NullSafe Functions: Ensuring Safe Variables

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Introduction

Values retrieved from DataSets, ViewState, Session State, or any number of other storage mechanisms can cause errors if they comes back as DBNull or Nothing. As an example, let's say you want to retrieve a String from a DataSet. Typically, you begin by checking if the value is DBNull.Value, then casting the value to a String (if you code with Option Strict On), then checking if it's Nothing, then trimming it. A similar process applies when retrieving a String from ViewState and Session State:

Dim firstName as string
' Check for DBNull
If ds.Tables(0).Rows("FirstName") is DBNull.Value Then
    firstName = String.Empty
Else
    firstName = CStr(ds.Tables(0).Rows("FirstName"))
    ' Check for Nothing
    If Not IsNothing(firstName) Then
        firstName = firstName.Trim
    End If
End If
' Now you can safely work with the value...

To overcome this cumbersome and repetitive process, I've created several "NullSafe" functions that handle the casting, trimming, and safety-checking of values.

Making Strings NullSafe

The first function is called NullSafeString. Notice that if the arg is DBNull, Nothing or String.Empty, the function defaults to returning an empty string, but you can override this behavior by passing a String of your choice as the second parameter:

'****************************************************************
' NullSafeString
'****************************************************************
Public Shared Function NullSafeString(ByVal arg As Object, _
Optional ByVal returnIfEmpty As String = "") As String

  Dim returnValue As String

  If (arg Is DBNull.Value) OrElse (arg Is Nothing) _
                   OrElse (arg Is String.Empty) Then
    returnValue = returnIfEmpty
  Else
    Try
      returnValue = CStr(arg).Trim
    Catch
      returnValue = returnIfEmpty
    End Try

  End If

  Return returnValue

End Function

Using the above function which resides in my Utility class, the code to pull FirstName from the DataSet becomes:

Dim firstName as string = _
    Utility.NullSafeString(ds.Tables(0).Rows("FirstName"), "Not Available")

This function guarantees that the value of firstName is safe to work with. In other words, method calls like firstName.Length, firstName.Replace will not throw exceptions.

Integers, Doubles, and Booleans

To ensure a value is strongly-typed upon its return from the NullSafe function, I create a new method for each data type. On most projects, I have four NullSafe methods, but on a few, I've had to extend to six to encompass Longs and Shorts. Below are my NullSafeInteger, NullSafeDouble, and NullSafeBoolean methods. You'll notice the default return value for the numeric functions is WILDCARD_ID, a constant I create with a value of -1. In the calling code, I can then check for this constant value to determine if the value is "valid."

'****************************************************************
' NullSafeInteger
'****************************************************************
Public Shared Function NullSafeInteger(ByVal arg As Object, _
  Optional ByVal returnIfEmpty As Integer = Constants.WILDCARD_ID) As Integer

  Dim returnValue As Integer

  If (arg Is DBNull.Value) OrElse (arg Is Nothing) _
                   OrElse (arg Is String.Empty) Then
    returnValue = returnIfEmpty
  Else
    Try
      returnValue = CInt(arg)
    Catch
      returnValue = returnIfEmpty
    End Try
  End If

  Return returnValue

End Function

'****************************************************************
'   NullSafeDouble
'****************************************************************
Public Shared Function NullSafeDouble(ByVal arg As Object, _
  Optional ByVal returnIfEmpty As Integer = Constants.WILDCARD_ID) As Double

  Dim returnValue As Double

  If (arg Is DBNull.Value) OrElse (arg Is Nothing) _
                   OrElse (arg Is String.Empty) Then
    returnValue = returnIfEmpty
  Else
    Try
      returnValue = CDbl(arg)
    Catch
      returnValue = returnIfEmpty
    End Try
  End If

  Return returnValue

End Function

'****************************************************************
' NullSafeBoolean
'****************************************************************
Public Shared Function NullSafeBoolean(ByVal arg As Object) As Boolean

  Dim returnValue As Boolean

  If (arg Is DBNull.Value) OrElse (arg Is Nothing) _
                   OrElse (arg Is String.Empty) Then
    returnValue = False
  Else
    Try
      returnValue = CBool(arg)
    Catch
      returnValue = False
    End Try
  End If

  Return returnValue

End Function

Conclusion

There is nothing magical about these functions; they simply encapsulate repetitive and cumbersome code into easy to use functions. Best of luck putting them to use!