System.NullReferenceException: Object reference not set to an instance of an object [duplicate]

问题

Possible Duplicate:
System.NullReferenceException: Object reference not set to an instance of an object

look at my code

if (ViewState["Edit"].ToString() == "new") { 
}
else {
      row = _section.GetBannerEntry(ViewState["Edit"].ToString()); 
}

I was getting the error on this line if (ViewState["Edit"].ToString() == "new") I replaced with if (ViewState["Edit"] != null && ViewState["Edit"].ToString() == "new") it works.

Now I am getting the same error here in else row = _section.GetBannerEntry(ViewState["Edit"].ToString());

please help

回答1:

Your View state is null Check for

if(ViewState["Edit"]!= null)
{
 if (ViewState["Edit"].ToString() == "new")
}

回答2:

If ViewState["Edit"] hasn't been set, .ToString() will throw an exception.

Try one of these:

if (ViewState["Edit"]!= null && ViewState["Edit"].ToString() == "new") //best one
if (ViewState["Edit"] + "" == "new")

回答3:

Well, either ViewState is null or ViewState["Edit"] is returning null...

回答4:

if (ViewState["Edit"] != null && ViewState["Edit"].ToString() == "new") {

or if whats supposed to be in the viewstate is a string, this should work:

if ((string)ViewState["Edit"] == "new") {

回答5:

Try

protected void btnSaveDetails_Click(object sender, EventArgs e) 
{         
DataRow row = null;          
 if (ViewState["Edit"] != null && ViewState["Edit"].ToString() == "new") 
 {
 }
}

来源：https://stackoverflow.com/questions/6213271/system-nullreferenceexception-object-reference-not-set-to-an-instance-of-an-obj

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nullreferenceexception