Improve conflicting member exception message

src/EFCore/EFCore.baseline.json

@@ -3845,7 +3845,10 @@
           "Member": "static string ConflictingKeylessAndPrimaryKeyAttributes(object? entity);"
         },
         {
-          "Member": "static string ConflictingPropertyOrNavigation(object? member, object? type, object? conflictingType);"
+          "Member": "static string ConflictingPropertyOrNavigationOnBaseType(object? member, object? type, object? conflictingMemberKind, object? conflictingType);"
+        },
+        {
+          "Member": "static string ConflictingPropertyOrNavigationWithKind(object? member, object? type, object? conflictingMemberKind);"
         },
         {
           "Member": "static string ConflictingRelationshipConversions(object? entityType, object? property, object? valueConversion, object? conflictingValueConversion);"

src/EFCore/Metadata/Builders/ReferenceNavigationBuilder.cs

@@ -238,8 +238,8 @@ private InternalForeignKeyBuilder WithOneBuilder(MemberIdentity reference)
             && ReferenceName == referenceName)
         {
             throw new InvalidOperationException(
-                CoreStrings.ConflictingPropertyOrNavigation(
-                    referenceName, RelatedEntityType.DisplayName(), RelatedEntityType.DisplayName()));
+                CoreStrings.ConflictingPropertyOrNavigationWithKind(
+                    referenceName, RelatedEntityType.DisplayName(), "navigation"));
         }
 
         var pointsToPrincipal = !foreignKey.IsSelfReferencing()

src/EFCore/Metadata/Internal/EntityType.cs

@@ -1451,16 +1451,14 @@ public virtual Navigation AddNavigation(MemberIdentity navigationMember, Foreign
             }
 
             throw new InvalidOperationException(
-                CoreStrings.ConflictingPropertyOrNavigation(
-                    name, DisplayName(), duplicateNavigation.DeclaringEntityType.DisplayName()));
+                duplicateNavigation.FormatConflictingMemberMessage(name, this));
         }
 
         var duplicateProperty = FindMembersInHierarchy(name).FirstOrDefault();
         if (duplicateProperty != null)
         {
             throw new InvalidOperationException(
-                CoreStrings.ConflictingPropertyOrNavigation(
-                    name, DisplayName(), ((IReadOnlyTypeBase)duplicateProperty.DeclaringType).DisplayName()));
+                duplicateProperty.FormatConflictingMemberMessage(name, this));
         }
 
         Check.DebugAssert(
@@ -1633,8 +1631,7 @@ public virtual IEnumerable<Navigation> GetNavigations()
         if (duplicateProperty != null)
         {
             throw new InvalidOperationException(
-                CoreStrings.ConflictingPropertyOrNavigation(
-                    name, DisplayName(), duplicateProperty.DeclaringType.DisplayName()));
+                duplicateProperty.FormatConflictingMemberMessage(name, this));
         }
 
         if (memberInfo != null)
@@ -2286,9 +2283,7 @@ public virtual ServiceProperty AddServiceProperty(
         if (duplicateMember != null)
         {
             throw new InvalidOperationException(
-                CoreStrings.ConflictingPropertyOrNavigation(
-                    name, DisplayName(),
-                    ((IReadOnlyTypeBase)duplicateMember.DeclaringType).DisplayName()));
+                duplicateMember.FormatConflictingMemberMessage(name, this));
         }
 
         ValidateClrMember(name, memberInfo, false);

src/EFCore/Metadata/Internal/InternalEntityTypeBuilder.cs

@@ -541,8 +541,7 @@ public override void RemoveMembersInHierarchy(string propertyName, Configuration
             if (conflictingNavigation.GetConfigurationSource() == ConfigurationSource.Explicit)
             {
                 throw new InvalidOperationException(
-                    CoreStrings.ConflictingPropertyOrNavigation(
-                        propertyName, Metadata.DisplayName(), conflictingNavigation.DeclaringEntityType.DisplayName()));
+                    conflictingNavigation.FormatConflictingMemberMessage(propertyName, Metadata));
             }
 
             var foreignKey = conflictingNavigation.ForeignKey;

src/EFCore/Metadata/Internal/PropertyBaseExtensions.cs

@@ -338,6 +338,56 @@ or PropertyAccessMode.FieldDuringConstruction
         return false;
     }
 
