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NickGravelyn

[.net] Validating Attribute Constructor Input?

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NickGravelyn    855
I'm working on a project and finally have a really good use for some custom attributes. Trouble is that I want to do some validation on the input of the constructor for the attribute. Basically this is what I want to have:
    [AttributeUsage(AttributeTargets.Class)]
    public class RequiredDataProviderAttribute : Attribute
    {
        Type dataProviderType;

        public Type DataProviderType
        {
            get { return dataProviderType; }
        }

        public RequiredDataProviderAttribute(Type type)
        {
            if (!typeof(IDataProvider).IsAssignableFrom(type))
                throw new Exception("Type is not derived from IDataProvider.");

            dataProviderType = type;
        }
    }

However that exception isn't thrown at compile time or runtime. Is there any way to make a validation like that and have it enforced? I would prefer to catch it at compile time, but either really works for me.

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mutex    1111
It looks like the attribute object is constructed when you try to retrieve it, and so the exception will occur at runtime:


[Asdf(null)]
class Program
{
static void Main(string[] args)
{
Type type = typeof(Program);
object[] attrs = type.GetCustomAttributes(false);
foreach (Attribute attr in attrs)
{
Console.WriteLine(attr.GetType().Name);
}
}
}

[AttributeUsage(AttributeTargets.All)]
public class AsdfAttribute : Attribute
{
public AsdfAttribute(string text)
{
if (text == null)
throw new ArgumentNullException("text");
}
}




Running results in ArgumentNullException thrown by AsdfAttribute inside the GetCustomAttributes call:

> TestAttribute.AsdfAttribute.AsdfAttribute(text = null) Line 29 C#
[Native to Managed Transition]
[Managed to Native Transition]
System.Reflection.CustomAttribute.CreateCaObject(module, ctor, blob = 11675394, blobEnd, namedArgs = 0) + 0x69 bytes
System.Reflection.CustomAttribute.GetCustomAttributes(decoratedModule = {TestAttribute.exe}, decoratedMetadataToken = 33554434, pcaCount = 0, attributeFilterType = {Name = "Object" FullName = "System.Object"}, mustBeInheritable = false, derivedAttributes = null) + 0x1eb bytes
System.Reflection.CustomAttribute.GetCustomAttributes(type, caType, inherit) + 0x103 bytes
System.RuntimeType.GetCustomAttributes(inherit) + 0x2e bytes
TestAttribute.Program.Main(args = {string[0]}) Line 14 + 0x9 bytes C#

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