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NullSafe Functions: Ensuring Safe Variables

, 7 Nov 2004
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In this article Rob Walling offers some functions that ensure variables returned from DataSets, ViewState, Session State, or Functions won't break your code.

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!

License

This article has no explicit license attached to it but may contain usage terms in the article text or the download files themselves. If in doubt please contact the author via the discussion board below.

A list of licenses authors might use can be found here

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rwalling

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Comments and Discussions

 
GeneralThis can be much simpler Pin
Sébastien Lorion7-Nov-04 20:27
memberSébastien Lorion7-Nov-04 20:27 
GeneralAlso ... Pin
Sébastien Lorion7-Nov-04 20:40
memberSébastien Lorion7-Nov-04 20:40 
GeneralRe: This can be much simpler Pin
rwalling9-Nov-04 20:13
memberrwalling9-Nov-04 20:13 
GeneralRe: This can be much simpler Pin
Sébastien Lorion9-Nov-04 23:25
memberSébastien Lorion9-Nov-04 23:25 
GeneralRe: This can be much simpler Pin
rwalling11-Nov-04 19:01
memberrwalling11-Nov-04 19:01 
GeneralRe: This can be much simpler Pin
Anonymous2-Feb-05 15:31
sussAnonymous2-Feb-05 15:31 
GeneralRe: This can be much simpler Pin
Anonymous2-Feb-05 19:08
sussAnonymous2-Feb-05 19:08 
GeneralRe: This can be much simpler Pin
robhughadams2-Nov-07 6:24
memberrobhughadams2-Nov-07 6:24 
Sadly point 1 isn't correct:
Sébastien Lorion wrote:
1.

To call the value type version of NVL, the internal type of the value argument must be a value type (ie the boxed type) AND the type of the emptyValue argument must also be a value type.


Dim o As Object

o = 1
NVL(o, 1) 'Call to value type version

o = "asdf"
NVL(o, 1) 'Call to object version

NVL(DBNull.Value, 1) 'Call to value type version
NVL(DBNull.Value, "asdf") 'Call to object value


The overload resolution is done at compile type and uses the type as declared, not the type of the object that is currently assigned to that variable.

You can see this if you change the test function to:
    Private Sub TestNVL()
        Dim o As Object = DBNull.Value
        Debug.Write("NVL values :     ")
        Debug.Write(NVL(o, "test1"))
        Debug.Write(", ")
        Debug.Write(NVL(o, 1))
        Debug.Write(", ")
        o = Nothing
        Debug.Write(NVL(o, "test2"))
        Debug.Write(", ")
        o = "val"
        Debug.Write(NVL(o, "test3"))
        Debug.Write(", ")
        o = 1234
        Debug.Write(NVL(o, 3))
        Debug.Write(", ")
        o = New Decimal(123.345)
        Debug.WriteLine(NVL(o, 4))
        Debug.WriteLine("Correct values : test1, 1, test2, val, 1234, 123.345")
    End Sub
Which produces this instead of the expected values:
NVL values :     , , , val, 1234, 123.345
More importantly if you attempt to cast the result instead of calling .ToString on it (which Debug.WriteLine does) an exception will be raised as the following proves:
    Private Sub TestNVL()
        Dim o As Object = DBNull.Value
        Debug.Write("NVL values :     ")
        Dim s As String = DirectCast(NVL(o, "test"), String)
        Debug.Write(NVL(s, "test"))
        Debug.WriteLine("Correct value : test")
    End Sub
So the NVL function will only work as expected if the input values have already been cast to the correct type - which defeats its purpose!

GeneralRe: This can be much simpler Pin
Matthew D3-Feb-05 8:06
memberMatthew D3-Feb-05 8:06 
GeneralRe: This can be much simpler Pin
Sébastien Lorion3-Feb-05 8:28
memberSébastien Lorion3-Feb-05 8:28 
GeneralRe: This can be much simpler Pin
Matthew D3-Feb-05 8:54
memberMatthew D3-Feb-05 8:54 
GeneralRe: This can be much simpler Pin
Sébastien Lorion3-Feb-05 9:10
memberSébastien Lorion3-Feb-05 9:10 
GeneralRe: This can be much simpler Pin
Matthew D3-Feb-05 9:20
memberMatthew D3-Feb-05 9:20 
GeneralRe: This can be much simpler Pin
Sébastien Lorion3-Feb-05 9:51
memberSébastien Lorion3-Feb-05 9:51 

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