tricks_of_IsNullOrEmpty.md

The tricks of IsNullorEmpty

I'm usually using a shared function from string type String.IsNullOrEmpty to determined that the user input string is nothing or is a empty string. And by works with the collection type, some time this String.IsNullOrEmpty liked operation is necessary as well:

• In most of situation of my data processing, such as text data parser, some text field is not exists in the source line, so that the parser may returns a empty string collection or nothing due to the exception
• In the object deserialization operations of the Xml, YAML or JSON, collection type as property may be nothing or is a empty collection after the serialization

So that a extension method IsNullOrEmpty of collection type is required for deal with these situation.

The init edition for collection

In the early time of my programming, I'm using the IsNullOrEmpty like this:

''' <summary>
''' This object collection is a null object or contains zero count items.
''' NOTE: Do not applied this function on the Linq Expression due to the performance issue.
''' (判断某一个对象集合是否为空，请注意，由于在这里是使用了集合的Count进行判断是否有元素，
''' 所以这个函数可能不是太适合用于Linq的非立即查询)
''' </summary>
''' <typeparam name="T"></typeparam>
''' <param name="source"></param>
''' <returns></returns>
''' <remarks></remarks>
<Extension>
Public Function IsNullOrEmpty(Of T)(source As IEnumerable(Of T)) As Boolean
    Return source Is Nothing OrElse source.Count = 0
End Function

As in the early time of my programming, collection type just involve in array, list, and dictionary. And the extension function show above just works fine, as the Count extension method just iterate the elements and give back the elements number.

Things get worse on Linq

The Linq expression results of two effects: LinqAPI.Exec(of T) in VisualBasic results a data collection, and the expression of linq just results a data producer.

• When iterate on the data collection, you just enumerate the datas, so that this iteration operation is just very rapid;
• When iterate on the data producer, you are not only enumerate all of the datas, and also producing new datas as your result data source.

Imports Microsoft.VisualBasic.Linq

' data collection result
Dim source As T() =
    LinqAPI.Exec(of T) <= From x As TIn
                          In dataSource
                          Let blablabla...
                          Where blablabla
                          Let blablabla
                          Select New T With {
                              .blablabla...
                          }
' data producer result
Dim source As IEnumerable(Of T) = 
    From x As TIn
    In dataSource
    Let blablabla...
    Where blablabla
    Let blablabla
    Select New T With {
        .blablabla...
    }

And this initial version IsNullOrEmpty is still works fine on the first Linq expression as source is a data collection, and Count method just enumerate the datas, and the procedure is very fast; But things get worsening on the second Linq expression, as the second expression result a data producer, you are not only enumerate the result datas, and also needs the expression to produce new objects as your result source. So that the Count function will enumerate the entire producer source and then reutrns the element counts, and this makes the program performance very worst.

Solved with For loop

As we can using For Each loop to enumerate all of the elements in the collection, and if the collection have not object, then the For loop will not happened, so that we can using a For loop to determined that the source collection have element or not without enumerate the entired collection, Here is the second edition for the generic collection IsNullOrEmpty:

''' <summary>
''' This object collection is a null object or contains zero count items.
''' </summary>
''' <typeparam name="T"></typeparam>
''' <param name="source"></param>
''' <returns></returns>
''' <remarks></remarks>
<Extension> Public Function IsNullOrEmpty(Of T)(source As IEnumerable(Of T)) As Boolean
    If source Is Nothing Then Return True

    Dim i As Integer = -1

    For Each x As T In source
        i += 1   ' 假若是存在元素的，则i的值会为零
        Return False  ' If is not empty, then this For loop will be used.
    Next

    ' 由于没有元素，所以For循环没有进行，i变量的值没有发生变化
    If i = -1 Then ' 使用count拓展进行判断或导致Linq被执行两次，现在使用FirstOrDefault来判断，主需要查看第一个元素而不是便利整个Linq查询枚举，从而提高了效率
        Return True  ' Due to the reason of source is empty, no elements, so that i value is not changed as the For loop didn't used.
    Else
        Return False
    End If

    Return False
End Function

Now using IsNullOrEmpty just works happy again with all of the enumeration collection type, both data collection and data producer works well with this extension method:

As at the first of the function, we just detecting that target is nothing or not? And then we using a For loop just needs iterate the first element in the source, instead of using Count method to enumerate all of the elements, so that this is works perfect for the two linq expression type. In the For loop, if the source collection is empty, then the For loop will never happend, and the value of variable i will not changed. So that we can determined that in this situation the source collection is Empty.

And here I have add some extensions method for the common specific cllection types, for the code performance improvements.

    <Extension> Public Function IsNullOrEmpty(Of TKey, TValue)(dict As Dictionary(Of TKey, TValue)) As Boolean
        If dict Is Nothing Then
            Return True
        End If
        Return dict.Count = 0
    End Function

    <Extension> Public Function IsNullOrEmpty(Of T)(queue As Queue(Of T)) As Boolean
        If queue Is Nothing Then
            Return True
        End If
        Return queue.Count = 0
    End Function

    <Extension> Public Function IsNullOrEmpty(Of T)(list As List(Of T)) As Boolean
        If list Is Nothing Then
            Return True
        End If
        Return list.Count = 0
    End Function

    ''' <summary>
    ''' This object array is a null object or contains zero count items.(判断某一个对象数组是否为空)
    ''' </summary>
    ''' <typeparam name="T"></typeparam>
    ''' <returns></returns>
    ''' <remarks></remarks>
    <Extension> Public Function IsNullOrEmpty(Of T)(array As T()) As Boolean
        Return array Is Nothing OrElse array.Length = 0
    End Function