单声道错误？实际使用变量时出现 CS0219 警告

Question

MonoDevelop（2.10.8）正在报告：

JPGCorruptForm.cs(20,20): Warning CS0219: The variable `myStream' is assigned but its value is never used (CS0219) (JPGCorrupt)

对于此功能：

    private void toolStripButtonChooseText_Click(object sender, EventArgs e)
    {
        Stream myStream = null;
        OpenFileDialog openFileDialog = new OpenFileDialog();

        openFileDialog.InitialDirectory = ".";
        openFileDialog.Filter = "txt files (*.txt)|*.txt|All files (*.*)|*.*";
        openFileDialog.FilterIndex = 1;
        openFileDialog.RestoreDirectory = false;

        if (openFileDialog.ShowDialog() == DialogResult.OK)
        {
            Stop();

            try
            {
                if ((myStream = openFileDialog.OpenFile()) != null)
                {
                    _settings.TextFile = openFileDialog.FileName;
                    CurrentTextFile = _settings.TextFile;
                }

            }
            catch (Exception ex)
            {
                MessageBox.Show("Error: Could not read file from disk. Original error: " + ex.Message);
            }
        }
    }

这是我的第一个单声道测试项目，我不确定这种事情是否正常。这当然不是致命的，但可能会变得烦人。

Answer 1

好吧，您正在为变量分配一个值，但您实际上从未读取它。换句话说，您可以轻松删除它，只需将中间表达式更改为：

if (openFileDialog.OpenFile() != null)

请注意，您现有的代码实际上并没有从变量中读取，即使您可能认为它在与 null 进行比较时也是如此。它更像是这样的：

Stream tmp = openFileDialog.OpenFile();
myStream = tmp;
if (tmp != null)

听起来你可能应该使用它，如果没有别的办法关闭流......尽管我会尽可能晚地声明它，就像这样：

using (Stream myStream = openFileDialog.OpenFile())
{
    if (myStream != null)
    {
        _settings.TextFile = openFileDialog.FileName;
        CurrentTextFile = _settings.TextFile;
    }
}

这是一个更简单的相同问题的示例，但方式：

using System;

class Test
{
    static void Main()
    {
        string x;

        if ((x = "Hello") != null)
        {
            Console.WriteLine("Yes");
        }
    }
}

请注意，对于警告级别 4（可能还有更低的警告级别），Microsoft C# 4 编译器也会识别该问题：

Test.cs(7,16): warning CS0219: The variable 'x' is assigned but its value is
        never used