Question

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 ???
     }
}

Answer 1

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;
}