C#读取文件，返回字符串形式的文件内容

/// <summary>
/// 读取文件，返回相应字符串
/// </summary>
/// <param name="fileName">文件路径</param>
/// <returns>返回文件内容</returns>
private string ReadFile(string fileName)
{
StringBuilder str = new StringBuilder();
using (FileStream fs = File.OpenRead(fileName))
{
long left = fs.Length;
int maxLength = 100;//每次读取的最大长度
int start = 0;//起始位置
int num = 0;//已读取长度
while (left > 0)
{
byte[] buffer = new byte[maxLength];//缓存读取结果
char[] cbuffer = new char[maxLength];
fs.Position = start;//读取开始的位置
num = 0;
if (left < maxLength)
{
num = fs.Read(buffer, 0, Convert.ToInt32(left));
}
else
{
num = fs.Read(buffer, 0, maxLength);
}
if (num == 0)
{
break;
}
start += num;
left -= num;
str = str.Append(Encoding.UTF8.GetString(buffer));
}
}
return str.ToString();
}