protected void Button1_Click(object sender, EventArgs e)
{
this.SqlDataSource1.Insert();
string filepath = "", filename = "", mfilename, mfilepath;
if(""!=FileUpload1.PostedFile.FileName)
{
filepath = FileUpload1.PostedFile.FileName;
filename = filepath.Substring(filepath.LastIndexOf(".") + 1);
try
{
mfilepath = Server.MapPath("ima/");
mfilename = filepath.Substring(filepath.LastIndexOf("\\") + 1);
string strsql = "insert into userinfo(imagepath)values('" + mfilepath + "')";
con = new SqlConnection(strCon);
con.Open();
SqlCommand cmd = new SqlCommand(strsql, con);
cmd.ExecuteNonQuery();
con.Close();//这句怎么不执行？？？？

}
catch { Response.Write("<scrip>alert('上传文件不存在')</scrip>"); }
}

this.SqlDataSource1.Insert();这是插入一条数据，原来的下面是插入一条数据
只要把原来的插入改为更新就可以了

string filepath = "", filename = "", mfilename, mfilepath;
if ("" != FileUpload1.PostedFile.FileName)
{
filepath = FileUpload1.PostedFile.FileName;
filename = filepath.Substring(filepath.LastIndexOf(".") + 1);

try
{
mfilepath = Server.MapPath("ima/");
mfilename = filepath.Substring(filepath.LastIndexOf("\\") + 1);
string strsql = "update userinfo set imagepath='ima\\"+mfilename+"'where username='"+TextBox1.Text+"'" ;
con = new SqlConnection(strCon);
con.Open();
SqlCommand cmd = new SqlCommand(strsql, con);
cmd.ExecuteNonQuery();

con.Close();

}