Question

Here is my attribute definition:

[AttributeUsage(AttributeTargets.Field)]
public class Optional : System.Attribute
{
    public Optional()
    {
    }
}

In MyClass:

[Optional] public TextBox Name;

Finally in another function:

typeof(MyClass).GetFields().ToList<FieldInfo>().ForEach(x => writer.WriteLine(
   x.FieldType + " is called " + 
   x.Name + " and has attributes " + 
   x.GetCustomAttributes(true)[0]
 ));

The problem is I get an error for index 0. I just want to check for fields where the attribute is applied. The error goes away when I remove x.GetCustomAttributes(true)[0].

Exact Error:

Exception Details: System.IndexOutOfRangeException: Index was outside the bounds of the array.

Source Error:

Line 63:             }
Line 64: 
Line 65:             typeof(T).GetFields().ToList<FieldInfo>().ForEach(x => writer.WriteLine(x.FieldType + " is called " + 
Line 66:                 x.Name + " and has attributes " + 
Line 67:                 x.GetCustomAttributes(true)[0]+ "</br>"));

Answer 1

Seems like there might be two questions here. To find all the fields that have your [Optional] attribute, you want:

typeof(MyClass).GetFields().Where(
   f => f.GetCustomAttributes(typeof(OptionalAttribute), true).Any())

To write out the custom attributes on all your fields, you want:

typeof(MyClass).ToList<FieldInfo>().ForEach(x => 
{
 writer.WriteLine(
   x.FieldType + " is called " + 
   x.Name + " and has attributes " + 
   string.Join(", ", x.GetCustomAttributes(true).Select(a => a.ToString()).ToArray()));
});