add PrepareLogout

Author: leastprivilege

src/IdentityModel.OidcClient/OidcClient.cs

@@ -108,6 +108,24 @@ public async Task<LoginResult> LoginAsync(LoginRequest request)
             return result;
         }
 
+        /// <summary>
+        /// Creates a logout URL.
+        /// </summary>
+        /// <param name="request">The logout request.</param>
+        /// <returns></returns>
+        public string PrepareLogout(LogoutRequest request = null)
+        {
+            if (request == null) request = new LogoutRequest();
+
+            var endpoint = _options.ProviderInformation.EndSessionEndpoint;
+            if (endpoint.IsMissing())
+            {
+                throw new InvalidOperationException("Discovery document has no end session endpoint");
+            }
+
+            return _authorizeClient.CreateEndSessionUrl(endpoint, request);
+        }
+
         /// <summary>
         /// Starts a logout.
         /// </summary>