Serialisation failure for overloaded properties

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In the following example, an error happens.

```powershell
$Google = Invoke-WebRequest 'google.com' 
$Google | ConvertTo-Json
```

> ConvertTo-Json: An item with the same key has already been added. Key: Content

Why?

On GitHub, [PowerShell/issues/11910](https://github.com/PowerShell/PowerShell/issues/11910) shows us the same error message, but doesn't currently contain a verbose explanation. 

Briefly: an object may inherit properties from a parent type, but if it attempts to overload a property that exists on the parent, then it cannot be seamlessly serialised by the ConvertTo-Json cmdlet.

Top Answer example

PeterVandivier

[The output type of Invoke-WebRequest][Invoke-WebRequest#output] is the child class [BasicHtmlWebResponseObject] which inherits from parent class [WebResponseObject]. A close comparison of those docs pages will reveal that many properties are inherited from parent to child, but that `Content` is _not_ inherited; instead - it is declared a second time. Not only this, but a deeper examination of the parent type will reveal that it is declared as a _different type_ in the child class than that of the same-named property in the parent type. Compare: 

[WebResponseObject.Content]

```cs
public byte[] Content { get; protected set; }
```

[BasicHtmlWebResponseObject.Content]

```cs
public new string Content { get; private set; }
```

This is notable, because if we take the example from [issues/11910] and modify it so that the overloaded Property is of the same type...

```diff
$type = @"
         public class B
         {
             public int A { get; set; }
         }
         public class C : B
         {
-             public new string A { get; set; }
+             public new int A { get; set; }
         }
"@
```

...then serialisation of on object of class `C` does not error.

```powershell
Add-Type $type
$obj = New-Object C
$obj | ConvertTo-Json
```

[Invoke-WebRequest#output]: https://docs.microsoft.com/en-us/powershell/module/microsoft.powershell.utility/invoke-webrequest#outputs
[BasicHtmlWebResponseObject]: https://docs.microsoft.com/en-us/dotnet/api/microsoft.powershell.commands.basichtmlwebresponseobject
[WebResponseObject]: https://docs.microsoft.com/en-us/dotnet/api/microsoft.powershell.commands.webresponseobject
[issues/11910]: https://github.com/PowerShell/PowerShell/issues/11910 

[WebResponseObject.Content]: https://github.com/PowerShell/PowerShell/blob/c146ff7/src/Microsoft.PowerShell.Commands.Utility/commands/utility/WebCmdlet/Common/WebResponseObject.Common.cs#L24
[BasicHtmlWebResponseObject.Content]: https://github.com/PowerShell/PowerShell/blob/c146ff7/src/Microsoft.PowerShell.Commands.Utility/commands/utility/WebCmdlet/Common/BasicHtmlWebResponseObject.Common.cs#L66

Answer #2 example

PeterVandivier

A trivial workaround exists: you can pipe the object through `Select *` which clobbers the strict typing. 

```powershell
PS> $Google.GetType()

IsPublic IsSerial Name                                     BaseType
-------- -------- ----                                     --------
True     False    BasicHtmlWebResponseObject               Microsoft.PowerShell.Commands.WebResponseObject

PS> ($Google | select *).GetType()

IsPublic IsSerial Name                                     BaseType
-------- -------- ----                                     --------
True     False    PSCustomObject                           System.Object

PS>
```

Once the object is freed of the constraints of its inheritence, then you may serialise it without error.

```powershell
($Google | select *) | ConvertTo-Json
```

---

HT to [@chrisdent on powershell slack](https://powershell.slack.com/archives/C1RCWRDL4/p1606739422257800) for the assist :-)

2 Answers