Introduction

I was working on a SkyTap REST API  client implementation in PowerShell and spent quite some time understanding the HTTPWebRequest object of the .Net Framework. The HTTPWebRequest object has a generic way to modify the HTTP Request headers and also some convenient properties to directly access the most common header values like for example the request method (GET, POST, PUT, DELETE etc.).

In my PowerShell scripts I wanted to use an convenient property, if it is available and if it isn’t modify the Headers collection instead. In order to do this I needed to check whether a property exists or not. This blog post is about my version of the Test-Member cmdlet.

Figure 1: The missing cmdlet

Features

This short cmdlet demonstrates some valuable advanced functions features:

• Advanced parameter binding
• Parameter sets
• Parameter validation
• PSUnit unit tests

Test-Member

Here is the script:

function Test-Member()
{
<# 
.Synopsis 
    Verifies whether a specific property or method member exists for a given .NET object 
.Description 
    Verifies whether a specific property or method member exists for a given .NET object  
.Example
    Test-Member -PropertyName "Context" -InputObject (new-object System.Collections.ArrayList)
.Example
    new-object "System.Collections.ArrayList" | Test-Member -PropertyName "ToArray"
.Parameter PropertyName
    Name of a Property to test for 
.Parameter MethodName
    Name of a Method to test for  
.ReturnValue 
    $True or $False 
.Link 
    about_functions_advanced 
    about_functions_advanced_methods 
    about_functions_advanced_parameters 
.Notes 
NAME:      Test-Member 
AUTHOR:    Klaus Graefensteiner 
LASTEDIT:  04/20/2010 12:12:42
#Requires -Version 2.0 
#> 
    
    [CmdletBinding(DefaultParameterSetName="Properties")]
    PARAM(
        
        [ValidateNotNull()]
        [Parameter(Position=0, Mandatory=$True, ValueFromPipeline=$True)]
        $InputObject,
        
        [ValidateNotNullOrEmpty()]
        [Parameter(Position=1, Mandatory=$True, ValueFromPipeline=$False, ParameterSetName="Properties")]
        [string] $PropertyName,
        
        [ValidateNotNullOrEmpty()]
        [Parameter(Position=1, Mandatory=$True, ValueFromPipeline=$False, ParameterSetName="Methods")]
        [string] $MethodName      
    )
    Process{
        
        switch ($PsCmdlet.ParameterSetName) 
        { 
            "Properties"
            {
                $Members = Get-Member -InputObject $InputObject;
                if ($Members -ne $null -and $Members.count -gt 0)
                {
                    foreach($Member in $Members)
                    {
                        if(($Member.MemberType -eq "Property" ) -and ($Member.Name -eq $PropertyName))
                        {
                            return $true
                        }
                    }
                    return $false
                }
                else
                {
                    return $false;
                }
            }
            "Methods"
            {
                $Members = Get-Member -InputObject $InputObject;
                if ($Members -ne $null -and $Members.count -gt 0)
                {
                    foreach($Member in $Members)
                    {
                        if(($Member.MemberType -eq "Method" ) -and ($Member.Name -eq $MethodName))
                        {
                            return $true
                        }
                    }
                    return $false
                }
                else
                {
                    return $false;
                }
            }
        }
    
    }# End Process
}


function Parse-XML([string] $XMLString, [String] $XPath, [int] $Status )
{
    $XMLResult = [XML] $XMLString;
    
    if($XMLResult -ne $null)
    {
        if($Status -eq 200)
        {
            $XPathSingle = $XPath.substring(0, $XPath.Length -1)
            return $XMLResult.$XPath.$XPathSingle;
        }
        else
        {
            return $XMLResult.Error;
        }
    }
}

Unit Tests

Here are the unit tests:

. PSUnit.ps1

function Test.InputObjectFromPipelineReturnsTrueWhenLookingForToArrayMethodInArrayList()
{
    #Arrange
    #Act
    $Result = new-object "System.Collections.ArrayList" | Test-Member -MethodName "ToArray"
    
    #Assert
    Assert-That -ActualValue $Result -Constraint {$ActualValue -eq $True}
}

function Test.InputObjectFromPipelineReturnsFalseWhenLookingForToArrayPropertyInArrayList()
{
    #Arrange
    #Act
    $Result = new-object "System.Collections.ArrayList" | Test-Member -PropertyName "ToArray"
    
    #Assert
    Assert-That -ActualValue $Result -Constraint {$ActualValue -eq $False}
}

function Test.InputObjectFromPipelineReturnsTrueWhenLookingForCountPropertyInArrayList()
{
    #Arrange
    #Act
    $Result = new-object "System.Collections.ArrayList" | Test-Member -PropertyName "Count"
    
    #Assert
    Assert-That -ActualValue $Result -Constraint {$ActualValue -eq $True}
}

function Test.InputObjectFromPipelineReturnsFalseWhenLookingForCountMethodInArrayList()
{
    #Arrange
    #Act
    $Result = new-object "System.Collections.ArrayList" | Test-Member -MethodName "Count"
    
    #Assert
    Assert-That -ActualValue $Result -Constraint {$ActualValue -eq $False}
}

Download

The Cmdlet can be downloaded here: Test-Member.zip

Ausblick

That’s it.