+    /// <summary>
+    ///     Builds the message for the diagnostic that fires when a member conflicts with an existing
+    ///     member on the structural type or one of its base types. The kind of the conflicting member
+    ///     is humanized via <see cref="GetMemberKindString" /> so the user-facing message uses stable
+    ///     labels like "property", "complex property", "navigation", "skip navigation", or
+    ///     "service property" regardless of whether the conflicting member came from a model or a
+    ///     runtime model.
+    /// </summary>
+    /// <remarks>
+    ///     This is an internal API that supports the Entity Framework Core infrastructure and not subject to
+    ///     the same compatibility standards as public APIs. It may be changed or removed without notice in
+    ///     any release. You should only use it directly in your code with extreme caution and knowing that
+    ///     doing so can result in application failures when updating to a new Entity Framework Core release.
+    /// </remarks>
+    public static string FormatConflictingMemberMessage(
+        this IReadOnlyPropertyBase conflictingMember,
+        string newMemberName,
+        IReadOnlyTypeBase owningType)
+    {
+        var conflictingMemberKind = GetMemberKindString(conflictingMember);
+        var owningTypeDisplayName = owningType.DisplayName();
+
+        // Compare the actual metadata instances rather than display names to avoid false positives when
+        // two distinct types share a simple name (e.g. same name in different namespaces or hierarchies).
+        return conflictingMember.DeclaringType == owningType
+            ? CoreStrings.ConflictingPropertyOrNavigationWithKind(newMemberName, owningTypeDisplayName, conflictingMemberKind)
+            : CoreStrings.ConflictingPropertyOrNavigationOnBaseType(
+                newMemberName,
+                owningTypeDisplayName,
+                conflictingMemberKind,
+                ((IReadOnlyTypeBase)conflictingMember.DeclaringType).DisplayName());
+    }
+
+    /// <summary>
+    ///     Returns a human-readable label for the kind of the given member (e.g. "property",
+    ///     "complex property", "navigation", "skip navigation", "service property"). Used to build
+    ///     user-facing diagnostic messages without coupling the message text to internal CLR class
+    ///     names (such as <c>RuntimeProperty</c> or <c>SkipNavigation</c>).
+    /// </summary>
+    private static string GetMemberKindString(IReadOnlyPropertyBase member)
+        => member switch
+        {
+            IReadOnlyComplexProperty => "complex property",
+            IReadOnlySkipNavigation => "skip navigation",
+            IReadOnlyNavigation => "navigation",
+            IReadOnlyServiceProperty => "service property",
+            IReadOnlyProperty => "property",
+            _ => member.GetType().Name
+        };
+
     private static string GetNoFieldErrorMessage(IPropertyBase propertyBase)
         => propertyBase.DeclaringType switch
         {

src/EFCore/Metadata/Internal/TypeBase.cs

@@ -634,9 +634,7 @@ private void CheckDiscriminatorProperty(Property? property)
         if (conflictingMember != null)
         {
             throw new InvalidOperationException(
-                CoreStrings.ConflictingPropertyOrNavigation(
-                    name, DisplayName(),
-                    ((IReadOnlyTypeBase)conflictingMember.DeclaringType).DisplayName()));
+                conflictingMember.FormatConflictingMemberMessage(name, this));
         }
 
         if (memberInfo != null)
@@ -1082,9 +1080,7 @@ public virtual IReadOnlyDictionary<string, FieldInfo> GetRuntimeFields()
         if (conflictingMember != null)
         {
             throw new InvalidOperationException(
-                CoreStrings.ConflictingPropertyOrNavigation(
-                    name, DisplayName(),
-                    conflictingMember.DeclaringType.DisplayName()));
+                conflictingMember.FormatConflictingMemberMessage(name, this));
         }
 
         if (memberInfo != null)

