delegates - What does "a field initializer cannot reference non static fields" mean in C#?

Question

I don't understand this error in C#

error CS0236: A field initializer cannot reference the non-static field, method, or property 'Prv.DB.getUserName(long)'

For the following code

public class MyDictionary<K, V>
{
    public delegate V NonExistentKey(K k);
    NonExistentKey nonExistentKey;

    public MyDictionary(NonExistentKey nonExistentKey_) { }
}

class DB
{
    SQLiteConnection connection;
    SQLiteCommand command;

    MyDictionary<long, string> usernameDict = new MyDictionary<long, string>(getUserName);

    string getUserName(long userId) { }
}

Answer 1

Any object initializer used outside a constructor has to refer to static members, as the instance hasn't been constructed until the constructor is run, and direct variable initialization conceptually happens before any constructor is run. getUserName is an instance method, but the containing instance isn't available.

To fix it, try putting the usernameDict initializer inside a constructor.