public override bool Equals(object other)
{
Thing o = other as Thing;
if (o == null) return false;
// generated in a loop based on the properties
if (!Id.Equals(o.Id)) return false;
if (!Description.Equals(o.Description)) return false;
return true;
}
}

This is possible with PostSharp 4.0 using the following code;
[PSerializable]
class EquatableAttribute : InstanceLevelAspect, IAdviceProvider
{
public List<ILocationBinding> Fields;
[ImportMember("Equals", IsRequired = true, Order = ImportMemberOrder.BeforeIntroductions)]
public Func<object, bool> EqualsBaseMethod;
[IntroduceMember(IsVirtual = true, OverrideAction = MemberOverrideAction.OverrideOrFail)]
public new bool Equals(object other)
{
// TODO: Define a smarter way to determine if base.Equals should be invoked.
if (this.EqualsBaseMethod.Method.DeclaringType != typeof(object) )
{
if (!this.EqualsBaseMethod(other))
return false;
}
object instance = this.Instance;
foreach (ILocationBinding binding in this.Fields)
{
// The following code is inefficient because it boxes all fields. There is currently no workaround.
object thisFieldValue = binding.GetValue(ref instance, Arguments.Empty);
object otherFieldValue = binding.GetValue(ref other, Arguments.Empty);
if (!object.Equals(thisFieldValue, otherFieldValue))
return false;
}
return true;
}
// TODO: Implement GetHashCode the same way.
public IEnumerable<AdviceInstance> ProvideAdvices(object targetElement)
{
Type targetType = (Type) targetElement;
FieldInfo bindingField = this.GetType().GetField("Fields");
foreach (
FieldInfo field in
targetType.GetFields(BindingFlags.DeclaredOnly | BindingFlags.Instance | BindingFlags.Public |
BindingFlags.NonPublic))
{
yield return new ImportLocationAdviceInstance(bindingField, new LocationInfo(field));
}
}
}

I'm afraid this can not be done with PostSharp. PostSharp "injects" aspects code in your clases but you have to code the aspects. The key here is indetify common behavior and cross cutting concern in your system and model it as Aspects.
In the example of IIdentifiable you can see how GUID is a unique identifier that can be use by a lot of different classes in your system. It is common code, it is cross cutting concern and you find yourself REPEATING code in all your class entities so Identificable can be modeled as Aspect and get rid of repeating code.
As diferent classes has diferent Equals implementation you can not "deatach" (convert to aspect) the implementation of Equals. Equals is not a common behavior. Equals is not cross cutting concern. Equals can not be an Aspect (without reflection).

