ExcelColumnCodeToColumnIndex.cs

void Main()
{
	Console.WriteLine(GetCode("A"));
	Console.WriteLine(GetCode("Z"));
	Console.WriteLine(GetCode("AA"));
	Console.WriteLine(GetCode("AZ"));
	Console.WriteLine(GetCode("AAA"));
}

// Define other methods and classes here
int GetCode(char c)
{
	return (int)c - 64;
}

int GetCode(string str)
{
	var val = str.ToUpper();
	var ind = val.Reverse().Select( (c,i) => new { Index = i, Code = c }).ToList();
	return ind.Aggregate(0, (i, x) => 
	{
		//NOTE: For Debugging
		//Console.WriteLine("Code: {0} Index: {1} Prev:{2}, ", x.Code, x.Index, i);
		var sum = i + GetCode(x.Code)*(int)Math.Pow(26,x.Index);
		return sum;
	});
}