Question

c#中 对象不能从 DBNull 转换为其他类型

SqlConnection con = DB.CreateConnectionForum();
con.Open();
SqlCommand cmd = new SqlCommand("select max(themeNum) from Forumtheme", con);
int count = Convert.ToInt32(cmd.ExecuteScalar()); //提示出错
count++;
con.Close();
string stheme = this.title.Text.ToString();
string scontent = this.content.Text.ToString();
string swriter = Session["tearchID"].ToString();
string sdate = this.Date.Text.ToString();
con.Open();
string sqlstr = "insert into Forumtheme(themeNum,theme,themeContent,themeReply,themeWrite,themeDate)";
sqlstr += " values(" + count.ToString() + ",'" + stheme + "','" + scontent + "'," + "0" + ",'" + swriter + "','" + sdate + "')";
DB.OperatingData(sqlstr, con);
this.title.Text = "";
this.content.Text = "";

Answer 1

select max(themeNum) from Forumtheme
执行的结果是 NULL

也就是 要么 Forumtheme 一条记录也没有
要么 Forumtheme 表里面 themeNum 的数据都是 NULL

int count = Convert.ToInt32(cmd.ExecuteScalar()); //提示出错
count++;

修改为：
int count = 0;
Object tmpVal = cmd.ExecuteScalar()
if( tmpVal != DBNull )
count = Convert.ToInt32( tmpVal );
)
count++;