[InvalidCastException: Unable to cast object of type 'System.DBNull' to type 'System.String'.] [duplicate]

问题

This question already has answers here:

Unable to cast object of type 'System.DBNull' to type 'System.String` (11 answers)

Closed 5 years ago.

The error line:

Source Error:

Line 37:         while (rdr.Read()==true)
Line 38:         {
Line 39:             if (TextBoxUserName.Text == (string)rdr["CUserName"]) 
Line 40:             {
Line 41:                 ClientScript.RegisterStartupScript(csType,"Error",scriptErrorUserId);

It pops up when I tried to register an account.

I use microsoft access as database.

Database - CUserName Session - sUserName and there's @eUserName

any idea guys?

回答1:

Replace this

if (TextBoxUserName.Text == (string)rdr["CUserName"])

with

if (TextBoxUserName.Text == rdr["CUserName"].ToString())

回答2:

You need to check if rdr["CUserName"] equals to System.DBNull.Value first before converting it to string. Change this:

if (TextBoxUserName.Text == (string)rdr["CUserName"])

to this:

string userName = rdr["CUserName"] != System.DBNull.Value ? (string)rdr["CUserName"] : string.Empty;
if (TextBoxUserName.Text == userName)

来源：https://stackoverflow.com/questions/21216641/invalidcastexception-unable-to-cast-object-of-type-system-dbnull-to-type-sy