Update schema for combining TargetFramework info to allow for invalid…

… xml names such as including '+' (#6699)

Fixes #6603

Context
The ability to gather extra properties from referenced projects was added in #5994. It used XML transformations to couch that logic in a way that MSBuild could understand.

Unfortunately, the XML-based logic assumed everything was a valid XML element. Ultimately, the goal of the change, at least as I understand it, was just to pass information.

Changes Made
TargetFrameworks had been XML elements, but some are not valid XML names. This was changed to being a property on the more generically named TargetFramework element. This also allows for escaping.

Testing
Verified that a simple case that had not previously worked now works and looked at what it produced in a debugger.

Notes
After this PR, the SDK can opt in. Then MSBuild can turn it on by default, and the SDK can stop specifying the property.

documentation/ProjectReference-Protocol.md

@@ -106,6 +106,8 @@ As of MSBuild 16.10, it is possible to gather additional properties from referen
 
 These properties will then be gathered via the `GetTargetFrameworks` call.  They will be available to the referencing project via the `AdditionalPropertiesFromProject` metadata on the `_MSBuildProjectReferenceExistent` item.  The `AdditionalPropertiesFromProject` value will be an XML string which contains the values of the properties for each `TargetFramework` in the referenced project.  For example:
 
+> :warning: This format is being changed. Soon, the schema will replace <net5.0> with <TargetFramework Name="net5.0">. You can opt into that behavior early by setting the _UseAttributeForTargetFrameworkInfoPropertyNames property to true. This property will have no effect after the transition is complete.
+
 ```xml
 <AdditionalProjectProperties>
   <net5.0>

ref/Microsoft.Build.Tasks.Core/net/Microsoft.Build.Tasks.Core.cs

@@ -164,6 +164,7 @@ public partial class CombineTargetFrameworkInfoProperties : Microsoft.Build.Task
         [Microsoft.Build.Framework.OutputAttribute]
         public string Result { get { throw null; } set { } }
         public string RootElementName { get { throw null; } set { } }
+        public bool UseAttributeForTargetFrameworkInfoPropertyNames { get { throw null; } set { } }
         public override bool Execute() { throw null; }
     }
     public partial class CombineXmlElements : Microsoft.Build.Tasks.TaskExtension

ref/Microsoft.Build.Tasks.Core/netstandard/Microsoft.Build.Tasks.Core.cs

@@ -94,6 +94,7 @@ public partial class CombineTargetFrameworkInfoProperties : Microsoft.Build.Task
         [Microsoft.Build.Framework.OutputAttribute]
         public string Result { get { throw null; } set { } }
         public string RootElementName { get { throw null; } set { } }
+        public bool UseAttributeForTargetFrameworkInfoPropertyNames { get { throw null; } set { } }
         public override bool Execute() { throw null; }
     }
     public partial class CombineXmlElements : Microsoft.Build.Tasks.TaskExtension

src/Tasks/CombineTargetFrameworkInfoProperties.cs

@@ -2,11 +2,7 @@
 // Licensed under the MIT license. See LICENSE file in the project root for full license information.
 
 using Microsoft.Build.Framework;
-using System;
-using System.Collections.Generic;
-using System.Linq;
-using System.Text;
-using System.Threading.Tasks;
+using Microsoft.Build.Shared;
 using System.Xml.Linq;
 
 namespace Microsoft.Build.Tasks
@@ -26,6 +22,11 @@ public class CombineTargetFrameworkInfoProperties : TaskExtension
         /// </summary>
         public ITaskItem[] PropertiesAndValues { get; set; }
 
+        /// <summary>
+        /// Opts into or out of using the new schema with Property Name=... rather than just specifying the RootElementName.
+        /// </summary>
+        public bool UseAttributeForTargetFrameworkInfoPropertyNames { get; set; } = false;
+
         /// <summary>
         /// The generated XML representation of the properties and values.
         /// </summary>
@@ -36,9 +37,11 @@ public override bool Execute()
         {
             if (PropertiesAndValues != null)
             {
-                XElement root = new XElement(RootElementName);
+                XElement root = UseAttributeForTargetFrameworkInfoPropertyNames ?
+                    new("TargetFramework", new XAttribute("Name", EscapingUtilities.Escape(RootElementName))) :
+                    new(RootElementName);
 
-                foreach (var item in PropertiesAndValues)
+                foreach (ITaskItem item in PropertiesAndValues)
                 {
                     root.Add(new XElement(item.ItemSpec, item.GetMetadata("Value")));
                 }

src/Tasks/CombineXmlElements.cs

@@ -2,11 +2,6 @@
 // Licensed under the MIT license. See LICENSE file in the project root for full license information.
 
 using Microsoft.Build.Framework;
-using System;
-using System.Collections.Generic;
-using System.Linq;
-using System.Text;
-using System.Threading.Tasks;
 using System.Xml.Linq;
 
 namespace Microsoft.Build.Tasks

src/Tasks/Microsoft.Common.CurrentVersion.targets

@@ -1913,9 +1913,14 @@ Copyright (C) Microsoft Corporation. All rights reserved.
       </_AdditionalTargetFrameworkInfoPropertyWithValue>
     </ItemGroup>
 
+    <PropertyGroup>
+      <_UseAttributeForTargetFrameworkInfoPropertyNames Condition="'$(_UseAttributeForTargetFrameworkInfoPropertyNames)' == ''">false</_UseAttributeForTargetFrameworkInfoPropertyNames>
+    </PropertyGroup>
+
     <CombineTargetFrameworkInfoProperties
         RootElementName="$(TargetFramework)"
-        PropertiesAndValues="@(_AdditionalTargetFrameworkInfoPropertyWithValue)">
+        PropertiesAndValues="@(_AdditionalTargetFrameworkInfoPropertyWithValue)"
+        UseAttributeForTargetFrameworkInfoPropertyNames="$(_UseAttributeForTargetFrameworkInfoPropertyNames)">
       <Output TaskParameter="Result"
               PropertyName="_AdditionalTargetFrameworkInfoProperties"/>
     </CombineTargetFrameworkInfoProperties>