aspnetcore/docs/ReferenceResolution.md

# `<Reference>` resolution

Most project files in this repo should use `<Reference>` instead of `<ProjectReference>` or `<PackageReference>`.
This was done to enable ASP.NET Core's unique requirements without requiring most ASP.NET Core contributors
to understand the complex rules for how versions and references should work. The build system will resolve
Reference items to the correct type and version of references based on our servicing and update rules.

See [ResolveReferences.targets](/eng/targets/ResolveReferences.targets) for the exact implementation of custom
`<Reference>` resolutions.

The requirements that led to this system are:

* Versions of external dependencies should be consistent and easily discovered.
* Newer versions of packages should not have lower dependency versions than previous releases.
* Minimize the cascading effect of servicing updates where possible by keeping a consistent baseline of dependencies.
* Servicing releases should not add or remove dependencies in existing packages.

As a minor point, the current system also makes our project files somewhat less verbose.

## Recommendations for writing a .csproj

* Use `<Reference>`.
* Do not use `<PackageReference>`.
* If you need to use a new package, add it to `eng/Dependencies.props` and `eng/Versions.props`.
* If the package comes from a partner team and needs to have versions automatically updated, also add an entry `eng/Version.Details.xml`.
* Only use `<ProjectReference>` in test projects.
* Name the .csproj file to match the assembly name.
* Run `eng/scripts/GenerateProjectList.ps1` (or `build.cmd /t:GenerateProjectList`) when adding new projects
* Use [eng/tools/BaseLineGenerator/](/eng/tools/BaselineGenerator/README.md) if you need to update baselines.
* If you need to make a breaking change to dependencies, you may need to add `<SuppressBaselineReference>`.

## Important files

* [eng/Baseline.xml](/eng/Baseline.xml) - this contains the 'baseline' of the latest servicing release for this branch.
  It should be modified and used to update the generated file, [eng/Baseline.Designer.props](eng/Baseline.Designer.props).
* [eng/Dependencies.props](/eng/Dependencies.props) - contains a list of all package references that might be used in the repo.
* [eng/ProjectReferences.props](/eng/ProjectReferences.props) - lists which assemblies or packages might be available to be referenced as a local project.
* [eng/Versions.props](/eng/Versions.props) - contains a list of versions which may be updated by automation. This is used by MSBuild to restore and build.
* [eng/Version.Details.xml](/eng/Version.Details.xml) - used by automation to update dependency variables in
  [eng/Versions.props](/eng/Versions.props) and, for SDKs and `msbuild` toolsets, [global.json](global.json).

## Example: adding a new project

Steps for adding a new project to this repo.

1. Create the .csproj
2. Run `eng/scripts/GenerateProjectList.ps1`
3. Add new project to AspNetCore.sln and any relevant `*.slnf` files

## Example: adding a new dependency

Steps for adding a new package dependency to an existing project. Let's say I'm adding a dependency on System.Banana.

1. Add the package to the .csproj file using `<Reference Include="System.Banana" />`
2. Add an entry to [eng/Dependencies.props](/eng/Dependencies.props) e.g. `<LatestPackageReference Include="System.Banana" />`
3. If this package comes from another dotnet team and should be updated automatically by our bot&hellip;
    1. Add an entry to [eng/Versions.props](/eng/Versions.props) like this `<SystemBananaPackageVersion>0.0.1-beta-1</SystemBananaPackageVersion>`.
    2. Add an entry to [eng/Version.Details.xml](/eng/Version.Details.xml) like this:

        ```xml
        <ProductDependencies>
          <!-- ... -->
          <Dependency Name="System.Banana" Version="0.0.1-beta-1">
            <Uri>https://github.com/dotnet/corefx</Uri>
            <Sha>000000</Sha>
          </Dependency>
          <!-- ... -->
        </ProductDependencies>
        ```

        If you don't know the commit hash of the source code used to produce "0.0.1-beta-1", you can use `000000` as a
        placeholder for `Sha` as its value will be updated the next time the bot runs.

