Fix setting ProjectMetadataElement.Name

src/Build.OM.UnitTests/Construction/ConstructionEditing_Tests.cs

@@ -3137,6 +3137,44 @@ public void AddMetadataAsAttributeAndAsElement()
             Helpers.VerifyAssertProjectContent(expected, project);
         }
 
+        [Fact]
+        public void SetMetadataName()
+        {
+            var project = ProjectRootElement.Create();
+            var itemGroup = project.AddItemGroup();
+
+            var item = itemGroup.AddItem("i1", "i");
+            var attributeMetadata = item.AddMetadata("A", "value_a", expressAsAttribute: true);
+            var elementMetadata = item.AddMetadata("B", "value_b", expressAsAttribute: false);
+
+            string expected = """
+                <Project ToolsVersion="Current" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+                  <ItemGroup>
+                    <i1 Include="i" A="value_a">
+                      <B>value_b</B>
+                    </i1>
+                  </ItemGroup>
+                </Project>
+                """;
+
+            Helpers.VerifyAssertProjectContent(expected, project);
+
+            attributeMetadata.Name = "A2";
+            elementMetadata.Name = "B2";
+
+            expected = """
+                <Project ToolsVersion="Current" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+                  <ItemGroup>
+                    <i1 Include="i" A2="value_a">
+                      <B2>value_b</B2>
+                    </i1>
+                  </ItemGroup>
+                </Project>
+                """;
+
+            Helpers.VerifyAssertProjectContent(expected, project);
+        }
+
         /// <summary>
         /// Legally modify a child whose parent is not parented (should not throw)
         /// </summary>

src/Build/Construction/ProjectElementContainer.cs

@@ -448,6 +448,23 @@ private void SetElementAsAttributeValue(ProjectElement child)
             ProjectXmlUtilities.SetOrRemoveAttribute(XmlElement, child.XmlElement.Name, value);
         }
 
+        /// <summary>
+        /// If child "element" is actually represented as an attribute, update the name in the corresponding Xml attribute
+        /// </summary>
+        /// <param name="child">A child element which might be represented as an attribute</param>
+        /// <param name="oldName">The old name for the child element</param>
+        internal void UpdateElementName(ProjectElement child, string oldName)
+        {
+            ErrorUtilities.VerifyThrow(Link == null, "External project");
+
+            if (child.ExpressedAsAttribute)
+            {
+                // To rename an attribute, we have to fully remove the old one and add a new one.
+                XmlElement.RemoveAttribute(oldName);
+                SetElementAsAttributeValue(child);
+            }
+        }
+
         /// <summary>
         /// If child "element" is actually represented as an attribute, update the value in the corresponding Xml attribute
         /// </summary>

src/Build/Construction/ProjectMetadataElement.cs

@@ -134,11 +134,14 @@ internal void ChangeName(string newName)
                 ValidateValidMetadataAsAttributeName(newName, Parent.ElementName, Parent.Location);
             }
 
+            string oldName = XmlElement.Name;
+
             // Because the element was created from our special XmlDocument, we know it's
             // an XmlElementWithLocation.
             XmlElementWithLocation newElement = XmlUtilities.RenameXmlElement(XmlElement, newName, XmlElement.NamespaceURI);
 
             ReplaceElement(newElement);
+            Parent.UpdateElementName(this, oldName);
         }
 
         internal static void ValidateValidMetadataAsAttributeName(string name, string parentName, IElementLocation parentLocation)