SaveAs support groupby collection merge cell expand

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TestIssue165.xlsx

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public static IEnumerable<IGrouping<TKey, TSource>> GroupBy<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector)
{
	return new GroupedEnumerable<TSource, TKey>(source, keySelector, null);
}

public interface IGrouping<out TKey, out TElement> : IEnumerable<TElement>, IEnumerable
{
	TKey Key
	{
		get;
	}
}

using System.Collections.Generic;

public GroupedEnumerable(IEnumerable<TSource> source, Func<TSource, TKey> keySelector, IEqualityComparer<TKey> comparer)
{
	if (source == null)
	{
		ThrowHelper.ThrowArgumentNullException(ExceptionArgument.source);
	}
	if (keySelector == null)
	{
		ThrowHelper.ThrowArgumentNullException(ExceptionArgument.keySelector);
	}
	_source = source;
	_keySelector = keySelector;
	_comparer = comparer;
}

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S.O : https://stackoverflow.com/questions/67768530/how-to-check-object-variable-is-linq-group?noredirect=1#comment119784177_67768530

c# - LINQ Grouping dynamically - Stack Overflow

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By Attribute option

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