Override same attributes found at class and property level in ApplicationSettingsBase

src/libraries/System.Configuration.ConfigurationManager/src/System/Configuration/ApplicationSettingsBase.cs

@@ -519,7 +519,7 @@ private SettingsProperty CreateSetting(PropertyInfo propertyInfo)
                     //       level and also property level, the latter overrides the former
                     //       for a given setting. This is exactly the behavior we want.
 
-                    settingsProperty.Attributes.Add(attribute.GetType(), attribute);
+                    settingsProperty.Attributes[attribute.GetType()] = attribute;
                 }
             }
 

src/libraries/System.Configuration.ConfigurationManager/tests/System/Configuration/ApplicationSettingsBaseTests.cs

@@ -96,6 +96,37 @@ public override void Initialize(string name, NameValueCollection config)
 
         }
 
+#nullable enable
+        public class SettingsWithNullableAttribute : ApplicationSettingsBase
+        {
+            [ApplicationScopedSetting]
+            public string StringProperty
+            {
+                get
+                {
+                    return (string)this[nameof(StringProperty)];
+                }
+                set
+                {
+                    this[nameof(StringProperty)] = value;
+                }
+            }
+
+            [UserScopedSetting]
+            public string? NullableStringProperty
+            {
+                get
+                {
+                    return (string)this[nameof(NullableStringProperty)];
+                }
+                set
+                {
+                    this[nameof(NullableStringProperty)] = value;
+                }
+            }
+        }
+#nullable disable
+
         private class PersistedSimpleSettings : SimpleSettings
         {
         }
@@ -315,5 +346,27 @@ public void OnSettingsLoaded_QueryProperty()
             Assert.Equal(newStringPropertyValue, settings.StringProperty);
             Assert.True(loadedFired);
         }
+
+        [SkipOnTargetFramework(TargetFrameworkMonikers.NetFramework, "Not fixed on NetFX")]
+        [Fact]
+        public void SettingsProperty_SettingsWithNullableAttributes_Ok()
+        {
+            SettingsWithNullableAttribute settings = new SettingsWithNullableAttribute();
+
+            Assert.Null(settings.NullableStringProperty);
+
+            string newValue = null;
+
+            settings.SettingChanging += (object sender, SettingChangingEventArgs e)
+                =>
+            {
+                newValue = (string)e.NewValue;
+            };
+
+            settings.NullableStringProperty = "test";
+
+            Assert.Equal("test", newValue);
+            Assert.Equal(newValue, settings.NullableStringProperty);
+        }
     }
 }