如何：从文件读取文本

更新：2007 年 11 月

下面的代码示例演示如何从文本文件中读取文本。第二个示例在检测到文件结尾时向您发出通知。通过使用 ReadAllLines 或 ReadAllText 方法也可以实现此功能。

示例

Imports System
Imports System.IO

Class Test
    Public Shared Sub Main()
        Try
            ' Create an instance of StreamReader to read from a file.
            Using sr As StreamReader = New StreamReader("TestFile.txt")
                Dim line As String
                ' Read and display the lines from the file until the end 
                ' of the file is reached.
                Do
                    line = sr.ReadLine()
                    Console.WriteLine(Line)
                Loop Until line Is Nothing
                sr.Close()
            End Using
        Catch E As Exception
            ' Let the user know what went wrong.
            Console.WriteLine("The file could not be read:")
            Console.WriteLine(E.Message)
        End Try
    End Sub
End Class

using System;
using System.IO;

class Test 
{
    public static void Main() 
    {
        try 
        {
            // Create an instance of StreamReader to read from a file.
            // The using statement also closes the StreamReader.
            using (StreamReader sr = new StreamReader("TestFile.txt")) 
            {
                String line;
                // Read and display lines from the file until the end of 
                // the file is reached.
                while ((line = sr.ReadLine()) != null) 
                {
                    Console.WriteLine(line);
                }
            }
        }
        catch (Exception e) 
        {
            // Let the user know what went wrong.
            Console.WriteLine("The file could not be read:");
            Console.WriteLine(e.Message);
        }
    }
}

Option Explicit On 
Option Strict On
Imports System
Imports System.IO
Public Class TextFromFile
    Private Const FILE_NAME As String = "MyFile.txt"
    Public Shared Sub Main()
        If Not File.Exists(FILE_NAME) Then
            Console.WriteLine("{0} does not exist.", FILE_NAME)
            Return
        End If
        Using sr As StreamReader = File.OpenText(FILE_NAME)
            Dim input As String
            input = sr.ReadLine()
            While Not input Is Nothing
                Console.WriteLine(input)
                input = sr.ReadLine()
            End While
            Console.WriteLine("The end of the stream has been reached.")
            sr.Close()
        End Using
    End Sub
End Class

using System;
using System.IO;
public class TextFromFile 
{
    private const string FILE_NAME = "MyFile.txt";
    public static void Main(String[] args) 
    {
        if (!File.Exists(FILE_NAME)) 
        {
            Console.WriteLine("{0} does not exist.", FILE_NAME);
            return;
        }
        using (StreamReader sr = File.OpenText(FILE_NAME))
        {
            String input;
            while ((input=sr.ReadLine())!=null) 
            {
                Console.WriteLine(input);
            }
            Console.WriteLine ("The end of the stream has been reached.");
            sr.Close();
        }
    }

可靠编程

此代码通过调用 File.OpenText 创建一个指向 MyFile.txt 的 StreamReader。StreamReader.ReadLine 将每行作为一个字符串返回。当不再有要读取的字符时，会有一条消息显示该情况，然后流关闭。

说明：
Visual Basic 用户可以选择使用由 My.Computer.FileSystem 对象提供的方法和属性进行文件 I/O。有关更多信息，请参见 My.Computer.FileSystem 对象。

请参见

任务

如何：创建目录列表

如何：对新建的数据文件进行读取和写入

如何：打开并追加到日志文件

如何：向文件写入文本

如何：从字符串中读取字符

如何：向字符串写入字符

概念

基本的文件 I/O

参考

StreamReader

File.OpenText

StreamReader.ReadLine