Reduce allocations in ProjectItemInstance.CommonConstructor by settin… #12978
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…g right capacity for list
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Pull request overview
This PR optimizes memory allocation in ProjectItemInstance.CommonConstructor by pre-sizing a list to avoid potential resizing operations. The issue identified through PRISM telemetry showed that when building the inheritedItemDefinitions list, the original code would create a list with exact capacity matching the input collection size, then potentially trigger a resize when adding one more item.
Key changes:
- Pre-allocate list capacity with
Count + 1to accommodate the potential addition of a project-level item definition - Use
AddRangeto populate the list instead of the collection constructor - Expand the ternary operator into an explicit if-else structure for clarity
AR-May
reviewed
Jan 13, 2026
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Microsoft.Build.Execution.ProjectItemInstance.CommonConstructor. InCommonConstructor,inheritedItemDefinitionsis built asnew List<ProjectItemDefinitionInstance>(itemDefinitions)(capacity ==itemDefinitions.Count) and theninheritedItemDefinitions.Add(itemDefinition)is called when the project-level item definition exists. This could lead to a list resize as seen in stack trace.List.Add -> EnsureCapacity -> set_Capacityframes seen in the stack trace that lead toTypeAllocated!Microsoft.Build.Execution.ProjectItemDefinitionInstance[].Best practices wiki
See related failure in PRISM
ADO work item