Pre-size MultiDictionary in CreateEvaluatedIncludeSnapshotIfRequested to avoid resize allocations

[nareshjo]

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• Issue: The allocation hot path ends in Microsoft.Build.Collections.MultiDictionary<string, ProjectItemInstance>.Add. In CreateEvaluatedIncludeSnapshotIfRequested, the MultiDictionary is constructed with default capacity (new MultiDictionary<string, ProjectItemInstance>(StringComparer.OrdinalIgnoreCase)) and then populated in a foreach loop over all project items.
  Each time the backing Dictionary<string, SmallList> exceeds its current capacity, Dictionary.Resize allocates a new Entry[] array — producing ~log₂(N) intermediate arrays that become immediate garbage. For a project with 4000 items this is ~12 resize allocations, with progressively larger arrays that can reach LOH thresholds.

• Issue type: Specify an up-front capacity for collections if one is known.

• Proposed fix: Add a capacity-aware constructor overload to MultiDictionary<K, V> and pass items.Count at the call site in CreateEvaluatedIncludeSnapshotIfRequested. The items parameter is ICollection so .Count is O(1). The capacity is an upper bound (unique key count ≤ item count), resulting in at most ~1–5% over-allocation — negligible compared to eliminating all resize allocations. MultiDictionary is an internal class; the existing constructor is preserved and the new overload is additive.
  This directly targets the Dictionary.Resize → Dictionary.Insert → MultiDictionary.Add → CreateEvaluatedIncludeSnapshotIfRequested frames seen in the stack trace that lead to TypeAllocated!Entry[System.String, MultiDictionary+SmallList[...]][].

Best practices wiki
See related failure in PRISM
ADO work item

[Copilot]

Pull request overview

This PR adds a capacity-aware constructor to the internal MultiDictionary<K, V> class and uses it in CreateEvaluatedIncludeSnapshotIfRequested to pre-size the backing dictionary, avoiding repeated resize allocations when populating from a known-size collection.

Changes:

• Added a new MultiDictionary(int capacity, IEqualityComparer<K> keyComparer) constructor that forwards the capacity to the backing Dictionary.
• Updated the CreateEvaluatedIncludeSnapshotIfRequested call site to pass items.Count as the initial capacity.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/Build/Collections/MultiDictionary.cs	Added capacity-aware constructor overload
src/Build/Instance/ProjectInstance.cs	Pass items.Count as capacity when constructing the MultiDictionary