Avoid index out of range

[HebaruSan]

Problem

Right after #4171, we got a report of an index out of bounds exception in the manage game instances dialog, which was fixed in 2ddf740. Today another came up in a different method of the same class.

Cause

Lines like this cause warnings (which we promote to errors) with nullable reference enabled:

CKAN/GUI/Dialogs/ManageGameInstancesDialog.cs

Line 352 in 82e5c35

ForgetButton.Enabled = HasSelections && (string)GameInstancesListView.SelectedItems[0].Tag != manager.CurrentInstance?.Name;

We need to check whether GameInstancesListView.SelectedItems[0].Tag is actually a string rather than null before we cast it, so I did that:

CKAN/GUI/Dialogs/ManageGameInstancesDialog.cs

Lines 361 to 366 in c13930f

	if (GameInstancesListView.SelectedItems[0].Tag is string instName)
	{
	RenameButton.Enabled = SelectButton.Enabled = SetAsDefaultCheckbox.Enabled = CloneGameInstanceMenuItem.Enabled = HasSelections;
	ForgetButton.Enabled = HasSelections && instName != manager.CurrentInstance?.Name;
	ImportFromSteamMenuItem.Enabled = manager.SteamLibrary.Games.Length > 0;
	}

But now we're evaluating GameInstancesListView.SelectedItems[0] before HasSelections runs to confirm that list is non-empty, so when it is empty, it's an array bounds error.

Ideal solution and why we can't do it

C# 11 provides a great way to simplify checking array bounds and eliminate hard coded indexes:

• https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/proposals/csharp-11.0/list-patterns

Initially I did a code search to find every instance of a hard coded array index and worked up a set of changes to replace them with list patterns, but I hit a snag when I tried to build it with Mono:

2>CSC : error CS1617: Invalid option '11' for /langversion. Use '/langversion:?' to list supported values.

$ csc /langversion:?
Supported language versions:
default
1
2
3
4
5
6
7.0
7.1
7.2
7.3
8.0
9.0 (default)
latestmajor
preview
latest

So we need a solution that works on C# 9.

Changes

Now every hard coded array index access is replaced by:

• A comment with a list pattern illustrating the intended behavior
• A set of explicit, non-list-pattern checks that do exactly the same thing
• My brief adventure in C# 11 enabled an "unnecessary lambda" message in vscode, so those have been addressed

If we are ever able to migrate to C# 11, we will have the option of switching to the list patterns that are already there in these comments.