[StructLayout(LayoutKind.Sequential, CharSet=CharSet.Ansi)]
public struct UserProfileData
{
    int userProfileRevision;
    [MarshalAs(UnmanagedType.LPStr)]
    public String firstName;
    [MarshalAs(UnmanagedType.LPStr)]
    public String lastName;
    [MarshalAs(UnmanagedType.LPStr)]
    public String memberids;
    [MarshalAs(UnmanagedType.LPStr)]
    public String emailAddress;
}

typedef struct userProfile
{
    int profileRevision;
    char *firstName;
    char *lastName;
    char *memberids;
    char *emailAddress;
} userProfile_t;

int getUserProfileData(userProfile_t *pUserProfile);

[StructLayout(LayoutKind.Sequential)]
public struct UserProfileData
{
    int userProfileRevision;
    public IntPtr firstName;
    public IntPtr lastName;
    public IntPtr memberids;
    public IntPtr emailAddress;
}

[DllImport(@"mydll.dll", CallingConvention=CallingConvention.???)]
private static extern int getUserProfileData(out UserProfileData userProfile);

string firstName = Marshal.PtrToStringAnsi(userProfile.firstName);