Error : Since Class has comimport attribute, method must be extern or abstract

Dear All,

I am working in a Excel RTD Server functionality.
My code looks like...

[ComImport]
[TypeLibType(2)]
[Guid("....................................")]
[ClassInterface()]

public class RTDFunctions : IRtdServer
{   
    public int ServerStart()
    {
     //Some body
    }
}

I am getting the following error.
Since 'RTDFunctions' has comimport attribute, 'ServerStart' must be extern or abstract.

Thanks in advance.
PMM :)