diff --git a/Casbin.UnitTests/ModelTests/RbacApiTest.cs b/Casbin.UnitTests/ModelTests/RbacApiTest.cs
index 32834db..036e224 100644
--- a/Casbin.UnitTests/ModelTests/RbacApiTest.cs
+++ b/Casbin.UnitTests/ModelTests/RbacApiTest.cs
@@ -423,8 +423,8 @@ public void TestGetImplicitPermissionsForUserWithDomain()
 
         TestGetImplicitPermissions(e, "alice", AsList(
                 AsList("alice", "domain1", "data2", "read"),
-                AsList("role:reader", "domain1", "data1", "read"),
-                AsList("role:writer", "domain1", "data1", "write")),
+                AsList("role:writer", "domain1", "data1", "write"),
+                AsList("role:reader", "domain1", "data1", "read")),
             "domain1");
     }
 
diff --git a/Casbin.UnitTests/Util/TestUtil.cs b/Casbin.UnitTests/Util/TestUtil.cs
index f19748d..7400a33 100644
--- a/Casbin.UnitTests/Util/TestUtil.cs
+++ b/Casbin.UnitTests/Util/TestUtil.cs
@@ -210,12 +210,11 @@ internal static void TestGetPermissions(IEnforcer e, string name, List<List<stri
         Assert.True(res.DeepEquals(myRes), message);
     }
 
-    internal static void TestGetImplicitPermissions(IEnforcer e, string name, List<List<string>> res,
+    internal static void TestGetImplicitPermissions(IEnforcer e, string name, List<List<string>> except,
         string domain = null)
     {
-        IEnumerable<IEnumerable<string>> myRes = e.GetImplicitPermissionsForUser(name, domain);
-        string message = "Implicit permissions for " + name + ": " + myRes + ", supposed to be " + res;
-        Assert.True(res.DeepEquals(myRes), message);
+        IEnumerable<IEnumerable<string>> actual = e.GetImplicitPermissionsForUser(name, domain);
+        Assert.True(except.DeepEquals(actual));
     }
 
     internal static void TestHasPermission(IEnforcer e, string name, List<string> permission, bool res)
diff --git a/Casbin/Rbac/Role.cs b/Casbin/Rbac/Role.cs
index a159c46..798c55f 100644
--- a/Casbin/Rbac/Role.cs
+++ b/Casbin/Rbac/Role.cs
@@ -1,4 +1,5 @@
 using System;
+using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.Linq;
 
@@ -9,7 +10,7 @@ namespace Casbin.Rbac
     /// </summary>
     public class Role
     {
-        private readonly Lazy<Dictionary<string, Role>> _roles = new();
+        private readonly Lazy<ConcurrentDictionary<string, Role>> _roles = new();
 
         public Role(string name)
         {
@@ -28,17 +29,7 @@ public Role(string name, string domain)
 
         public void AddRole(Role role)
         {
-            if (_roles.IsValueCreated is false)
-            {
-                _roles.Value.Add(role.Name, role);
-            }
-
-            if (_roles.Value.ContainsKey(role.Name))
-            {
-                return;
-            }
-
-            _roles.Value.Add(role.Name, role);
+            _roles.Value.TryAdd(role.Name, role);
         }
 
         public void DeleteRole(Role role)
@@ -47,11 +38,7 @@ public void DeleteRole(Role role)
             {
                 return;
             }
-
-            if (_roles.Value.ContainsKey(role.Name))
-            {
-                _roles.Value.Remove(role.Name);
-            }
+            _roles.Value.TryRemove(role.Name, out _);
         }
 
         public bool HasRole(string name, int hierarchyLevel, Func<string, string, bool> matchingFunc = null)
@@ -71,7 +58,8 @@ public bool HasRole(string name, int hierarchyLevel, Func<string, string, bool>
                 return false;
             }
 
-            return _roles.Value.Values.Any(role => role.HasRole(name, hierarchyLevel - 1));
+            return _roles.Value.Values.Any(role =>
+                role.HasRole(name, hierarchyLevel - 1));
         }
 
         public bool HasDirectRole(string name, Func<string, string, bool> matchingFunc = null)
@@ -86,7 +74,7 @@ public bool HasDirectRole(string name, Func<string, string, bool> matchingFunc =
                 return _roles.Value.ContainsKey(name);
             }
 
-            
+
             foreach (var role in _roles.Value.Values)
             {
                 if (name == role.Name || matchingFunc(role.Name, name) && role.Name != name)
@@ -101,10 +89,5 @@ public IEnumerable<string> GetRoles()
         {
             return _roles.IsValueCreated ? _roles.Value.Keys : Enumerable.Empty<string>();
         }
-
-        public override string ToString()
-        {
-            return $"{Name}{string.Join(",", _roles.Value)}";
-        }
     }
 }