System.InvalidCastException when a field is blank in the Dataset VB.Net

Question

I can’t seem to resolve this: I’ve got a dataset holding address records from an Access database. I’ve set up navigation buttons to scroll through the records (first, last, next, previous) and all is well until I hit a record that has a blank field and then I get:

System.InvalidCastException ‘Conversion from type ‘DBNull’ to type ‘String’ is not valid

I realise that I need to test the value of the dataset field and bypass it if it is empty but I can’t find the syntax to do this, can you help?

This is the Sub that is erroring:
Private Sub NavigateRecords()
txtFirstName.Text = ds.Tables(“AddressBook”).Rows(inc).Item(1)
txtLastName.Text = ds.Tables(“AddressBook”).Rows(inc).Item(2)
txtAddress1.Text = ds.Tables(“AddressBook”).Rows(inc).Item(3)
txtAddress2.Text = ds.Tables(“AddressBook”).Rows(inc).Item(4)
txtAddress3.Text = ds.Tables(“AddressBook”).Rows(inc).Item(5)
txtPostCode.Text = ds.Tables(“AddressBook”).Rows(inc).Item(6)
txtPhone.Text = ds.Tables(“AddressBook”).Rows(inc).Item(7)
txtEMail.Text = ds.Tables(“AddressBook”).Rows(inc).Item(8)
txtNotes.Text = ds.Tables("AddressBook").Rows(inc).Item(9)

End Sub

Answer 1

Hi FRANCESMACKENZIE-1253,
Thank you for posting here.

In order to detect a field is blank in the Dataset , you use the following code to do it.
For example.

If Not IsDBNull(ds.Tables("AddressBook").Rows(inc).Item(1)) Then  
    txtFirstName.Text = ds.Tables("AddressBook").Rows(inc).Item(1)  
End If  

Or

txtFirstName.Text = If(IsDbNull(ds.Tables("AddressBook").Rows(inc).Item(1)), String.Empty, ds.Tables("AddressBook").Rows(inc).Item(1).ToString)  

Another quick way to fix it : just add 'ToString()' method at the end.

txtFirstName.Text = ds.Tables("AddressBook").Rows(inc).Item(1).ToString()  

Hope them could be helpful.

Best Regards,
Xingyu Zhao

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