How do I get the parameters passed in to the constructor of an attribute?

Question

I'm working on a documentation generator. MSDN documentation shows the parameters passed to Attributes when they are applied. Such as [ComVisibleAttribute(true)]. How would I get those parameter values and/or the constructor called in my c# code via reflection, pdb file or otherwise?

To clarify> If someone has documented a method that has an attribute on it like so:

/// <summary> foo does bar </summary>
[SomeCustomAttribute("a supplied value")]
void Foo() {
  DoBar();
}

I want to be able to show the signature of the method in my documentation like so:

Signature:

[SomeCustomAttribute("a supplied value")]
void Foo();

Are you asking about coding up your own attribute that takes parameters, or are you looking to find out through reflection the way someone else's attribute has been constructed? — Sergey Kalinichenko, Feb 21 '13 at 22:58
through reflection the way someone else's attribute has been constructed — Charles Lambert, Feb 21 '13 at 23:05
Hmm, thanks for the clarification. I apologize for misunderstanding your question. My method obviously wouldn't give that. You'd probably have to inspect the IL, but I'm not sure how to go about doing that. — p.s.w.g, Feb 21 '13 at 23:13

score 6 · Accepted Answer · answered Feb 21 '13 at 23:14

If you have a member for which you want to get the custom attributes and the constructor arguments, you can use the following reflection code:

MemberInfo member;      // <-- Get a member

var customAttributes = member.GetCustomAttributesData();
foreach (var data in customAttributes)
{
    // The type of the attribute,
    // e.g. "SomeCustomAttribute"
    Console.WriteLine(data.AttributeType);

    foreach (var arg in data.ConstructorArguments)
    {
        // The type and value of the constructor arguments,
        // e.g. "System.String a supplied value"
        Console.WriteLine(arg.ArgumentType + " " + arg.Value);
    }
}

To get a member, start with getting the type. There are two ways to get a type.

If you have an instance obj, call Type type = obj.GetType();.
If you have a type name MyType, do Type type = typeof(MyType);.

Then you can find, for example, a particular method. Look at the reflection documentation for more info.

MemberInfo member = typeof(MyType).GetMethod("Foo");

What about class level attributes? The member part is easy. I haven't figured out how to reflect on class level attribute parameters. — jwize, Feb 18 '14 at 06:04

p.s.w.g · Answer 2 · 2013-02-22T17:53:12.663

For the ComVisibileAttribute, the parameter passed to the constructor becomes the Value property.

[ComVisibleAttribute(true)]
public class MyClass { ... }

...

Type classType = typeof(MyClass);
object[] attrs = classType.GetCustomAttributes(true);
foreach (object attr in attrs)
{
    ComVisibleAttribute comVisible = attr as ComVisibleAttribute;
    if (comVisible != null)
    {
        return comVisible.Value // returns true
    }
}

Other attributes will follow a similar design pattern.

EDIT

I found this article about Mono.Cecil that describes how to do something very similar. This looks it ought to do what you need.

foreach (CustomAttribute eca in classType.CustomAttributes)
{
    Console.WriteLine("[{0}({1})]", eca, eca.ConstructorParameters.Join(", "));
}

How do I get the parameters passed in to the constructor of an attribute?

2 Answers2