小编arz*_*arz的帖子

我看到了这个链接：

https://learn.microsoft.com/en-us/dotnet/api/system.idisposable?view=net-6.0

代码部分是：

if (!File.Exists(filename))
    throw new FileNotFoundException("The file does not exist.");

this.filename = filename;
string txt = String.Empty;
StreamReader sr = null;
try
{
    sr = new StreamReader(filename);
    txt = sr.ReadToEnd();
}
finally
{
    if (sr != null) sr.Dispose();
}

StreamReader什么时候变成null？