I just wanted to add that anything can break it; I had an enum in one of mine driving me crazy trying to fix.

One more thing, [ExecuteInEditMode] breaks the class as well - I wouldn't be surprised if other, similar flags break things.

Here's the answer:

• If there are two classes in a file then it works fine without a namespace

• If there are two classes in a file then putting them both in a namespace fails

• If, in the example, I put WaitForRelease as a child of ActivatedObject it works fine, so nested classes work

• If the class has methods with default parameters it fails [Added due to answer below]

So for me it wasn't just default params, but also named params! (i.e. namespaces AND named params - if you use either one alone you shouldn't have problems)

This will fail:

 namespace MyNamespace
 {
    public class Settings : ScriptableObject
    {
         public static void IntMethod(int x) { }
         public static void Test()
         {
             IntMethod(@x: 10); // <-- Named param
         }
    }
 }

 // Somewhere else...
 Debug.Log(ScriptableObject.CreateInstance<Settings>()); // prints "()"

This will succeed:

 namespace MyNamespace
 {
    public class Settings : ScriptableObject
    {
         public static void IntMethod(int x) { }
         public static void Test()
         {
             IntMethod(10); // <-- Pass arg normally
         }
    }
 }

 // Somewhere else...
 Debug.Log(ScriptableObject.CreateInstance<Settings>()); // prints a valid value
 

What Works (4.5.5+)

I came across this issue in testing with versions of Unity between 4.5.5 and 5.1.1. It seems that the following are no longer limitations (in at least 4.5.5+):

• Two classes in the same file, both within a namespace.

• Having a method with a default parameter within the MonoBehaviour/ ScriptableObject subclass that's in a namespace.

I have not tested with named parameters and therefore cannot speak to them.

What Doesn't Work (as late as 5.1.1)

However, I did run afoul of an issue with namespaces and MonoBehaviours/ScriptableObjects. The following WILL NOT WORK:

• A non-MonoBehaviour/ScriptableObject class that has a method with a default parameter appears before the MonoBehaviour/ScriptableObject subclass in the script file.

The following examples are expected to be in a file called TestClass.cs.

Works: No Namespaces

The SupportClass is simply a wrapper for an int. This works.

 using UnityEngine;
 
 [System.Serializable]
 public class SupportClass
 {
     public int anInt;
 
     public void SetTheInt(int i = 1)
     {
         anInt = i;
     }
 }
 
 public class TestClass : MonoBehaviour
 {
     public SupportClass someVal;
 
     public int TestMethod(int i = 1)
     {
         return i;
     }
 }
 

Doesn't Work: Add a Namespace

Both classes are within the namespace.

 using UnityEngine;
 
 namespace TestNamespace
 {
     [System.Serializable]
     public class SupportClass
     {
         public int anInt;
 
         public void SetTheInt(int i = 1)
         {
             anInt = i;
         }
     }
 
     public class TestClass : MonoBehaviour
     {
         public SupportClass someVal;
 
         public int TestMethod(int i = 1)
         {
             return i;
         }
     }
 }

Works: Reorder Classes Within the Namespace

Simply changing the order of the classes causes this to work again.

 using UnityEngine;
 
 namespace TestNamespace
 {
     public class TestClass : MonoBehaviour
     {
         public SupportClass someVal;
         
         public int TestMethod(int i = 1)
         {
             return i;
         }
     }
 
     [System.Serializable]
     public class SupportClass
     {
         public int anInt;
 
         public void SetTheInt(int i = 1)
         {
             anInt = i;
         }
     }
 }

Works: Remove the Default Parameter from the Support Class

Simply removing the default parameter from the support class when it appears first, also fixes the problem.

 using UnityEngine;
 
 namespace TestNamespace
 {
     [System.Serializable]
     public class SupportClass
     {
         public int anInt;
 
         public void SetTheInt(int i)
         {
             anInt = i;
         }
     }
 
     public class TestClass : MonoBehaviour
     {
         public SupportClass someVal;
 
         public int TestMethod(int i = 1)
         {
             return i;
         }
     }

Final Note

You can have multiple support classes defined before the MonoBehaviour/ScriptableObject subclass. This will work 100% fine until you add a default parameter to any method in any support class that appears in the script file before the MonoBehaviour/ScriptableObject subclass. In fact, if you have multiple support classes, you can put those without default parameters in their methods before the MonoBehaviour/ScriptableObject subclass and those with them after. This also works fine.