How can I access web.config values from within a class attribute?

Question

I have the following EnableCors attribute on a WebApi controller class:

[EnableCors(origins:"http://localhost:57280", headers: "*", methods: "*")]
public class PhysicalActivityController : ApiController

But I don't want that url hard-coded. I want to fetch it from web.config.

Something like this:

[EnableCors(origins: System.Web.Configuration.WebConfigurationManager.AppSettings["ModellingApiUrl"], headers: "*", methods: "*")]
public class PhysicalActivityController2 : ApiController

The problem is that I can't access methods like that from within an attribute. The context of the code is not the same as being within an ordinary method. When I write the code as in the second example, Visual Studio highlights "AppSettings" in red, and gives the error "Cannot resolve symbol 'AppSettings'".

Any ideas on how I can achieve this? I tried writing a custom attribute that inherits from EnableCors, but sadly System.Web.Http.Cors.EnableCorsAttribute is a sealed class, so I can't inherit from it.

Answer 1

I solved this in the end by a wholesale copy of the code for the EnableCors attribute, and then I modified it to fetch values from the config.

The problem is that the string which is passed into the attribute is an argument to its constructor, and needs to be a compile-time constant. Therefore I don't believe that what I wanted to do was possible - you have to edit the code behind the attribute.

My solution:

[AttributeUsage(AttributeTargets.Class | AttributeTargets.Method, AllowMultiple = false)]
[SuppressMessage("Microsoft.Design", "CA1019:DefineAccessorsForAttributeArguments", Justification = "Attribute arguments are accessible as collections.")]
public sealed class EnableCorsUsingConfigAttribute : Attribute, ICorsPolicyProvider
{        
    public EnableCorsUsingConfigAttribute(string configKeyForOrigins, string headers, string methods)
        : this(configKeyForOrigins, headers, methods, null)
    {
    }

    public EnableCorsUsingConfigAttribute(string configKeyForOrigins, string headers, string methods, string exposedHeaders)
    {
        string origins = System.Web.Configuration.WebConfigurationManager.AppSettings[configKeyForOrigins];

(the rest of the code is a direct copy of the EnableCors attribute)

Usage:

[EnableCorsUsingConfig(configKeyForOrigins: "ModellingApiClientUrls", headers: "*", methods: "*")]
public class PhysicalActivityController : ApiController
{

Answer 2

System.Configuration.ConfigurationManager.ConnectionStrings["ModellingApiUrl"].ToString();

use this code i hope this is useful to you

Answer 3

you just need to add :

ConfigurationManager.AppSettings["AllowedOriginsCors"].ToString();

the external file :

  <appSettings>
    <add key="AllowedOriginsCors" value="http://localhost:4200" />
  </appSettings>