try catch塊內的變量范圍

Question

嘗試在Finally代碼塊中關閉文件時出現錯誤：

static void Main(string[] args)
{
    try
    {
        StreamReader myReader = new StreamReader("c:\\j\\MyFile1.txt");

        string line = "";

        while (line != null)
        {
            line = myReader.ReadLine();
            Console.WriteLine(line);
        }

        //myReader.Close();
    }
    catch (FileNotFoundException e)
    {
        Console.WriteLine("sorry file not found! {0}", e.Message);
    }
    catch (DirectoryNotFoundException e)
    {
        Console.WriteLine("You have given the wrong path for the file! {0}", e.Message);
    }
    catch (Exception e)
    {
        Console.WriteLine("Sorry you typed in a wrong file name! {0}", e.Message);
    }
    finally
    { 
        myReader.Close();
    }

    Console.ReadLine();
}

Answer 1

您需要在try上方聲明StreamReader 。

話雖如此，在這種情況下，我建議使用using語句，而不是try / finally，因為它是專門為資源清理而設計的。

using (StreamReader myReader = new StreamReader("c:\\j\\MyFile1.txt"))
{
    try
    {
        string line = "";

        while (line != null)
        {
            line = myReader.ReadLine();
            Console.WriteLine(line);
        }
    }
    catch (FileNotFoundException e)
    {
        Console.WriteLine("sorry file not found! {0}", e.Message);
    }
    catch (DirectoryNotFoundException e)
    {
        Console.WriteLine("You have given the wrong path for the file! {0}", e.Message);

    }
    catch (Exception e)
    {
        Console.WriteLine("Sorry you typed in a wrong file name! {0}", e.Message);
    }
}

Console.ReadLine();

這將確保StreamReader是關閉的，但是可以使用更加符合ADOMATIC的C＃方式。 StreamReader的IDisposable.Dispose實現將關閉流

Answer 2

嘗試之前聲明變量：

StreamReader myReader = null;

等等。然后在try塊中進行設置。

Answer 3

您需要在try / catch / finally塊之外聲明StreamReader實例。

static void Main(string[] args)
{
    using (StreamReader myReader = null)
    {
    try
    {
        myReader = new StreamReader("c:\\j\\MyFile1.txt");

        string line = "";

        while (line != null)
        {
            line = myReader.ReadLine();
            Console.WriteLine(line);
        }

        //myReader.Close();
    }
    catch (FileNotFoundException e)
    {
        Console.WriteLine("sorry file not found! {0}", e.Message);
    }
    catch (DirectoryNotFoundException e)
    {
        Console.WriteLine("You have given the wrong path for the file! {0}", e.Message);
    }
    catch (Exception e)
    {
        Console.WriteLine("Sorry you typed in a wrong file name! {0}", e.Message);
    }
    finally
    { 
        myReader.Close();
    }
}
    Console.ReadLine();
}

Answer 4

只需在try塊之外定義myReader ，然后在調用close的finally塊中檢查null

StreamReader myReader = null;
try
{
    myReader = new StreamReader("c:\\j\\MyFile1.txt");
    //.....

在最后一塊

finally
{ 
    // Performs the operations that should be accomplished for eg closing the connections, file, database
    if(myReader != null)
        myReader.Close();
}

Answer 5

像這樣做

StreamReader myReader = null;
try
        {

           myReader  = new StreamReader("c:\\j\\MyFile1.txt");
            string line = "";

            while (line != null)
            {
                line = myReader.ReadLine();
                Console.WriteLine(line);


            }

            //myReader.Close();
        }

        catch (FileNotFoundException e)
        {
            Console.WriteLine("sorry file not found! {0}", e.Message);

        }

        catch (DirectoryNotFoundException e)
        {
            Console.WriteLine("You have given the wrong path for the file! {0}", e.Message);

        }



        catch (Exception e)
        {
            Console.WriteLine("Sorry you typed in a wrong file name! {0}", e.Message);


        }

        finally
        { 
            // Performs the operations that should be accomplished for eg closing the connections, file, database
            if(myReader !=null)
            myReader.Close();

        }



        Console.ReadLine();


    }
}