空值检查

Question

  public string AgentVersion
  {
    get { return m_version; }
  } // property: Enabled
  private string m_version = null;

The below declaration coding i did in Constructor

  string keySpoPath = SpoRegistry.SpoAgentRoot;
  RegistryKey regkey = Registry.LocalMachine.OpenSubKey(keySpoPath);
  m_version = (string)regkey.GetValue(SpoRegistry.regValue_CurrentVersion);

here are my doubts

---

1. Do I need private string m_version = null; in property declaration in this context? If I remove that one, are there any probs?

2. If AgentVersion is null or not getting any value or any strings other than numeric values I want to assign AgentVersion to value '0.0.0.0' otherwise i will display the numeric value which is coming. Is this code below sufficient here string.IsNullOrEmpty(AgentVersion) ? "0.0.0.0" : AgentVersion; string.IsNullOrEmpty(AgentVersion) ? "0.0.0.0" : AgentVersion; If then where and how can I implement in 'Property'

Answer 1

1. 由于m_version的值始终在构造函数中分配，因此无需将其设置为null 。
2. get { return string.IsNullOrEmpty(m_version) ? "0.0.0.0" : m_version; }

Answer 2

1. You can use Resharper, it will show you where you have redundant declarations. In this case it is redundant.

2. If String.Empty is a valid value you can implement it using

   public string AgentVersion { get { return m_version ?? "0.0.0.0"; } }

otherwise you are correct use String.IsNullOrEmpty

public string AgentVersion
{
    get { return String.IsNullOrEmpty(m_version) ? "0.0.0.0":m_version; }
}

Answer 3

you can use this code.

public string AgentVersion 
 {
    get
    { 
        if(string.isNullOrEmpty(m_version))
        {
            string keySpoPath = SpoRegistry.SpoAgentRoot;
            RegistryKey regkey = Registry.LocalMachine.OpenSubKey(keySpoPath);
            m_version = (string)regkey.GetValue(SpoRegistry.regValue_CurrentVersion);
        }
             m_version = string.isNullOrEmpty(m_version) ? m_version : "0.0.0.0";
                 return m_version;
        } 

}
string m_version;

Answer 4

1) You don't need to instantiate m_version as null in this context. You usually only need to do this when declaring a local variable, to avoid the 'Use of unassigned local variable [name] ' compile error. So private string m_version; will suffice.

2) For checking null and empty values, what you have with String.IsNullOrEmpty() is fine. If you want to go check your property returns strings in the correct format, you can use Regex.IsMatch() to ensure the property only ever returns strings in version number-format.

[DefaultValue("0.0.0.0")]
public string AgentVersion
{
    get
    {
        return System.Text.RegularExpressions.Regex.IsMatch(m_version ?? String.Empty, @"\A\d+[.]\d+[.]\d+[.]\d+\z") 
            ? m_version 
            : "0.0.0.0";
    }
}

private string m_version;

My regular expressions are a bit rusty, so someone can probably improve on what I have here, but it's the general gist of what you need.