Declare OpenIddictClientRegistration.ConfigurationManager as a nullable property

src/OpenIddict.Abstractions/OpenIddictResources.resx

@@ -1572,6 +1572,9 @@ To apply post-logout redirection responses, create a class implementing 'IOpenId
   <data name="ID0421" xml:space="preserve">
     <value>At least one subject type must be supported.</value>
   </data>
+  <data name="ID0422" xml:space="preserve">
+    <value>A configuration manager must be attached to the client registration to be able to resolve the server configuration.</value>
+  </data>
   <data name="ID2000" xml:space="preserve">
     <value>The security token is missing.</value>
   </data>

src/OpenIddict.Client/OpenIddictClientHandlers.Protection.cs

@@ -305,7 +305,8 @@ public static partial class OpenIddictClientHandlers
                     // If validation failed because of an unrecognized key identifier and a client
                     // registration is available, inform the configuration manager that the configuration
                     // MAY have be refreshed by sending a new discovery request to the authorization server.
-                    if (context.Registration is not null && result.Exception is SecurityTokenSignatureKeyNotFoundException)
+                    if (result.Exception is SecurityTokenSignatureKeyNotFoundException &&
+                        context.Registration.ConfigurationManager is not null)
                     {
                         context.Registration.ConfigurationManager.RequestRefresh();
                     }

src/OpenIddict.Client/OpenIddictClientHandlers.cs

@@ -423,10 +423,17 @@ public static partial class OpenIddictClientHandlers
                 throw new InvalidOperationException(SR.GetResourceString(SR.ID0408));
             }
 
+            // Resolve and attach the server configuration to the context if none has been set already.
+            if (context.Configuration is null)
+            {
+                if (context.Registration.ConfigurationManager is null)
+                {
+                    throw new InvalidOperationException(SR.GetResourceString(SR.ID0422));
+                }
+
                 try
                 {
-                // Resolve and attach the server configuration to the context if none has been set already.
+                    context.Configuration = await context.Registration.ConfigurationManager
-                context.Configuration ??= await context.Registration.ConfigurationManager
                         .GetConfigurationAsync(context.CancellationToken)
                         .WaitAsync(context.CancellationToken) ??
                         throw new InvalidOperationException(SR.GetResourceString(SR.ID0140));
@@ -444,6 +451,7 @@ public static partial class OpenIddictClientHandlers
 
                     return;
                 }
+            }
 
             // Ensure the selected grant type, if explicitly set, is listed as supported in the configuration.
             if (!string.IsNullOrEmpty(context.GrantType) &&
@@ -1019,7 +1027,15 @@ public static partial class OpenIddictClientHandlers
             // Note: if the static registration cannot be found in the options, this may indicate
             // the client was removed after the authorization dance started and thus, can no longer
             // be used to authenticate users. In this case, throw an exception to abort the flow.
-            context.Registration = await _service.GetClientRegistrationByIdAsync(context.RegistrationId, context.CancellationToken);
+            context.Registration ??= await _service.GetClientRegistrationByIdAsync(context.RegistrationId, context.CancellationToken);
+
+            // Resolve and attach the server configuration to the context if none has been set already.
+            if (context.Configuration is null)
+            {
+                if (context.Registration.ConfigurationManager is null)
+                {
+                    throw new InvalidOperationException(SR.GetResourceString(SR.ID0422));
+                }
 
                 try
                 {
@@ -1044,6 +1060,7 @@ public static partial class OpenIddictClientHandlers
                 }
             }
         }
+    }
 
     /// <summary>
     /// Contains the logic responsible for ensuring the issuer parameter, if available, matches the expected issuer.
@@ -4209,10 +4226,17 @@ public static partial class OpenIddictClientHandlers
                 throw new InvalidOperationException(SR.GetResourceString(SR.ID0408));
             }
 
