Passing input object to ConvertTo-Html

Question

Why is the output of following code blocks different? The object that ConvertTo-Html cmdlet takes as input is not the same in both cases? The output of code block 1 is what I was expecting.

Code 1:

$psdrive = Get-PSDrive
$psdrive | ConvertTo-Html

Code 2:

$psdrive = Get-PSDrive
ConvertTo-Html -InputObject $psdrive

Here the outputs.

Output 1:

 ...
     <tr>
        <th>Used</th>
        <th>Free</th>
        <th>CurrentLocation</th>
        <th>Name</th>
        <th>Provider</th>
        <th>Root</th>
        <th>Description</th>
        <th>MaximumSize</th>
        <th>Credential</th>
        <th>DisplayRoot</th>
    </tr>
    <tr>
        <td></td>
        <td></td>
        <td></td>
        <td>Alias</td>
        <td>Microsoft.PowerShell.Core\Alias</td>
        <td></td>
        <td>Drive that contains a view of the aliases stored in a session state</td>
        <td></td>
        <td>System.Management.Automation.PSCredential</td>
        <td></td>
    </tr>
    <tr>
        <td>72876965888</td>
        <td>12914372608</td>
        <td>Windows\system32</td>
        <td>C</td>
        <td>Microsoft.PowerShell.Core\FileSystem</td>
        <td>C:\</td>
        <td></td>
        <td></td>
        <td>System.Management.Automation.PSCredential</td>
        <td></td>
    </tr>
  ...

Output 2:

   ...
      <tr>
        <th>Count</th>
        <th>Length</th>
        <th>LongLength</th>
        <th>Rank</th>
        <th>SyncRoot</th>
        <th>IsReadOnly</th>
        <th>IsFixedSize</th>
        <th>IsSynchronized</th>
    </tr>
    <tr>
        <td>12</td>
        <td>12</td>
        <td>12</td>
        <td>1</td>
        <td>System.Object[]</td>
        <td>False</td>
        <td>True</td>
        <td>False</td>
    </tr>
</table>
 ...

Answer 1

That is how the pipeline behaves.

Get-PsDrive will return an array (if there is more than one drive).

$psdrive.GetType()

IsPublic IsSerial Name                                     BaseType
-------- -------- ----                                     --------
True     True     Object[]                                 System.Array

By piping $psdive (which should be called $psdrives, plural), you pass every single item of the array to ConvertTo-Html, so they will be formatted properly.

By passing the actual array as -InputObject it will convert the array itself into HTML, that's why you see the array members such as Count, Length etc. You would get the same result if you passed an empty array: ConvertTo-Html -InputObject @()

You can easily see that behavior, check this out:

# will return the members of the drives (Root, MaximumSize, ...)
$psdrive | Get-Member

# will return the members of the array that contains the drives (Add, Count, ...)
Get-Member -InputObject $psdrive

Conclusion: The first version, using the pipeline, is the correct way to do it.