Use Reflection

Not recommended, but you could possibly access any private variable of any type using another extension method like so:

public static T GetFieldValue<T>(this object obj, string name) {
    var field = obj.GetType().GetField(name, BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance);
    return (T)field?.GetValue(obj);
}

And then access a private field of an arbitrary type:

Foo foo = new Foo();
string privateBar = foo.GetFieldValue<string>("_bar");