VB.NET抛出“对象引用未设置为对象的实例”异常

Question

Why do I get this Exception?

Object reference not set to an instance of an object.

With:

wb.Document.GetElementById("formfieldv1").InnerText = "some value"

Where wb is the name of the WebBrowser control.

Here is all the code:

Private Sub btnSend_Click(sender As System.Object, e As System.EventArgs) Handles btnSend.Click
Dim strFrom As String = txtFrom.Text
Dim strTo As String = txtTo.Text
Dim strMsg As String = txtMsg.Text
wb.Document.GetElementById("formfieldv1").InnerText = strFrom ' strFrom fills fine
End Sub

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Update

As suggested in comments I modified code like this:

  Dim doc = wb.Document

  If (doc Is Nothing) Then
     MsgBox("doc is nothing")
  End If


  Dim el = wb.Document.GetElementById("formfieldv1")

  If (el Is Nothing) Then
     MsgBox("el is nothing")
  Else
     el.InnerText = strFrom
  End If

With that I am getting el is nothing . How do I solve this now ?

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Alternatively if you guys can help me out with this that will solve my problem too:

How to fill html form using web browser control

Answer 1

I think this is a great example of why it's good to break down operations into several lines, instead of trying to do many operations in one line, especially when null values can be returned.

If you take wb.Document.GetElementById("formfieldv1").InnerText = "some value"

and break it down into

var document = wb.Document;
var element = document.GetElementById("formfieldv1");
element.InnerText = "some value";

When the exception is thrown, it will be much more apparent what is failing. It is also easier to inspect the result of each operation when stepping through code. From a compilation standpoint, it will make no difference, it will ultimately be compiled down to the same IL.

I think there tends to be a natural desire to do as much as possible in a single line of code, but I think in many cases it hurts readability and debug-ability.