[C#] Consider replacing LINQ expression when accessing columns by name

[georgevanburgh]

Describe the enhancement requested

We've recently been doing some profiling of code that uses the C# Arrow library, and which frequently accesses columns by name in a tight loop - and found the following LINQ expression makes a significant contribution to execution time.

arrow/csharp/src/Apache.Arrow/Schema.cs

Lines 81 to 86 in 2df7b23

	public int GetFieldIndex(string name, IEqualityComparer<string> comparer = default)
	{
	comparer ??= StringComparer.CurrentCulture;
	return _fieldsList.IndexOf(_fieldsList.First(x => comparer.Equals(x.Name, name)));
	}

Obviously there are workarounds for this, such as creating our own cached mapping of column names to ordinals - but it would be nice if the built-in method was a little more efficient. In addition to the use of linq, we're also iterating through _fieldsList twice (once to find a match, and then another to find the index of the match).

We've also noticed that usage of StringComparer.CurrentCulture causes an allocation every time this method is called (assuming a value for comparer is not provided). I'm assuming the choice to use a culture-aware comparison by default is intentional?

Component(s)

C#

[CurtHagenlocher]

The use of StringComparer.CurrentCulture is definitely a head-scratcher. It apparently dates back to the very first checkin of the file. It's especially surprising because the very same checkin used StringComparer.OrdinalIgnoreCase to build the dictionary. While I ordinarily argue against making breaking changes, I think it might be justified this time.

I would expect the allocation to be a result of the closure that needs to be created to pass to First.

[CurtHagenlocher]

Issue resolved by pull request 44576
#44576

[vthemelis]

The use of StringComparer.CurrentCulture is definitely a head-scratcher. It apparently dates back to the very first checkin of the file. It's especially surprising because the very same checkin used StringComparer.OrdinalIgnoreCase to build the dictionary. While I ordinarily argue against making breaking changes, I think it might be justified this time.

I would expect the allocation to be a result of the closure that needs to be created to pass to First.

Added #44650 for this as it's fairly confusing.