C# 我用下面的方法读取文本文件显示到textbox里 怎么显示是乱码啊

textBox1.Clear();
if (ofdialoginput.ShowDialog() == DialogResult.OK)
{
using (FileStream filestream = File.OpenRead(ofdialoginput.FileName))
{
using (StreamReader streamreader=new StreamReader(filestream))
{
string line = null;
while ((line = streamreader.ReadLine()) != null)
{
textBox1.Text += line.ToString();
line = streamreader.ReadLine();

}
}