Question

我有一个Message类，它有三个属性Content，Type和UniqueId。创建Message对象时，Content和Type是已知的，因此我可以将它们传递给类的构造函数，并使Content和Type属性只读，这样它们的值就不能再被更改了。但是，对于UniqueId，我需要在创建对象后在我的代码中计算它，并将值赋给UniqueId属性。由于我无法将UniqueId传递给构造函数并将此属性设为只读，我想知道有没有这样的方法，一旦设置了属性UniqueId，它的值就不能再被更改了？

public class Message
{
    private readonly string content;
    private readonly AuditMessageType type;
    private Guid messageUId;

    public Message(string syslogMessage, AuditMessageType messageType, Guid messageUniqueId = new Guid())
    {
        content = syslogMessage;
        type = messageType;
        messageUId = messageUniqueId;
    }

    public string Message
    {
        get { return message; }
    }

    public AuditMessageType Type
    {
        get { return type; }
    }

    public Guid MesageUniqueId
    {
        get { return messageUId; }
        set { messageUId = value; } // How to make UniqueId property set once here? It cannot be pass in the constructor, as it needs to computed in the code after the object has been created. 
    }
}

Answer 1

难道你不能简单地创建一个警卫旗吗？

bool wasSetMessageId = false;
public Guid MesageUniqueId
{
    get { return messageUId; }
    set 
    {
       if (!wasSetMessageId) 
       {
          messageUId = value;
          wasSetMessageId = true;
       } 
       else
       {
          throw new InvalidOperationException("Message id can be assigned only once");
       }
    } 
}

Answer 2

如何做到这一点：

如果Guid.Empty无效，则为MessageUniqueId

  public Guid MesageUniqueId
    {
        get { return messageUId; }
        set {
             if (messageUId  == Guid.Empty) 
                messageUId = value; 
        } 
    }

如果您可以使用Nullable Guid代替Guid

 public Guid ? MesageUniqueId
        {
            get { return messageUId; }
            set {
                 if (messageUId  == null) 
                    messageUId = value; 
            } 
        }

如果您不能同时执行上述操作，请使用私有变量：

  private bool messageUniqueIdhasBeenSet = false ; 
 public Guid  MesageUniqueId
        {
            get { return messageUId; }
            set {
                 if (!messageUniqueIdhasBeenSet ) 
                  {
                    messageUId = value;
                    messageUniqueIdhasBeenSet = true ; 
                   } 
            } 
        }

Answer 3

private bool idHasBeenSet = false;
public Guid MessageUniqueId
{
    get { return messageUId; }
    set {
        if (idHasBeenSet) return; //or throw an exception if you need to
        messageUId = value;
        idHasBeenSet = true;
    }
}

Answer 4

c＃中没有这样的直接功能，但你可以自己编写代码，如下所示： -

private Guid? messageUId;
 public Guid MesageUniqueId
    {
        get { return messageUId; }
        set {   
              if (null == messageUId ) 
              { 
                   messageUId = value;              
              }
              else  
              { 
                  throw new InvalidOperationException("Message id can be assigned only once");
              }
     }
    }

如何设置属性一次又不能更改？

4 个答案: