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GH-36816: [C#] Reduce allocations (#36817)

### What changes are included in this PR?

A small refactoring to make a small improvement in memory utilization.

### Are these changes tested?

Existing test coverage should be sufficient.

* Closes: #36816

Authored-by: Curt Hagenlocher <curt@hagenlocher.org>
Signed-off-by: David Li <li.davidm96@gmail.com>