Adapter pattern without the use of interfaces

Adapter pattern without the use of interfaces

this is my first question here so i'm sorry for any mistakes
i'm trying to understand if the Adapter pattern implementation is
dependent on interface realizations. from what i read, i should use
adapter when i need to Convert the interface of a class into another
interface clients expect.
i have been told the classic example of the 3 pronged plug using an
adapter to connect to a two pronged plug...
so, from what i understand, if some method in a class has a method with a
specific paramater signature, and i want to wrap it with logic that
require more (or less) paramaters, then i should implement the adapter
pattern.
from all the examples iv'e seen, the proper way to do this is to create an
adapter class that realizes an interface with the desired method
signature, and to hold the adoptee as a member in the adapter class. then,
i can call the adpotee method through the realized interface method and
input the logic.
the question is, if i only need to use one adoptee class, why use an
interface? is it not simpler to drop the interface bit and just hold the
adoptee as a member and implement the desired logic in a "stand alone"
method?
is it required to realize an interface method here for the adapter pattern
to be valid?
public class PostStatusAdapter
{
public interface IpostStatus
{
void Post(string i_Post, string i_Password);
}
public class UserAdapter : IpostStatus
{
User Adoptee = new User();
void IpostStatus.Post(string i_Post, string i_Password)
{
if (PasswordCorrect(i_Password))
{
Adoptee.PostStatus(i_Post);
}
}
private bool PasswordCorrect(string i_Password) { ... }
}
}