Add safe copy for enumeration

src/Microsoft.AspNetCore.Authentication.Core/AuthenticationSchemeProvider.cs

@@ -2,7 +2,9 @@
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System;
+using System.Collections.Concurrent;
 using System.Collections.Generic;
+using System.Linq;
 using System.Threading.Tasks;
 using Microsoft.AspNetCore.Http;
 using Microsoft.Extensions.Options;
@@ -49,6 +51,9 @@ protected AuthenticationSchemeProvider(IOptions<AuthenticationOptions> options,
 
  private readonly IDictionary<string, AuthenticationScheme> _schemes;
  private readonly List<AuthenticationScheme> _requestHandlers;
+ // Used as a safe return value for enumeration apis
+ private IEnumerable<AuthenticationScheme> _schemesCopy = new AuthenticationScheme[0];
+ private IEnumerable<AuthenticationScheme> _requestHandlersCopy = new AuthenticationScheme[0];
 
  private Task<AuthenticationScheme> GetDefaultSchemeAsync()
  => _options.DefaultScheme != null
@@ -123,7 +128,7 @@ public virtual Task<AuthenticationScheme> GetSchemeAsync(string name)
  /// </summary>
  /// <returns>The schemes in priority order for request handling</returns>
  public virtual Task<IEnumerable<AuthenticationScheme>> GetRequestHandlerSchemesAsync()
- => Task.FromResult<IEnumerable<AuthenticationScheme>>(_requestHandlers);
+ => Task.FromResult(_requestHandlersCopy);
 
  /// <summary>
  /// Registers a scheme for use by <see cref="IAuthenticationService"/>. 
@@ -144,8 +149,10 @@ public virtual void AddScheme(AuthenticationScheme scheme)
  if (typeof(IAuthenticationRequestHandler).IsAssignableFrom(scheme.HandlerType))
  {
  _requestHandlers.Add(scheme);
+ _requestHandlersCopy = _requestHandlers.ToList();
  }
  _schemes[scheme.Name] = scheme;
+ _schemesCopy = _schemes.Values.ToList();
  }
  }
 
@@ -164,13 +171,19 @@ public virtual void RemoveScheme(string name)
  if (_schemes.ContainsKey(name))
  {
  var scheme = _schemes[name];
- _requestHandlers.Remove(scheme);
- _schemes.Remove(name);
+ if (_requestHandlers.Remove(scheme))
+ {
+ _requestHandlersCopy = _requestHandlers.ToList();
+ }
+ if (_schemes.Remove(name))
+ {
+ _schemesCopy = _schemes.Values.ToList();
+ }
  }
  }
  }
 
  public virtual Task<IEnumerable<AuthenticationScheme>> GetAllSchemesAsync()
- => Task.FromResult<IEnumerable<AuthenticationScheme>>(_schemes.Values);
+ => Task.FromResult(_schemesCopy);
  }
 }