using in .NET

using 的使用并不少见，但是最近看到这种用法还是觉得有些奇怪：

static IEnumerable<int> LoadNumbers(string filePath)
{
    using StreamReader reader = File.OpenText(filePath);
    
    var numbers = new List<int>();
    string line;
    while ((line = reader.ReadLine()) is not null)
    {
        if (int.TryParse(line, out int number))
        {
            numbers.Add(number);
        }
    }
    return numbers;
}

如上面的代码示例，using并没有带大括号，微软的文档说明是：

When declared in a using declaration, a local variable is disposed at the end of the scope in which it's declared. In the preceding example, disposal happens at the end of a method.

A variable declared by the using statement or declaration is readonly. You cannot reassign it or pass it as a ref or out parameter.

https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/statements/using