How to check if an appSettings key exists?

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>开发者_StackOverflow;

and in the codefile

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

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
}

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

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);

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

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..
}

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

I think the LINQ expression may be best:

   const string MyKey = "myKey"

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

I liked codebender's answer, but needed it to work in C++/CLI. This is what I ended up with. There's no LINQ usage, but works.

generic <typename T> T MyClass::ReadAppSetting(String^ searchKey, T defaultValue) {
  for each (String^ setting in ConfigurationManager::AppSettings->AllKeys) {
    if (setting->Equals(searchKey)) { //  if the key is in the app.config
      try {                           // see if it can be converted
        auto converter = TypeDescriptor::GetConverter((Type^)(T::typeid)); 
        if (converter != nullptr) { return (T)converter->ConvertFromString(ConfigurationManager::AppSettings[searchKey]); }
      } catch (Exception^ ex) {} // nothing to do
    }
  }
  return defaultValue;
}

Using the new c# syntax with TryParse worked well for me:

  // TimeOut
  if (int.TryParse(ConfigurationManager.AppSettings["timeOut"], out int timeOut))
  {
     this.timeOut = timeOut;
  }