How can I get a Span from a List while avoiding needless copies?

In .Net 5.0, you can use CollectionsMarshal.AsSpan() (source, GitHub issue) to get the underlying array of a List<T> as a Span<T>.

Keep in mind that this is still unsafe: if the List<T> reallocates the array, the Span<T> previously returned by CollectionsMarshal.AsSpan won’t reflect any further changes to the List<T>. (Which is why the method is hidden in the System.Runtime.InteropServices.CollectionsMarshal class.)