        If the new dependency comes from dotnet/runtime and you are updating dotnet/aspnetcore-tooling, add a
        `CoherentParentDependency` attribute to the `<Dependency>` element as shown below. This example indicates the
        dotnet/runtime dependency version for System.Banana should be determined based on the dotnet/aspnetcore build
        that produced the chosen Microsoft.CodeAnalysis.Razor. That is, the dotnet/runtime and dotnet/aspnetcore
        dependencies should be coherent.

        ```xml
        <Dependency Name="System.Banana" Version="0.0.1-beta-1" CoherentParentDependency="Microsoft.CodeAnalysis.Razor">
          <!-- ... -->
        </Dependency>
        ```

        The attribute value should be `"Microsoft.CodeAnalysis.Razor"` for dotnet/runtime dependencies in
        dotnet/aspnetcore-tooling.

## Example: make a breaking change to references

If Microsoft.AspNetCore.Banana in 2.1 had a reference to `Microsoft.AspNetCore.Orange`, but in 3.1 or 5.0 this reference
is changing to `Microsoft.AspNetCore.BetterThanOrange`, you would need to make these changes to the .csproj file

```diff
<!-- in Microsoft.AspNetCore.Banana.csproj -->
  <ItemGroup>
-    <Reference Include="Microsoft.AspNetCore.Orange" /> <!-- the old dependency -->
+    <Reference Include="Microsoft.AspNetCore.BetterThanOrange" /> <!-- the new dependency -->
+    <SuppressBaselineReference Include="Microsoft.AspNetCore.Orange" /> <!-- suppress as a known breaking change -->
  </ItemGroup>
```

## A darc cheatsheet

`darc` is a command-line tool that is used for dependency management in the dotnet ecosystem of repos. `darc` can be installed using the `darc-init` scripts located inside the `eng/common` directory. Once `darc` is installed, you'll need to set up the appropriate access tokens as outlined [in the official Darc docs](https://github.com/dotnet/arcade/blob/master/Documentation/Darc.md#setting-up-your-darc-client).

Once `darc` is installed and set-up, it can be used to modify the subscriptions and dependencies in a project.

**Getting the list of subscriptions in a repo**

Subscriptions are objects that define the ecosystem repos we are listening for updates to, the frequency we are looking for updates, and more.

```bash
darc get-subscriptions --target-branch master --target-repo aspnetcore$ --regex
```

**Disable/enable a subscription**

```bash
darc subscription-status --id {subscriptionIdHere} --enable
darc subscription-status --id {subscriptionIdHere} --disable
```

**Trigger a subscription**

Triggering a subscription will search for updates in its dependencies and open a PR in the target repo via the dotnet-maestro bot with these changes.

```bash
darc trigger-subscriptions --id {subscriptionIdHere}
```

**Manually update dependencies**

If the `dotnet-maestro` bot has not correctly updated the dependencies, `darc update-dependencies` may be used to update the dependencies manually. Note, you'll need to run the commands below in a separate branch and submit a PR with the changes. These are the things that the bot should do for you if you use `trigger-subscriptions` or automatically (when the subscription fires e.g. about 15 minutes after a dependency's build completes if `Update Frequency: EveryBuild`).

```bash
darc update-dependencies --channel '.NET Core 3.1 Release'
darc update-dependencies --channel '.NET 5 Dev' --source-repo efcore
```

Generally, using `trigger-subscriptions` is preferred for creating dependency updates instead of manually updating dependencies in your own PR.

**Toggling batchability of subscription**

Subscriptions can be batched. When a dependency update is detected, `darc` will bundle the commits for that update with existing dependency PRs. To toggle whether a subscription is batched or not, you will need to use the `update-subscription` command.

```bash
darc update-subscription --id {subscriptionIdHere}
```

Your shell's default editor will open and allow you to edit the metadata of the subscription.

To disable batching, set `Batchable` to `False` and update the `Merge Policies` section with the following YAML.

```
  - Name: Standard
    Properties: {}
```

To enable batching, set `Batchable` to `True` and remove any `Merge Policies` set on the subscription.

Note: Merge policies can only be set on unbatched subscriptions. Be sure to set/unset the `Merge Policies` field properly as you toggle batchability.