AppSettings 后备/默认值？

Question

ASP.NET

For each appSetting I use, I want to specify a value that will be returned if the specified key isn't found in the appSettings. I was about to create a class to manage this, but I'm thinking this functionality is probably already in the .NET Framework somewhere?

Is there a NameValueCollection/Hash/etc-type class in .NET that will let me specify a key and a fallback/default value -- and return either the key's value, or the specified value?

If there is, I could put the appSettings into an object of that type before calling into it (from various places).

Answer 1

I don't believe there's anything built into .NET which provides the functionality you're looking for.

You could create a class based on Dictionary<TKey, TValue> that provides an overload of TryGetValue with an additional argument for a default value, eg:

public class MyAppSettings<TKey, TValue> : Dictionary<TKey, TValue>
{
    public void TryGetValue(TKey key, out TValue value, TValue defaultValue)
    {
        if (!this.TryGetValue(key, out value))
        {
            value = defaultValue;
        }
    }
}

You could probably get away with string s instead of keeping in generic.

There's also DependencyObject from the Silverlight and WPF world if those are options.

Of course, the simplest way is something like this with a NameValueCollection :

string value = string.IsNullOrEmpty(appSettings[key]) 
    ? defaultValue 
    : appSettings[key];

key can be null on the string indexer. But I understand it's a pain to do that in multiple places.

Answer 2

This is what I do.

WebConfigurationManager.AppSettings["MyValue"] ?? "SomeDefault")

For Boolean and other non-string types...

bool.Parse(WebConfigurationManager.AppSettings["MyBoolean"] ?? "false")

Answer 3

You can make a custom configuration section and provide default values using the DefaultValue attribute. Instructions for that are available here .

Answer 4

ASP.NET's ConfigurationManager provides that functionality. You can bind your configuration section (retrieved with .GetSection("MySection") ) to an object with either .Get<MySectionType>() or .Bind(mySectionTypeInstance) . This also has the benefit, that it does conversion for you (see integers in the example).

Example (NET 6)

appsettings.json

{
  "MySection": {
    "DefinedString": "yay, I'm defined",
    "DefinedInteger": 1337
  }
}

MySection.cs

// could also be a struct or readonly struct
public class MySectionType
{
  public string DefinedString { get; init; } = "default string";
  public int DefinedInteger { get; init; } = -1;
  public string OtherString { get; init; } = "default string";
  public int OtherInteger { get; init; } = -1;

  public override string ToString() => 
    $"defined string :   \"{DefinedString}\"\n" +
    $"defined integer:   {DefinedInteger}\n" +
    $"undefined string : \"{OtherString}\"\n" +
    $"undefined integer: {OtherInteger}";
}

Program.cs

ConfigurationManager configuration = GetYourConfigurationManagerHere();

// either
var mySection = configuration.GetSection("MySection").Get<MySectionType>();

// or
var mySection = new MySectionType();
configuration.GetSection("MySection").Bind(mySection);

Console.WriteLine(mySection);

// output:
// defined string :   "yay, I'm defined"
// defined integer:   1337
// undefined string : "default string"
// undefined integer: -1

Answer 5

I think the machine.config under C:\\%WIN%\\Microsoft.NET will do this. Add keys to that file as your default values.

http://msdn.microsoft.com/en-us/library/ms228154.aspx

Answer 6

You can build logic around ConfigurationManager to get a typed way to retrieve your app setting value. Using here TypeDescriptor to convert value.

/// /// Utility methods for ConfigurationManager /// public static class ConfigurationManagerWrapper {

    /// <summary>
    /// Use this extension method to get a strongly typed app setting from the configuration file.
    /// Returns app setting in configuration file if key found and tries to convert the value to a specified type. In case this fails, the fallback value
    /// or if NOT specified - default value - of the app setting is returned
    /// </summary>
    /// <typeparam name="T"></typeparam>
    /// <param name="appsettingKey"></param>
    /// <param name="fallback"></param>
    /// <returns></returns>
    public static T GetAppsetting<T>(string appsettingKey, T fallback = default(T))
    {
        string val = ConfigurationManager.AppSettings[appsettingKey] ?? "";
        if (!string.IsNullOrEmpty(val))
        {
            try
            {
                Type typeDefault = typeof(T);
                var converter = TypeDescriptor.GetConverter(typeof(T));
                return converter.CanConvertFrom(typeof(string)) ? (T)converter.ConvertFrom(val) : fallback;
            }
            catch (Exception err)
            {
                Console.WriteLine(err); //Swallow exception
                return fallback;
            }
        }
        return fallback;
    }

}

}