How to check if DataReader value is not null?

Question

I\'m writing a VB.Net code that reads an Oracle Table through an SQL query.

The SQL query may return some null columns. I\'m trying to check if these columns are nul

Answer 1

You need to check if the field is null before you convert the value to date.

If (Reader.IsDBNull(0)) Then
    'Null: Do not call GetDateTime
End If

If (Not Reader.IsDBNull(0)) Then
    'Not null: Retrieve the datetime.
    Dim dt As DateTime = Reader.GetDateTime(0)
End If

Answer 2

Using Generic function with extension, will make it easier.

Imports System.Runtime.CompilerServices

<Extension()>
Public Module DataReaderExtensions
  Public Function GetValue(Of T)(ByVal drVar As Object) As T
    If drVar.Equals(DBNull.Value) Then
        ' Value is null, determine the return type for a default
        If GetType(T).Equals(GetType(String)) Then
            Return CType(CType("", Object), T)
        Else
            ' If it's anything else just return nothing
            Return CType(Nothing, T)
        End If
    Else
        ' Cast the value into the correct return type
        Return CType(drVar, T)
    End If
  End Function
End Module

And you can call it like

dr.Item("abc").GetValue(string)
dr.Item("def").GetValue(Nullable(of Date))

Answer 3

Nothing means an object has not been initialized, DBNull means the data is not defined/missing. There are several ways to check:

' The VB Function
If IsDBNull(Reader.Item(0)) Then...

The GetDateTime method is problematic because you are asking it to convert a non value to DateTime. Item() returns Object which can be tested easily before converting.

 ' System Type
 If System.DBNull.Value.Equals(...)

You can also the DbReader. This only works with the ordinal index, not a column name:

If myReader.IsDbNull(index) Then 

Based on that, you can put together functions either as Shared class members or reworked into Extensions to test for DBNull and return a default value:

Public Class SafeConvert
    Public Shared Function ToInt32(Value As Object) As Integer
        If DBNull.Value.Equals(Value) Then
            Return 0
        Else
            Return Convert.ToInt32(Value)
        End If
    End Function

    Public Shared Function ToInt64(Value As Object) As Int64
        If DBNull.Value.Equals(Value) Then
            Return 0
        Else
            Return Convert.ToInt64(Value)
        End If
    End Function

    ' etc
End Class

Usage:

myDate = SafeConvert.ToDateTime(Reader.Item(0))

For a DateTime converter, you'd have to decide what to return. I prefer to do those individually.