DBNull filtering with LINQ

Question

Why NULLdoes the following query cause an error below for a string with a value for barrel when I explicitly filter these strings in a sentence Where?

Dim query = From row As dbDataSet.conformalRow In dbDataSet.Tables("conformal") _
            Where Not IsDBNull(row.Cal) AndAlso tiCal_drop.Text = row.Cal _
            AndAlso Not IsDBNull(row.Tran) AndAlso tiTrans_drop.Text = row.Tran _
            AndAlso Not IsDBNull(row.barrel) _
            Select row.barrel
If query.Count() > 0 Then tiBarrel_txt.Text = query(0)

Run-time exception thrown : System.Data.StrongTypingException - The value for column 'barrel' in table 'conformal' is DBNull.

How should my request / condition be rewritten to work as I expected?

Answer 1

By default, in strongly typed datasets, properties throw this exception if the field is NULL. You need to use the generated method Is[Field]Null:

Dim query = From row As dbDataSet.conformalRow In dbDataSet.Tables("conformal") _
            Where Not row.IsCalNull() AndAlso tiCal_drop.Text = row.Cal _
            AndAlso Not row.IsTranNull() AndAlso tiTrans_drop.Text = row.Tran _
            AndAlso Not row.IsbarrelNull() _
            Select row.barrel
If query.Count() > 0 Then tiBarrel_txt.Text = query(0)

Or the DataRow.IsNull method:

Dim query = From row As dbDataSet.conformalRow In dbDataSet.Tables("conformal") _
            Where Not row.IsNull("Cal") AndAlso tiCal_drop.Text = row.Cal _
            AndAlso Not row.IsNull("Tran") AndAlso tiTrans_drop.Text = row.Tran _
            AndAlso Not row.IsNull("barrel") _
            Select row.barrel
If query.Count() > 0 Then tiBarrel_txt.Text = query(0)

Answer 2

It worked for me.

Dim query = From row As dbDataSet.conformalRow
        In dbDataSet.Tables("conformal") _ 
        Where row.Cal.Length > 0 AndAlso tiCal_drop.Text = row.Cal _ 
            AndAlso row.Tran.Length > 0 AndAlso tiTrans_drop.Text = row.Tran _ 
            AndAlso row.barrel.Length > 0 _ 
        Select row.barrel