src/EFCore/Properties/CoreStrings.resx

@@ -438,8 +438,11 @@
   <data name="ConflictingKeylessAndPrimaryKeyAttributes" xml:space="preserve">
     <value>The entity type '{entity}' has both [Keyless] and [PrimaryKey] attributes; one must be removed.</value>
   </data>
-  <data name="ConflictingPropertyOrNavigation" xml:space="preserve">
-    <value>The property or navigation '{member}' cannot be added to the '{type}' type because a property or navigation with the same name already exists on the '{conflictingType}' type.</value>
+  <data name="ConflictingPropertyOrNavigationOnBaseType" xml:space="preserve">
+    <value>The member '{member}' cannot be added to the '{type}' type because a {conflictingMemberKind} with the same name already exists on the '{conflictingType}' type. Remove the existing {conflictingMemberKind} first.</value>
+  </data>
+  <data name="ConflictingPropertyOrNavigationWithKind" xml:space="preserve">
+    <value>The member '{member}' cannot be added to the '{type}' type because a {conflictingMemberKind} with the same name already exists. Remove the existing {conflictingMemberKind} first.</value>
   </data>
   <data name="ConflictingRelationshipConversions" xml:space="preserve">
     <value>The property '{entityType}.{property}' participates in several relationship chains that have conflicting conversions: '{valueConversion}' and '{conflictingValueConversion}'.</value>

test/EFCore.Specification.Tests/ModelBuilding/ModelBuilderTest.OneToOne.cs

@@ -3050,7 +3050,7 @@ public virtual void Throws_on_duplicate_navigation_when_self_referencing()
             var modelBuilder = CreateModelBuilder();
 
             Assert.Equal(
-                CoreStrings.ConflictingPropertyOrNavigation("SelfRef1", nameof(SelfRef), nameof(SelfRef)),
+                CoreStrings.ConflictingPropertyOrNavigationWithKind("SelfRef1", nameof(SelfRef), "navigation"),
                 Assert.Throws<InvalidOperationException>(()
                     => modelBuilder.Entity<SelfRef>().HasOne(e => e.SelfRef1).WithOne(e => e.SelfRef1)).Message);
         }

test/EFCore.Tests/Metadata/Internal/EntityTypeTest.BaseType.cs

@@ -370,7 +370,7 @@ public void Adding_property_throws_when_parent_type_has_property_with_same_name(
         b.BaseType = a;
 
         Assert.Equal(
-            CoreStrings.ConflictingPropertyOrNavigation("G", typeof(B).Name, typeof(A).Name),
+            CoreStrings.ConflictingPropertyOrNavigationOnBaseType("G", typeof(B).Name, "property", typeof(A).Name),
             Assert.Throws<InvalidOperationException>(() => b.AddProperty("G")).Message);
     }
 
@@ -389,7 +389,7 @@ public void Adding_property_throws_when_grandparent_type_has_property_with_same_
         d.BaseType = c;
 
         Assert.Equal(
-            CoreStrings.ConflictingPropertyOrNavigation("G", typeof(D).Name, typeof(A).Name),
+            CoreStrings.ConflictingPropertyOrNavigationOnBaseType("G", typeof(D).Name, "property", typeof(A).Name),
             Assert.Throws<InvalidOperationException>(() => d.AddProperty("G")).Message);
     }
 
@@ -406,7 +406,7 @@ public void Adding_property_throws_when_child_type_has_property_with_same_name()
         b.AddProperty(A.GProperty);
 
         Assert.Equal(
-            CoreStrings.ConflictingPropertyOrNavigation("G", typeof(A).Name, typeof(B).Name),
+            CoreStrings.ConflictingPropertyOrNavigationOnBaseType("G", typeof(A).Name, "property", typeof(B).Name),
             Assert.Throws<InvalidOperationException>(() => a.AddProperty(A.GProperty)).Message);
     }
 
@@ -426,7 +426,7 @@ public void Adding_property_throws_when_grandchild_type_has_property_with_same_n
         d.AddProperty(A.GProperty);
 
         Assert.Equal(
-            CoreStrings.ConflictingPropertyOrNavigation("G", typeof(A).Name, typeof(D).Name),
+            CoreStrings.ConflictingPropertyOrNavigationOnBaseType("G", typeof(A).Name, "property", typeof(D).Name),
             Assert.Throws<InvalidOperationException>(() => a.AddProperty(A.GProperty)).Message);
     }