Get class DisplayName attribute value

using your example I got it working doing this:

 var displayName = typeof(Opportunity)
    .GetCustomAttributes(typeof(DisplayNameAttribute), true)
    .FirstOrDefault() as DisplayNameAttribute;

if (displayName != null)
    Console.WriteLine(displayName.DisplayName);

This outputted "Opportunity".

Or for the more generic way you seem to be doing it:

public static string GetDisplayName<T>()
{
    var displayName = typeof(T)
      .GetCustomAttributes(typeof(DisplayNameAttribute), true)
      .FirstOrDefault() as DisplayNameAttribute;

    if (displayName != null)
        return displayName.DisplayName;

     return "";
}

Usage:

string displayName = GetDisplayName<Opportunity>();

GetCustomAttributes() returns an object[], so you need to apply the specific cast first before accessing the required property values.