C# WinForm 尝试读取或写入受保护的内存。这通常指示其他内存已损坏。

初学C#，先查询出datagridview中的数据，再导出为txt，重复导出同样的数据是正常的，但改变查询的数据后就出现如标题提示的错误，代码如下，请大师指点。
using (SaveFileDialog sfdTxt = new SaveFileDialog())
{
Stream txtStream = null;
sfdTxt.Filter = "txt文本文件(*.txt)|*.txt";
sfdTxt.RestoreDirectory = true;
sfdTxt.FileName = string.Format(DateTime.Now.ToString("yyyyMMdd"));
//错误时指向下面这句。
if (sfdTxt.ShowDialog() == DialogResult.OK)
{
if ((txtStream = sfdTxt.OpenFile()) != null)
{
using (StreamWriter streamWriter = new StreamWriter(@txtStream, System.Text.Encoding.Unicode))
{
for (int i = 0; i < this.dgvSale.Rows.Count; i++)
{
streamWriter.WriteLine(this.dgvSale.Rows[i].Cells[3].Value.ToString() + " " + this.dgvSale.Rows[i].Cells[8].Value.ToString() + " " + this.dgvSale.Rows[i].Cells[10].Value.ToString());
}
streamWriter.Close();
}
}
}
else
{
return;
}
sfdTxt.Dispose();
txtStream.Close();
}

using (SaveFileDialog sfdTxt = new SaveFileDialog())
                {
                    sfdTxt.Filter = "txt文本文件(*.txt)|*.txt";
                    if (DialogResult.OK==sfdTxt.ShowDialog())
                    {
                       using (StreamWriter streamWriter = new StreamWriter(sfdTxt.FileName, System.Text.Encoding.Unicode))
                            {
                                for (int i = 0; i < this.dgvSale.Rows.Count; i++)
                                {
                                    streamWriter.WriteLine(this.dgvSale.Rows[i].Cells[3].Value.ToString() + " " + this.dgvSale.Rows[i].Cells[8].Value.ToString() + " " + this.dgvSale.Rows[i].Cells[10].Value.ToString());
                                }
                                streamWriter.Close();
                            } 
                   }
                    else
                    {
                        return;
                    }
                    txtStream.Dispose();
                }

if(ofdlinsertbools.showdialog() == dialogresult.ok)

{

    string name = ofdinsertbooks.filename //全路径，包括图片名称

}

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