from http://github.com/BLK10/Singleton

Singleton Toolbox

Aggregating singletons: the Toolbox

A Toolbox component

[ExecuteInEditMode]
[DisallowMultipleComponent]
[RequireComponent(typeof(Toolbox))]
public class ToolComponent : MonoBehaviour, IToolComponent
{


    public static IToolComponent Instance
    {
        get
        {
            IToolbox tool = Toolbox.Instance;
            if (tool != null)
                return (tool.GetOrAddComponent<ToolComponent>() as IToolComponent);

            throw new NullReferenceException("Singleton \"Toolbox\" could not be null.");
        }
    }
}

A RuntimeToolbox component

[DisallowMultipleComponent]
[RequireComponent(typeof(RuntimeToolbox))]
public class RuntimeToolComponent : MonoBehaviour, IRuntimeToolComponent
{


    public static IRuntimeToolComponent Instance
    {
        get
        {
            IToolbox rtool = RuntimeToolbox.Instance;

            if (rtool != null)
                return (rtool.GetOrAddComponent<RuntimeToolComponent>() as IRuntimeToolComponent);
            
            throw new NullReferenceException("Singleton \"RuntimeToolbox\" could not be null.");                
        }
    }
}