Add ReferenceExpression to the External Parameters article

[alistairmatthews]

Summary

Adds a new section describing how you can use reference expressions to manipulate connection strings in .NET Aspire 9.2 and later.

Fixes #2710

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docs/fundamentals/external-parameters.md	External parameters

[alistairmatthews]

@IEvangelist can you check that I've done the code snippets correctly? They are in the snippets/referenceexpressions folder. Thanks!

[IEvangelist]

Looks great, just want these ranges cleaned up. Try id if possible, will likely require a bit of refactoring in the Program.cs.

[alistairmatthews]

Looks great, just want these ranges cleaned up. Try id if possible, will likely require a bit of refactoring in the Program.cs.

Done.

Question about IDs though: Do you know whether, if you put two blocks with the same ID in a code file, then referenced that ID in the triple-colon link, will it render both blocks? That would be useful. The guidance says you can nest them but nothing else.

[IEvangelist]

That's a fair question, I'm not sure, but you could try it if you'd like. Here's how it renders now:

[alistairmatthews]

That's a fair question, I'm not sure, but you could try it if you'd like. Here's how it renders now:

That looks good in the context of this article where we've had previous examples of complete app host program.cs files. I'll experiment with other examples though. Thanks!