Msbuild: Implement a way to merge one item into another

[cdmihai]

Given item types A and B, there should be an easy way in MSBuild to merge B into A.

Possible ways to do merging:

1. Metadata merging
   
   
   
   1. Additive: Apply the metadata from the elements of B[B∩A] to the elements A[B∩A]
   
   
   2. Overwrite: Overwrite the metadata from the elements of B[B∩A] to the elements A[B∩A]
   
   
2. Item merging:
   
   
   
   1. Intersection: Merge (additive or overwrite) the metadata from the elements of B[B∩A] to the elements A[B∩A]
   
   
   2. Union: Merge (additive or overwrite) the metadata from the elements of B[B∩A] to the elements A[B∩A], and insert the elements B\A into A
   
   

Probably the most useful merging is intersection item merging with additive metadata. Possible syntax implementations for only this type of merging:

• reuse the current Update operation. If the Update query is an item reference, and there is no metadata defined, do merging instead.

<Foo Update="@(Bar)"> // applies all the metadata from Bar into Foo
<Foo Update="@(Bar)" KeepMetadata="Tar"> // applies only metadata named "Tar" from Bar into Foo
<Foo Update="@(Bar)" RemoveMetadata="Tar"> // applies all metadata except "Tar" from Bar into Foo

The drawback is that MSBuild currently treats <Foo Update="@(Bar)"/> as a noop (i.e. for all items in Foo that match in Bar, do nothing), so we'll break the existing noop behaviour. Since it's a pretty benign behaviour, it probably doesn't happen too often.

• add a new Merge operation

<Foo Merge="@(Bar)"> // applies all the metadata from Bar into Foo
<Foo Merge="@(Bar)" KeepMetadata="Tar"> // applies only metadata named "Tar" from Bar into Foo
<Foo Merge="@(Bar)" RemoveMetadata="Tar"> // applies all metadata except "Tar" from Bar into Foo

The advantages of Merge:

• it disambiguates behaviour by item operations. This way each operation does something consistent (Update single metadata via a matching query Vs Merging all metadata of one item type into another item type).
• it allows extensibility to the other merging types via configuration attributes (e.g. <Foo Merge="@(Bar)" MetadataMergeStrategy="overwrite" ItemMergeStrategy="Union">)

[vatsan-madhavan]

Are there patterns for how any of these can be implemented today within targets, esp. _Intersection_ with merging of metadata?

/cc @nguerrera, @rainersigwald

[rainersigwald]

I would implement a task @vatsan-madhavan. It's often possible to do it in pure MSBuild but IME the output is incomprehensible even to the author as of about half an hour after writing it.