Get rid of s_compatOids in favor of a switch

[vcsjones]

s_compatOids is a dictionary that is never updated at runtime and has contained 11 items for more than 5 years.

Rather than use a dictionary, let's use a switch here. The compiler will generate an efficient lookup for the handful of switch cases. This means we are doing less work in the .cctor of OidLookup to create the dictionary. This dictionary is not frequently read from, either, so the eagerly created dictionary was wasted work, most of the time.

[Copilot]

Pull Request Overview

This PR optimizes the OID lookup mechanism by replacing a static dictionary s_compatOids with a switch expression. The change eliminates the overhead of creating an 11-item dictionary during static construction, which was rarely accessed, in favor of a more efficient compile-time optimized switch statement.

• Replaces dictionary lookup with switch expression for better performance
• Removes static dictionary initialization overhead
• Maintains identical functionality with cleaner code structure

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[EgorBo]

Unrelated to this PR: we have quite a few static readonly Dictionary fields initialized with some constant values - I wonder if we can propose an analyzer suggesting to convert them to plain switches? It should be more efficient even for string literals given they have special support from the Roslyn side via Trie-like lookup, while for Dictionary we have to compute the hashcode (O(N) over string's content).

Or just replace them all manually at least in the BCL

cc @jkotas

[EgorBo]

This not only removes the pretty slow static init, but also makes lookup faster: sharplab (trie)

[vcsjones]

There is definitely more I think we can do in this OidLookup, I will keep chipping away at it.

[stephentoub]

special support from the Roslyn side via Trie-like lookup, while for Dictionary we have to compute the hashcode (O(N) over string's content).

Only in some cases. Others will fall back to a binary search still based on computing a hash code, e.g SharpLab

switch also only works for the equivalent of StringComparer.Ordinal.