Extension methods can only see public (or C# with Example

internal) members of the extended class 
public class SomeClass 
{ 
public void DoStuff() 
{ 
} 
protected void DoMagic() 
{ 
} 
} 
public static class SomeClassExtensions 
{ 
public static void DoStuffWrapper(this SomeClass someInstance) 
{ 
someInstance.DoStuff(); // ok 
} 
public static void DoMagicWrapper(this SomeClass someInstance) 
{ 
someInstance.DoMagic(); //  compilation  error 
} 
} 
Extension methods are just a syntactic sugar, and are not actually members of the class they extend. This means 
that they cannot break encapsulation —they only have access to public (or when implemented in the same 
assembly, internal) fields, properties and methods.