David Callan Thanks for posting your question in Microsoft Q&A. Currently, Azure Functions supports the below list of versions for Isolated model (refer Languages; Supported versions doc) and .NET 9 is not supported yet. To answer your question, the specific versions of .NET such as .NET 9
cannot be installed via extension. Instead, I suggest you using the supported versions.
I hope this helps and let me know if any questions.
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for others to benefit from your experience. Or simply add a comment tagging me and would be happy to answer your questions.