当Checkbox值为null时。 程序抛出此错误“对象不能从DBNull强制转换为其他类型。”

Question

When the checkbox value in database is null it throws error. When it is 0 or 1 it works as expected.

if (Request.QueryString["Sckey"] != null && Request.QueryString["Sckey"].ToString() != "")
{
    string str_sql1 = "select ScenarioSeq,ScenarioName,FileName,ScenarioNotes,IsUpperCase from Scenario where ScenarioKey=@ScenarioKey;";
    SqlCommand cm = new SqlCommand(str_sql1);
    cm.Parameters.AddWithValue("@ScenarioKey", str_ScKey);
    DataSet ds2 = this.DA.GetDataSet(cm);

    DataRow dr = ds2.Tables[0].Rows[0];
    txt_ScenarioName.Text = dr["ScenarioName"].ToString();
    txt_ScenarioNotes.Text = dr["ScenarioNotes"].ToString();
    ch_Uppercase.Checked = Convert.ToBoolean(dr["IsUpperCase"]);     
}

Answer 1

Null in the terms of RDBMS has a special meaning , it stands for unknown , invalid , can't be applied to etc., and that's why can't be cast (what bool value ( true , false ) should be used for unknown ?). Try dr.IsNull("IsUpperCase") :

 ch_Uppercase.Checked = !dr.IsNull("IsUpperCase") && Convert.ToBoolean(dr["IsUpperCase"]);

Answer 2

Do something like this..

ch_Uppercase.Checked = 
     Convert.ToBoolean(dr["IsUpperCase"] == DBNull.Value? 0: dr["IsUpperCase"]);

Answer 3

int number;
bool result = Int32.TryParse(dr["IsUpperCase"], out number);
ch_Uppercase.Checked = number;

Answer 4

像这样更正它。

ch_Uppercase.Checked = !dr.IsNull ? Convert.ToBoolean(dr["IsUpperCase"]) : false;