Pass arguments in to a Type that is supplied to an Attribute

I have an attribute that I am using to decorate object properties with. The attribute identifies the properties as needing validation to be performed on them. I am essentially implementing the Strategy Pattern and building all of the validation (really only about 6 types) in to individual objects that I can use across multiple classes. What I want to do, is provide parameters to the validation classes, without having to create an attribute for each validation object variation.
My attribute looks like this:
[AttributeUsage(AttributeTargets.Property)]
public class ValidationRuleAttribute : Attribute
{
public ValidationRuleAttribute(Type validationRule, string customFailureMessage = "")
{
if (typeof(IValidationRule).IsAssignableFrom(validationRule))
{
this.ValidationRule = string.IsNullOrEmpty(customFailureMessage)
? Activator.CreateInstance(validationRule, customFailureMessage) as IValidationRule
: Activator.CreateInstance(validationRule) as IValidationRule;
}
else
{
throw new ArgumentException(
string.Format(
"ValidationRule attributes can only be used with IValidationRule implementations. The '{0}' Tyoe is not supported.",
validationRule.Name));
}
}
public IValidationRule ValidationRule { get; private set; }
}
As an example, I have a simple StringIsNotNull validation object. I want to expand on it by allowing me to specify a minimum string length requirement. So the StringIsNotEmptyValidation would become StringHasMinimumLengthValidation
public class StringIsNotEmptyValidation : IValidationRule
{
private readonly string customErrorMessage;
public StringIsNotEmptyValidation()
{
}
public StringIsNotEmptyValidation(string customErrorMessage)
{
this.customErrorMessage = customErrorMessage;
}
public string ResultMessage { get; private set; }
public IValidationMessage Validate(System.Reflection.PropertyInfo property, IValidatable sender)
{
string value = property.GetValue(sender).ToString();
// Validate
bool isFailed = string.IsNullOrWhiteSpace(value);
if (isFailed)
{
if (string.IsNullOrEmpty(this.customErrorMessage))
{
DisplayNameAttribute displayName = property.GetCustomAttribute<DisplayNameAttribute>(true);
string errorMessage = displayName == null
? string.Format("You can not leave {0} empty.", property.Name)
: string.Format("You can not leave {0} empty.", displayName.DisplayName);
this.ResultMessage = errorMessage;
return new ValidationErrorMessage(errorMessage);
}
else
{
this.ResultMessage = this.customErrorMessage;
return new ValidationErrorMessage(customErrorMessage);
}
}
this.ResultMessage = string.Empty;
return null;
}
}
Within my model, I decorate my property with the attribute and validation object.
[RepositoryParameter(DbType.String)]
[ValidationRule(typeof(StringIsNotEmptyValidation))]
public string WorkDescription
{
get
{
return this.workDescription ?? string.Empty;
}
set
{
this.SetPropertyByReference(ref this.workDescription, value);
if (this.HasValidationMessageType<ValidationErrorMessage>(this.GetPropertyName(p => p.WorkDescription)))
{
this.Validate();
}
}
}
What I want to do, is write my attribute usage like this:
[ValidationRule(new StringIsNotEmptyValidation(minimumLength: 4))]
Since you can't instance objects in an attribute constructor, I'm forced to provide the attributes in my attribute constructor like this:
[ValidationRule(typeof(StringIsNotEmptyValidation), minLength: 4)]
I don't like this because if I have a ObjectIsNotNull or a StringIsInRange I will need to do two things:
Create a new attribute for each parameter variation (or a lot of overloads)
Set up the validation rule instances within the constructor, which will have varying property names.
The Validation object implements the following interface
public interface IValidationRule
{
string ResultMessage { get; }
IValidationMessage Validate(PropertyInfo property, IValidatable sender);
}
I don't want to bloat my interface with a large number of properties that might be used or might not be used depending on the Rule implementing it. It also makes it difficult to assign attribute params to the rule object.
So my question is how can I provide parameters to the IValidationRule concrete classes, without creating multiple attribute types to facilitate this? This is being used so that I an do cross-object validation. The PropertyInfo passed in to the validation rule is from a cache of PropertyInfo's. I need to keep the amount of reflection used down, otherwise I'd just use attributes for each rule parameter and use reflection on sender to figure out what ranges to use.
Update
After discussing this with Corey, it does indeed appear that attributes are supported in Universal Apps and it is only the DataAnnotations namespace that is missing. In order to get access to the attributes, I had to add a using statement to System.Reflection in order to gain access to a series of extension methods that expose the GetCustomAttribute methods. They are now extension methods and not built in to the Type class.
So I suppose in the end, I can just create my validation logic within the attributes, instead of individual objects. I can't think of any downsides to going this route.
In order to access the attributes in a Universal App, you have to include System.Reflection as a using statement, then access via the GetRuntimeProperties() extension method.
var validationRule = this
.GetType()
.GetRuntimeProperties() // Can be GetRuntimeFields or GetRuntimeMethods as well.
.FirstOrDefault(p => p.GetCustomAttribute<IntegerInRangeAttribute>() != null);

