why we use nested using statement in c#?-CodePudding

using (StreamReader outFile = new StreamReader(outputFile.OpenRead())) 
{    
   StreamReader resFile = new StreamReader(resultFile.OpenRead())    
   {       //Some Codes    }
}

Why does the above resFile object not close automatically?

I wrote the resFile object inside the using statement also. Please explain the using statement.

CodePudding user response：

You didn't use nested using. There's only one using statement.

An example of nested using:

using (...)
{
     using (...)
     {
         ...
     }
}

The reason why you may want to use nested using is that you have more than one declaration that need to be disposed.

CodePudding user response：

Find the official explanation for the using statement here: https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/using-statement

In your example you are not having a nested using statement as Mark said correctly in his answer. (Though you should have a second using for the resFile Streamreader.

They have to be nested, if you want to use both stream readers in the "// Some Codes" part, as the instances for outFile and resFile are only available inside the curly brackets.

Starting from C#8 there is a new possibility for usings that "avoids" the nesting. See: https://learn.microsoft.com/de-de/dotnet/csharp/language-reference/proposals/csharp-8.0/using

CodePudding user response：

using is syntactic sugar and basically compiles to the following:

Original code:

using (var a = b)
{    
  c();
  d();
}

Desugared code:

{
  var a;
  try {
    a = b;
    c();
    d();
  } finally {
    if (a != null) {
      ((IDisposable)a).Dispose();
    }
  }
}