Using Reflection.Emit to copy a custom attribute to another method

Question

I am trying to generate a new set of wcf interfaces based on existing interfaces. I am using the Reflection.Emit namespace to accomplish this. My problem is how to copy the old custom attributes from one method to the new method. Every example I have seen of SetCustomAttributes() requires knowing the attribute type beforehand. I need to discover the attribute type at runtime. Any thoughts?

Answer 1

The answer you (frjames) posted is close, but doesn't account for property initializers like...

[ServiceBehavior(Name="ServiceName")]

However, the idea of converting CustomAttributeData to a CustomAttributeBuilder for use in Reflection.Emit is right on.

I ended up having to do this for an open source project (Autofac) and came up with this extension method:

public static CustomAttributeBuilder ToAttributeBuilder(this CustomAttributeData data)
{
  if (data == null)
  {
    throw new ArgumentNullException("data");
  }

  var constructorArguments = new List<object>();
  foreach (var ctorArg in data.ConstructorArguments)
  {
    constructorArguments.Add(ctorArg.Value);
  }

  var propertyArguments = new List<PropertyInfo>();
  var propertyArgumentValues = new List<object>();
  var fieldArguments = new List<FieldInfo>();
  var fieldArgumentValues = new List<object>();
  foreach (var namedArg in data.NamedArguments)
  {
    var fi = namedArg.MemberInfo as FieldInfo;
    var pi = namedArg.MemberInfo as PropertyInfo;

    if (fi != null)
    {
      fieldArguments.Add(fi);
      fieldArgumentValues.Add(namedArg.TypedValue.Value);
    }
    else if (pi != null)
    {
      propertyArguments.Add(pi);
      propertyArgumentValues.Add(namedArg.TypedValue.Value);
    }
  }
  return new CustomAttributeBuilder(
    data.Constructor,
    constructorArguments.ToArray(),
    propertyArguments.ToArray(),
    propertyArgumentValues.ToArray(),
    fieldArguments.ToArray(),
    fieldArgumentValues.ToArray());
}

That one accounts for all the ways to initialize the attribute.

Answer 2

Here is the answer I came up with after some more research.

CustomAttributeBuilder ct = AddAttributesToMemberInfo(methodInfo);
if (ct != null)
{
    methodBuilder.SetCustomAttribute(ct);
}

CustomAttributeBuilder AddAttributesToMemberInfo(MemberInfo oldMember)
{
    CustomAttributeBuilder ct = null;
    IList<CustomAttributeData> customMethodAttributes = CustomAttributeData.GetCustomAttributes(oldMember);
    foreach (CustomAttributeData att in customMethodAttributes)
    {
        List<object> namedFieldValues = new List<object>();
        List<FieldInfo> fields = new List<FieldInfo>();
        List<object> constructorArguments = new List<object>();
        foreach (CustomAttributeTypedArgument cata in att.ConstructorArguments)
        {
            constructorArguments.Add(cata.Value);
        }
        if (att.NamedArguments.Count > 0)
        {
            FieldInfo[] possibleFields = att.GetType().GetFields();

            foreach (CustomAttributeNamedArgument cana in att.NamedArguments)
            {
                for (int x = 0; x < possibleFields.Length; x++)
                {
                    if (possibleFields[x].Name.CompareTo(cana.MemberInfo.Name) == 0)
                    {
                        fields.Add(possibleFields[x]);
                        namedFieldValues.Add(cana.TypedValue.Value);
                    }
                }


            }
        }

        if (namedFieldValues.Count > 0)
        {
            ct = new CustomAttributeBuilder(att.Constructor, constructorArguments.ToArray(), fields.ToArray(), namedFieldValues.ToArray());
        }
        else
        {
            ct = new CustomAttributeBuilder(att.Constructor, constructorArguments.ToArray());
        }


    }
    return ct;
}