So there are a few options here.
First, and often used, is to have a different attribute for each type of rule you want to process. You are already building classes for each of your rules, so instead of having some encapsulating attribute that instantiates them all just make each rule an attribute:
[StringMinLengthRule(5)]
public string SomeString { get; set; }
Build the validation logic into your attributes - say with a base attribute that does the bulk of the work, calling a virtual method to do the actual validation. Then you can just enumerate the rule attributes and call them from your validation method.
Next, you can have a number of different properties on your attribute that can be set during declaration to provide the properties for your various rules:
[Validation(RuleType.StringMinLength, MinLength = 5)]
public string SomeString { get; set; }
You could still have the rules be processed in the ValidationAttribute itself, or create IValidationRule instances at run-time to process the actual validations. Unfortunately there's nothing to stop you from adding a Validation attribute that sets the wrong properties for the rule type, resulting in errors at run-time when you try to validate an instance.
Finally, something that works but probably shouldn't... and it's kinda ugly:
[AttributeUsage(AttributeTargets.Field | AttributeTargets.Property)]
public class ValidationRuleAttribute : Attribute
{
public IValidationRule ValidationRule { get; private set; }
public ValidationRuleAttribute(RuleType type, params object[] parms)
{
if (type == RuleType.NotNull)
{
if (parms.Length != 0)
throw new ArgumentException("RuleType.NotNull requires 0 parameters", "parms");
ValidationRule = new NotNullValidation();
}
if (type == RuleType.StringMinLength)
{
if (parms.Length != 1)
throw new ArgumentException("RuleType.StringMinLength requires 1 parameter", "parms");
if (!(parms[0] is int))
throw new ArgumentException("RuleType.StringMinLength requires an integer", "parms");
ValidationRule = new StringLengthValidation((int)parms[0]);
}
}
}
The biggest problem with it is that it won't complain until you try to instantiate a class at run-time that has a bad Validation attribute. Your code can run quite happily up until the point where it tries to create an instance of that bad class, at which point all of the attributes will actually be constructed and those ArgumentExceptions start flying.
In fact only the first option doesn't suffer from run-time problems, because you can control the types of parameters being supplied by using the correct constructor formats. You can still tell it to do silly things - like requiring that strings must have less than 0 length for instance - but that's up to you :P

Create Attribute to represent multiple sub attributes in C#

I have the following type of code sample across one of my projects...
[Obfuscation(Exclude = true)]
[UsedImplicitly]
public DelegateCommand<object> OpenXCommand { get; private set; }
I am finding the attributes are adding a lot of "noise" to the code - I also see it in a way violating the DRY principle since I may have several properties like this in one class, all with the same attribute decoration.
Q: Is there some way I can set up an attribute that will represent a combination of sub attributes?
Ideally I would like something that looks like this..
[MyStandardCommandAttribute]
public DelegateCommand<object> OpenXCommand { get; private set; }
I have not implemented my own attributes before so I am unsure if this is possible. Any suggestions?

No. Your one attribute cannot "be" Obfuscation and UsedImplicitly at the same time (there's no multiple inheritance in C#).
Code looking for e.g. UsedImplicitlyAttribute has no way of knowing that MyStandardCommandAttribute is supposed to represent UsedImplicitlyAttribute (unless you control all of the code using all of these attributes).

Unfortunately, there's no way to do this in C#.
But, if you control the places that read these attributes (with reflection), you can do it by convention.
For example, you can have a marker interface that will "annotate" your attribute with the attributes it proxies (sounds like a meta-attribute):
public interface AttributeProxy<T>
where T : Attribute {}
public class MyStandardCommandAttribute :
Attribute,
AttributeProxy<ObfuscationAttribute>,
AttributeProxy<UsedImplicitlyAttribute> {}
(Of course, you also have to match the right AttributeUsages. And you can't set properties on the proxied attributes like this.)
Now, you could go a step further and use an IL manipulation library, like Mono.Cecil, to actually transfer the attributes appropriately in a post-compilation step. In this case, it would work even if it weren't you reflecting on these attributes.
Update: still in the reflect-your-own-attributes scenario, you can use the below code to get to proxied attributes, even setting properties values:
public interface IAttributeProxy {
Attribute[] GetProxiedAttributes();
}
public class MyStandardCommandAttribute : Attribute, IAttributeProxy {
public Attribute[] GetProxiedAttributes() {
return new Attribute[] {
new ObfuscationAttribute { Exclude = true },
new UsedImplicitlyAttribute()
};
}
}
Use this extension method on your reflection code:
public static object[] GetCustomAttributesWithProxied(this MemberInfo self, bool inherit) {
var attributes = self.GetCustomAttributes(inherit);
return attributes.SelectMany(ExpandProxies).ToArray();
}
private static object[] ExpandProxies(object attribute) {
if (attribute is IAttributeProxy) {
return ((IAttributeProxy)attribute).GetProxiedAttributes().
SelectMany(ExpandProxies).ToArray(); // don't create an endless loop with proxies!
}
else {
return new object[] { attribute };
}
}