How to check if an appSettings key exists?

问题:

How do I check to see if an Application Setting is available?

i.e. app.config

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
  <appSettings>
    <add key ="someKey" value="someValue"/>
  </appSettings>
</configuration>

and in the codefile

if (ConfigurationManager.AppSettings.ContainsKey("someKey"))
{
  // Do Something
}else{
  // Do Something Else
}

回答1:

MSDN: Configuration Manager.AppSettings

if (ConfigurationManager.AppSettings[name] != null)
{
// Now do your magic..
}

or

string s = ConfigurationManager.AppSettings["myKey"];
if (!String.IsNullOrEmpty(s))
{
    // Key exists
}
else
{
    // Key doesn't exist
}

回答2:

if (ConfigurationManager.AppSettings.AllKeys.Contains("myKey"))
{
    // Key exists
}
else
{
    // Key doesn't exist
}

回答3:

Safely returned default value via generics and LINQ.

public T ReadAppSetting<T>(string searchKey, T defaultValue, StringComparison compare = StringComparison.Ordinal)
{
    if (ConfigurationManager.AppSettings.AllKeys.Any(key => string.Compare(key, searchKey, compare) == 0)) {
        try
        { // see if it can be converted.
            var converter = TypeDescriptor.GetConverter(typeof(T));
            if (converter != null) defaultValue = (T)converter.ConvertFromString(ConfigurationManager.AppSettings.GetValues(searchKey).First());
        }
        catch { } // nothing to do just return the defaultValue
    }
    return defaultValue;
}

Used as follows:

string LogFileName = ReadAppSetting("LogFile","LogFile");
double DefaultWidth = ReadAppSetting("Width",1280.0);
double DefaultHeight = ReadAppSetting("Height",1024.0);
Color DefaultColor = ReadAppSetting("Color",Colors.Black);

回答4:

If the key you are looking for isn't present in the config file, you won't be able to convert it to a string with .ToString() because the value will be null and you'll get an "Object reference not set to an instance of an object" error. It's best to first see if the value exists before trying to get the string representation.

if (!String.IsNullOrEmpty(ConfigurationManager.AppSettings["myKey"]))
{
    String myKey = ConfigurationManager.AppSettings["myKey"].ToString();
}

Or, as Code Monkey suggested:

if (ConfigurationSettings.AppSettings["myKey"] != null)
{
// Now do your magic..
}

回答5:

Upper options gives flexible to all manner, if you know key type try parsing them bool.TryParse(ConfigurationManager.AppSettings["myKey"], out myvariable);

回答6:

I think the LINQ expression may be best:

   const string MyKey = "myKey"

   if (ConfigurationManager.AppSettings.AllKeys.Any(key => key == MyKey))
          {
              // Key exists
          }

回答7:

var isAlaCarte = ConfigurationManager.AppSettings.AllKeys.Contains("IsALaCarte") && bool.Parse(ConfigurationManager.AppSettings.Get("IsALaCarte"));