Custom CodeDomSerializer causes casting exception during serialization in Designer

[Jonasmarien]

Environment

Version 17.9.7

.NET version

.NET 8

Did this work in a previous version of Visual Studio and/or previous .NET release?

No response

Issue description

When I create a custom CodeDomSerializer and specify it on a Component using the DesignerSerializerAttribute, a casting exception occurs whenever serialization is triggered in the Visual Studio Designer.

Project file

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>WinExe</OutputType>
    <TargetFramework>net8.0-windows</TargetFramework>
    <Nullable>enable</Nullable>
    <UseWindowsForms>true</UseWindowsForms>
    <ImplicitUsings>enable</ImplicitUsings>
  </PropertyGroup>

</Project>

Steps to reproduce

1. Create a new WinForms project targeting .NET 8

2. Add a WinForms UserControl

3. Create a new MyComponent class which inherits System.ComponentModel.Component

   internal class MyComponent : Component
   {
   }

4. Create a new MySerializer class which inherits System.ComponentModel.Design.Serialization.CodeDomSerializer

   
   using System.ComponentModel.Design.Serialization;

public class MySerializer : CodeDomSerializer { }

5. Explicitly assign MySerializer to MyComponent using the `DesignerSerializerAttribute`

using System.ComponentModel; using System.ComponentModel.Design.Serialization;

[DesignerSerializer(typeof(MySerializer), typeof(CodeDomSerializer))] internal class MyComponent : Component { }

6.  Right-click MyControl and select 'View Designer'
![image](https://github.com/dotnet/winforms/assets/14920486/aea1c34b-5278-4e4b-a278-eb3c63bbec7f)

7. Open up Toolbox and drag MyComponent into the Designer view

**Note:** Here's a pre-made solution to start with. Simply start from step 6.
[https://github.com/Jonasmarien/winforms-codedomserializer-issue-reproduction](https://github.com/Jonasmarien/winforms-codedomserializer-issue-reproduction)

### Diagnostics

```text
[09:14:36.547921] fail: Request failures: Sessions/GetUpdatedRootComponentCodeDom.
                        Microsoft.DotNet.DesignTools.Client.DesignToolsServerException: Unable to cast object of type 'WInForms.CustomSerializer.Project.MySerializer' to type 'Microsoft.DotNet.DesignTools.Serialization.CodeDomSerializer'.

                        For information on how to troubleshoot the designer refer to the guide at https://aka.ms/winforms/designer/troubleshooting.

[elachlan]

@Olina-Zhang Can your team please test this?

[Olina-Zhang]

@Jonasmarien this issue can be resolved using Nuget: Microsoft.WinForms.Designer.SDK, and need to refactor the namespaces from System.ComponentModel.Design to Microsoft.DotNet.DesignTools.Designers, referring to blog: https://devblogs.microsoft.com/dotnet/custom-controls-for-winforms-out-of-process-designer/

Here is my updated app based on your steps: WinFormsApp22.zip

Video:

https://github.com/dotnet/winforms/assets/26474449/5e73df26-71fa-4ead-bbcc-7b3b4ae7fc2f

[Jonasmarien]

@Olina-Zhang Using the SDK does resolve the casting issue but causes another exception if I add the following to the serializer:

public class MySerializer : CodeDomSerializer
{
    public override object Serialize(IDesignerSerializationManager manager, object value)
    {
        SerializeToExpression(manager, value);
        return null;
    }
}

Same happens when I try to call the GetExpression method, even though both are defined in CodeDomSerializerBase.

Any idea why this exception is thrown?

[Olina-Zhang]

@Jonasmarien What about using component item in Winforms app directly?

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