检查配置文件中的空值

Question

Which way is better to check whether a config value is null?

if(ConfigurationManager.AppSettings["configValue"]!=null)
{
  var _queue = ConfigurationManager.AppSettings["configValue"]
}

or this way?

var _queue=ConfigurationManager.AppSettings["configValue"] ?? null;

Answer 1

something along these lines

string val = ConfigurationManager.AppSettings["configValue"];
if (val == null)
    Console.WriteLine("Missing appSettings configuration 'configValue'");
else if (val == string.Empty)
    Console.WriteLine("appSettings configuration 'configValue' not set");
else
    Console.WriteLine("appSettings configuration 'configValue' is " + val);

But usually, even if someone has not set value, you want your application to still function...

string val = ConfigurationManager.AppSettings["configValue"];
if (string.IsNullOrWhiteSpace(val))
    val = "default value";

Answer 2

I use these extensions. This is abbreviated. There are a few other methods for parsing values to other types.

This way I can be explicit with the error messages so that if a setting is required but missing it doesn't fail silently or throw a vague exception.

public static class AppSettingsExtensions
{
    public static string Required(this NameValueCollection appSettings, string key)
    {
        var settingsValue = appSettings[key];
        if (string.IsNullOrEmpty(settingsValue))
            throw new MissingAppSettingException(key);
        return settingsValue;
    }

    public static string ValueOrDefault(this NameValueCollection appSettings, string key, string defaultValue)
    {
        return appSettings[key] ?? defaultValue;
    }
}

public class MissingAppSettingException : Exception
{
    internal MissingAppSettingException(string key, Type expectedType)
        : base(string.Format(@"An expected appSettings value with key ""{0}"" and type {1} is missing.", key, expectedType.FullName))
    { }
    public MissingAppSettingException(string key)
        : base(string.Format(@"An expected appSettings value with key ""{0}"" is missing.", key))
    { }
}

Usage:

    var setting = ConfigurationManager.AppSettings.Required("thisCantBeMissing");
    var optionalSetting = ConfigurationManager.AppSettings.ValueOrDefault("thisCanBeMissing", "default value");

The second one is handy because I often never need to create the appSettings key. I can just use the default.