Reduce allocations in ProjectItemInstance.CommonConstructor by settin…

[nareshjo]

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• Issue: The allocation hot path ends in Microsoft.Build.Execution.ProjectItemInstance.CommonConstructor. In CommonConstructor, inheritedItemDefinitions is built as new List<ProjectItemDefinitionInstance>(itemDefinitions) (capacity == itemDefinitions.Count) and then inheritedItemDefinitions.Add(itemDefinition) is called when the project-level item definition exists. This could lead to a list resize as seen in stack trace.
• Issue type: Specify an up-front capacity for collections if one is known
• Proposed fix: Pre-sizing the list to the final size (itemDefinitions.Count + 1) is a safe and localized minimal change in this method and directly targets the List.Add -> EnsureCapacity -> set_Capacity frames seen in the stack trace that lead to TypeAllocated!Microsoft.Build.Execution.ProjectItemDefinitionInstance[].

Best practices wiki
See related failure in PRISM
ADO work item

[Copilot]

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 + 1 to accommodate the potential addition of a project-level item definition
• Use AddRange to populate the list instead of the collection constructor
• Expand the ternary operator into an explicit if-else structure for clarity

[AR-May]

There is no need to have this list variable.

[nareshjo]

I think AI agent used a list variable to be able to call AddRange(), which is not available on inheritedItemDefinitions which is IList.
I agree it can be done without the variable by casting since we just created it as a List. Were you suggesting something like this?

inheritedItemDefinitions = (itemDefinitions == null) ? null : new List<ProjectItemDefinitionInstance>(itemDefinitions.Count + 1);
((List<ProjectItemDefinitionInstance>)inheritedItemDefinitions)?.AddRange(itemDefinitions);