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c#attributes
Tekno
Tekno

Reputation: 193

Attributes and classes

I'm searching to how can I know inside an attribute definition if the class I apply the attribute to, has another attribute
Example: 

[My1Attribute]  
public class MyClass  
{   
    [My2Attribute]  
    int aux{get;set;}  

}        

internal sealed class My1Attribute : Attribute
{ 
     public My1Attribute
     {
           // How can  I Know if 'MyClass' has My2Attribute applied ???
     }
}

Upvotes: 2

Views: 305

Answers (4)

Reddog
Reddog

Reputation: 15579

The attribute itself will not know about the class to which it's attached. You will need to use some other service/helper function/whatever to pair them up.

However, you might find the following useful:

public static bool HasAttribute<T, TAttribute>() where TAttribute : Attribute
{
    return typeof (T).GetCustomAttributes(typeof (TAttribute), true).Any();
}

Edit: For finding attributes on member fields

/// <summary>
/// Returns all the (accessible) fields or properties that for a given type that
/// have the "T" attribute declared on them.
/// </summary>
/// <param name="type">Type object to search</param>
/// <returns>List of matching members</returns>
public static List<MemberInfo> FindMembers<T>(Type type) where T : Attribute
{
    return FindMembers<T>(type, MemberTypes.Field | MemberTypes.Property);
}

/// <summary>
/// Returns all the (accessible) fields or properties that for a given type that
/// have the "T" attribute declared on them.
/// </summary>
/// <param name="type">Type object to search</param>
/// <returns>List of matching members</returns>
public static List<MemberInfo> FindMembers<T>(Type type, MemberTypes memberTypesFlags) where T : Attribute
{
    const BindingFlags FieldBindingFlags = BindingFlags.Instance | BindingFlags.NonPublic | BindingFlags.Public;

    List<MemberInfo> members = new List<MemberInfo>();
    members.AddRange(type.FindMembers(
                            memberTypesFlags,
                            FieldBindingFlags,
                            HasAttribute<T>, // Use delegate from below...
                            null)); // This arg is ignored by the delegate anyway...

    return members;
}

public static bool HasAttribute<T>(MemberInfo mi) where T : Attribute
{
    return GetAttribute<T>(mi) != null;
}

public static bool HasAttribute<T>(MemberInfo mi, object o) where T : Attribute
{
    return GetAttribute<T>(mi) != null;
}

Upvotes: 2

T. Kiley
T. Kiley

Reputation: 2802

I am guessing you mean in general find out if any class with MyAttribute1 has My2Attribute (rather than specifically MyClass). The only way I can think of doing it is getting a list of all classes from reflection and iterating through them checking which ones have Attribute1 and then check if they have Attribute2.

I don't think you can do anything clever like automatically retrieve a list of classes with current attribute.

Upvotes: 0

Matthew Abbott
Matthew Abbott

Reputation: 61589

In this instance, you would need to define your rules about how you determine what members you are going to check. In your example, you're using the attributed decorated on a property, so given that you have an instance of Type for MyClass (e.g. typeof(MyClass)), you can grab the properties:

var property = type.GetProperty("aux", BindingFlags.Instance | BindingFlags.NonPublic);
if (property.IsDefined(typeof(My1Attribute))) 
{
    // Property has the attribute.
}

(This is assuming you actually want to grab that non-public instance property, if not adjust your BindingFlags).

If you actually want to use the attribute:

var attib = property.GetCustomAttributes(typeof(My1Attribute), false)[0];
// Do something with the attribute instance.

Upvotes: 1

Nano Taboada
Nano Taboada

Reputation: 4182

Have you tried Reflection?, plus here's a related question that you might find helpful: How to check if C# class has security attribute used

Upvotes: 0

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