Simplifying creation of inherited classes

class Entity {
    public string Field1 { get; set; }
    public string Field2 { get; set; }
    public string Field3 { get; set; }
    public int? FieldA { get; set; }
    public int? FieldB { get; set; }

}

class Base {
    public string Field1 { get; set; }
    public string Field2 { get; set; }
    public string Field3 { get; set; }
}

class A : Base {
    public int FieldA { get; set; }
}

class B : Base {
    public int FieldB { get; set; }
}

Entity entity = GetEntityFromDatabase();
if (entity.Field1 == "A") {
    return new A {
        Field1 = entity.Field1,
        Field2 = entity.Field2,
        Field3 = entity.Field3,
        FieldA = entity.FieldA.Value
    };
} else if (entity.Field2 == "B") {
    return new B {
        Field1 = entity.Field1,
        Field2 = entity.Field2,
        Field3 = entity.Field3,
        FieldB = entity.FieldB.Value
    };
}