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Visual Basic .NET The Complete Reference phần 5

Khi bạn cần phải sửa đổi các biến cho các phương pháp khác trong lớp hoặc cho các hoạt động quan tâm đến toàn bộ lớp, sau đó sử dụng các biến lớp. Nếu không có phương pháp khác đòi hỏi phải truy cập vào biến hoặc nhu cầu sử dụng nó, sau đó khai báo địa phương để phương pháp này. | Flags Figure 8-1 Using WinCV to look at the built-in enumerations Using the Enum type we can similarly represent a collection of bit flags for any application. Looking at the enumeration shown in Figure 8-1 one can conclude the following an expression that checks for IsUnique and IsKey seems perfectly feasible as does one that verifies whether a file IsCompressed IsEncrypted or IsHidden AND IsReadOnly with the FileAttributes enumeration. We can do the same for the message flag constants demonstrated in the GetMessages example listed in Chapter 5. Remember the following collection of flags Dim messageFlag As Integer 2 Dim isAccessed As Integer 4 Dim isArchived As Integer 8 Dim isDeleted As Integer 16 Dim newMessage As Integer 32 This collection would be better represented inside an enumeration. We can call the enumeration MessageFlagEnum as depicted in this code Public Enum MessageFlagEnum MessageFlag 0x0001 IsAccessed 0x0002 IsArchived 0x0004 IsDeleted 0x0008 NewMessage 0x0016 End Enum We don t have to initialize the symbols with hex values although it doesn t hurt and the symbol values can still be easily stored in the database. We also don t necessarily need to represent the bit flags in any special sequencinghere I have raised each bit flag to the power of two. However as you have seen the Enum semantics don t typically lend themselves to bit-flag semantics in which it s normal to write an expression evaluating the combined state of more than one symbol. So when you execute the following code Dim MsgState As MessageFlagEnum.IsAccessed MessageFlagEnum.IsArchived Debug.WriteLine MsgState.ToString you are going to get the value 0x0006 to the console instead of the combination of the two symbols. Thus if the intention is to write IsAccessed IsArchived to the Debug window it will not happen. To force the enumeration to return the latter you can pass the F argument listed in Table 8-2 to the Format method or 254 Final Words on Enums to the ToString method as shown in