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!