UserInfo user = new UserInfo() {
    Id = 1,
    Name = "MyName",
    Address = new AddressInfo() {
        Street = "MyStreet",
        ZIP = 123456
    }
};
try {
    byte[] bytes = Proto.Encode<UserInfo>(user);
    UserInfo newUser = Proto.Decode<UserInfo>(bytes);
    Debug.Log(newUser);
} catch (Exception e) {
    Debug.Log(e);
}

[ProtoContract]
public class UserInfo {
    [ProtoMember(1)]
    public int Id;

    [ProtoMember(2)]
    public string Name;

    [ProtoMember(3)]
    public AddressInfo Address;
}

[ProtoContract]
public class AddressInfo {
    [ProtoMember(1)]
    public string Street;

    [ProtoMember(2)]
    public int ZIP;
}