How to define a bitmap at varible

[C# Programming]

hi All,
I would like to define at c# a bitmap in a struct
Define spesific bit to be use
I don't know how to do it


at c++ i should define it as
typedef struct
{
unsigned long m_opcode :2;
unsigned long m_id :3;
unsigned long m_src :4;
unsigned long m_address :3;
unsigned long m_timer :18;
unsigned long m_reserved :1;
} WiMAXBlock1HeaderT;


how can i define it at C#?

thanks
ronen