How to add Compile Update item to csproj?

I'm trying to use Microsoft Build SDK to add a `Compile Update` item to the SDK style csproj file. What I need is to basically generate an XML like the following:

```xml

Class1.cs

```

However, I couldn't find an easy way to do this. If I simply use `ProjectItem.SetMetadataValue`, I get the following exception:

```
System.InvalidOperationException: 'Cannot modify an evaluated object originating in an imported file "C:\Program Files\dotnet\sdk\3.1.201\Sdks\Microsoft.NET.Sdk\targets\Microsoft.NET.Sdk.DefaultItems.props".'
```

And if I try to use APIs like `Project.AddItem`, I have to go through this:

```cs
var item = project.AddItem("Compile", "Class1.Generated.cs", new []
{
new KeyValuePair("DependentUpon", "Class1.cs")
}).Single();
item.Xml.Include = null;
item.Xml.Update = "Class1.Generated.cs";
```

Because `Project.AddItem` requires an "unevaluated include".

Am I missing something? Is there a better way to do this?

Start by examining ProjectItem.SetMetadataValue and Project.AddItem, then trace the exception for evaluated items originating in Microsoft.NET.Sdk.DefaultItems.props. Compare the requested Compile Update XML with the generated project state and determine whether an existing API supports it; done means identifying a supported approach or documenting the limitation.

Written by the indexing model from the issue text.