IndexOutOfRangeException in InjectSnippetsIntoFile when snippet insertion is at end of file

[gavinbarron]

Bug

InjectSnippetsIntoFile crashes with System.ArgumentOutOfRangeException when the snippet insertion point is at or near the end of a documentation file.

Stack trace

System.AggregateException: One or more errors occurred.
  (Index was out of range. Must be non-negative and less than the size of the collection. (Parameter 'index'))
 ---> System.ArgumentOutOfRangeException: Index was out of range.
   at System.Collections.Generic.List1.get_Item(Int32 index)
   at System.Linq.Enumerable.ElementAt[TSource](IEnumerable1 source, Int32 index)
   at ApiDoctor.ConsoleApp.Program.InjectSnippetsIntoFile(MethodDefinition method, String codeSnippets)
     in Program.cs:line 2513
   at ApiDoctor.ConsoleApp.Program.GenerateSnippetsAsync(GenerateSnippetsOptions options, ...)
     in Program.cs:line 2002

Root cause

After Splice removes the old snippet tab section, the collection shrinks. The code then unconditionally accesses updatedFileContents.ElementAt(insertionLine + 1) without checking bounds. When the insertion point is at or near the end of the file (no trailing content after the code block), insertionLine + 1 exceeds the collection size.

Affected code

// Program.cs, in the InsertSnippets block:
updatedFileContents = updatedFileContents.Splice(insertionLine, snippetsTabSectionEndLine - insertionLine);
if (!string.IsNullOrWhiteSpace(codeSnippets))
{
    // BUG: no bounds check before ElementAt
    if (!string.IsNullOrWhiteSpace(updatedFileContents.ElementAt(insertionLine + 1)))
        codeSnippets = $"{codeSnippets}\r\n";
    updatedFileContents = FileSplicer(updatedFileContents.ToArray(), insertionLine, codeSnippets);
}

Fix

Add a bounds check before accessing ElementAt:

if (insertionLine + 1 < updatedFileContents.Count()
    && !string.IsNullOrWhiteSpace(updatedFileContents.ElementAt(insertionLine + 1)))

This skips appending the trailing newline when the insertion point is at the end of the file, which is the correct behavior — there's no subsequent content to separate from.