+            // Resolve and attach the server configuration to the context if none has been set already.
+            if (context.Configuration is null)
+            {
+                if (context.Registration.ConfigurationManager is null)
+                {
+                    throw new InvalidOperationException(SR.GetResourceString(SR.ID0422));
+                }
+
                 try
                 {
-                // Resolve and attach the server configuration to the context if none has been set already.
+                    context.Configuration = await context.Registration.ConfigurationManager
-                context.Configuration ??= await context.Registration.ConfigurationManager
                         .GetConfigurationAsync(context.CancellationToken)
                         .WaitAsync(context.CancellationToken) ??
                         throw new InvalidOperationException(SR.GetResourceString(SR.ID0140));
@@ -4232,6 +4256,7 @@ public static partial class OpenIddictClientHandlers
                 }
             }
         }
+    }
 
     /// <summary>
     /// Contains the logic responsible for negotiating the best flow
@@ -5852,10 +5877,17 @@ public static partial class OpenIddictClientHandlers
                 throw new InvalidOperationException(SR.GetResourceString(SR.ID0408));
             }
 
+            // Resolve and attach the server configuration to the context if none has been set already.
+            if (context.Configuration is null)
+            {
+                if (context.Registration.ConfigurationManager is null)
+                {
+                    throw new InvalidOperationException(SR.GetResourceString(SR.ID0422));
+                }
+
                 try
                 {
-                // Resolve and attach the server configuration to the context if none has been set already.
+                    context.Configuration = await context.Registration.ConfigurationManager
-                context.Configuration ??= await context.Registration.ConfigurationManager
                         .GetConfigurationAsync(context.CancellationToken)
                         .WaitAsync(context.CancellationToken) ??
                         throw new InvalidOperationException(SR.GetResourceString(SR.ID0140));
@@ -5875,6 +5907,7 @@ public static partial class OpenIddictClientHandlers
                 }
             }
         }
+    }
 
     /// <summary>
     /// Contains the logic responsible for attaching the client identifier to the sign-out request.

src/OpenIddict.Client/OpenIddictClientRegistration.cs

@@ -143,7 +143,7 @@ public sealed class OpenIddictClientRegistration
     /// <summary>
     /// Gets or sets the configuration manager used to retrieve and cache the server configuration.
     /// </summary>
-    public IConfigurationManager<OpenIddictConfiguration> ConfigurationManager { get; set; } = default!;
+    public IConfigurationManager<OpenIddictConfiguration>? ConfigurationManager { get; set; }
 
     /// <summary>
     /// Gets or sets the URI of the configuration endpoint exposed by the server.

src/OpenIddict.Client/OpenIddictClientService.cs

@@ -154,6 +154,11 @@ public sealed class OpenIddictClientService
         }
 
         var registration = await GetClientRegistrationAsync(issuer, cancellationToken);
+        if (registration.ConfigurationManager is null)
+        {
+            throw new InvalidOperationException(SR.GetResourceString(SR.ID0422));
+        }
+
         return await registration.ConfigurationManager
             .GetConfigurationAsync(cancellationToken)
             .WaitAsync(cancellationToken) ??
@@ -182,6 +187,11 @@ public sealed class OpenIddictClientService
         }
 
         var registration = await GetClientRegistrationAsync(provider, cancellationToken);
+        if (registration.ConfigurationManager is null)
+        {
+            throw new InvalidOperationException(SR.GetResourceString(SR.ID0422));
+        }
+
         return await registration.ConfigurationManager
             .GetConfigurationAsync(cancellationToken)
             .WaitAsync(cancellationToken) ??
@@ -207,6 +217,11 @@ public sealed class OpenIddictClientService
         }
 
         var registration = await GetClientRegistrationByIdAsync(identifier, cancellationToken);
+        if (registration.ConfigurationManager is null)
+        {
+            throw new InvalidOperationException(SR.GetResourceString(SR.ID0422));
+        }
+
         return await registration.ConfigurationManager
             .GetConfigurationAsync(cancellationToken)
             .WaitAsync(cancellationToken) ??