System.IO.StreamReader myFile =
   new System.IO.StreamReader("c:\\test.txt");
string myString = myFile.ReadToEnd();

string content = System.IO.File.ReadAllText( @"C:\file.txt" );

System.IO.StreamReader myFile =
   new System.IO.StreamReader("c:\\test.txt");
string myString = myFile.ReadToEnd();

@Cris抱歉，这是微软的MSDN

方法

在这个实验中，将对两个班级进行比较。StreamReader和FileStream类将被指示从应用程序目录中读取两个10K和200K的完整文件。

StreamReader (VB.NET)

sr = New StreamReader(strFileName)
Do
  line = sr.ReadLine()
Loop Until line Is Nothing
sr.Close()

FileStream (VB.NET)

Dim fs As FileStream
Dim temp As UTF8Encoding = New UTF8Encoding(True)
Dim b(1024) As Byte
fs = File.OpenRead(strFileName)
Do While fs.Read(b, 0, b.Length) > 0
    temp.GetString(b, 0, b.Length)
Loop
fs.Close()

结果

在这个测试中，FileStream明显更快。StreamReader读取小文件需要额外50%的时间。对于大文件，它需要额外的27%的时间。

StreamReader是专门寻找换行符，而FileStream没有。这将占用一些额外的时间。

建议

根据应用程序需要对某段数据做什么，可能会有额外的解析，这将需要额外的处理时间。考虑这样一个场景，其中文件有数据列，行是CR/LF分隔的。StreamReader将沿着文本行查找CR/LF，然后应用程序将执行额外的解析以查找数据的特定位置。(你认为弦。子字符串没有价格?)

另一方面，FileStream以块的形式读取数据，积极主动的开发人员可以编写更多的逻辑来使用流。如果需要的数据位于文件中的特定位置，这当然是一种方法，因为它可以降低内存使用量。

FileStream在速度上是更好的机制，但需要更多的逻辑。

看一下File.ReadAllText()方法

一些重要备注:

This method opens a file, reads each line of the file, and then adds each line as an element of a string. It then closes the file. A line is defined as a sequence of characters followed by a carriage return ('\r'), a line feed ('\n'), or a carriage return immediately followed by a line feed. The resulting string does not contain the terminating carriage return and/or line feed. This method attempts to automatically detect the encoding of a file based on the presence of byte order marks. Encoding formats UTF-8 and UTF-32 (both big-endian and little-endian) can be detected. Use the ReadAllText(String, Encoding) method overload when reading files that might contain imported text, because unrecognized characters may not be read correctly. The file handle is guaranteed to be closed by this method, even if exceptions are raised

你也可以从文本文件中读取文本到字符串，如下所示

string str = "";
StreamReader sr = new StreamReader(Application.StartupPath + "\\Sample.txt");
while(sr.Peek() != -1)
{
  str = str + sr.ReadLine();
}