It is impossible to create a Windows Forms class without loading the assembly before parsing the file containing the class

Prerequisites

• Write a descriptive title.
• Make sure you are able to repro it on the latest released version
• Search the existing issues.
• Refer to the FAQ.
• Refer to Differences between Windows PowerShell 5.1 and PowerShell.

Steps to reproduce

1. Create a simple Windows Forms class in a .ps1 :

using namespace System.Windows.Forms
Add-Type -Assembly System.Windows.Forms

class MyForm : Form {
    MyForm() {
        [Application]::EnableVisualStyles()
        
        $this.Text = 'My Form'
        $this.Size = New-Object System.Drawing.Size(300, 200)
        $this.StartPosition = 'CenterScreen'
    }
}

$myForm = [MyForm]::new()
$myForm.ShowDialog()

2. Try to run it :
   PS> .\myform.ps1

Expected behavior

Empty form opens up

Actual behavior

PS> .\myform.ps1
ParserError: C:\myform.ps1:5
Line |
   5 |  class MyForm : Form {
     |                 ~~~~
     | Unable to find type [Form].

Note that this class works correctly if i instead do this :

PS> Add-Type -Assembly System.Windows.Forms
PS> .\myform.ps1

Removing "using namespace" and specifying "System.Windows.Forms" on each type does not change anything to this behavior, nor does adding a "using assembly" statement (which is supposedly made for this if i understand the documentation about it correctly).

using assembly 'C:\Program Files\PowerShell\7\System.Windows.Forms.dll'
Add-Type -Assembly System.Windows.Forms

class MyForm : System.Windows.Forms.Form {
    MyForm() {
        [System.Windows.Forms.Application]::EnableVisualStyles()
        
        $this.Text = 'My Form'
        $this.Size = New-Object System.Drawing.Size(300, 200)
        $this.StartPosition = 'CenterScreen'
    }
}

$myForm = [MyForm]::new()
$myForm.ShowDialog()

PS> .\myform.ps1
ParserError: C:\myform.ps1:4
Line |
   4 |  class MyForm : System.Windows.Forms.Form {
     |                 ~~~~~~~~~~~~~~~~~~~~~~~~~
     | Unable to find type [System.Windows.Forms.Form].

Error details

Exception             :
    Type        : System.Management.Automation.ParseException
    Errors      :
        Extent  : Form
        ErrorId : TypeNotFound
        Message : Unable to find type [Form].

        Extent  : Application
        ErrorId : TypeNotFound
        Message : Unable to find type [Application].
    Message     : At C:\myform.ps1:5 char:16
                  + class MyForm : Form {
                  +                ~~~~
                  Unable to find type [Form].

                  At C:\myform.ps1:7 char:10
                  +         [Application]::EnableVisualStyles()
                  +          ~~~~~~~~~~~
                  Unable to find type [Application].
    ErrorRecord :
        Exception             :
            Type    : System.Management.Automation.ParentContainsErrorRecordException
            Message : At C:\myform.ps1:5 char:16
                      + class MyForm : Form {
                      +                ~~~~
                      Unable to find type [Form].

                      At C:\myform.ps1:7 char:10
                      +         [Application]::EnableVisualStyles()
                      +          ~~~~~~~~~~~
                      Unable to find type [Application].
            HResult : -2146233087
        CategoryInfo          : ParserError: (:) [], ParentContainsErrorRecordException
        FullyQualifiedErrorId : TypeNotFound
        InvocationInfo        :
            ScriptLineNumber : 5
            OffsetInLine     : 16
            HistoryId        : 2
            ScriptName       : C:\myform.ps1
            Line             : class MyForm : Form {

            Statement        : Form
            PositionMessage  : At C:\myform.ps1:5 char:16
                               + class MyForm : Form {
                               +                ~~~~
            PSScriptRoot     : C:\
            PSCommandPath    : C:\myform.ps1
            CommandOrigin    : Internal
        ScriptStackTrace      : at <ScriptBlock>, <No file>: line 1
    TargetSite  :
        Name          : Create
        DeclaringType : scriptblock
        MemberType    : Method
        Module        : System.Management.Automation.dll
    Data        : System.Collections.ListDictionaryInternal
    Source      : System.Management.Automation
    HResult     : -2146233087
    StackTrace  :
   at System.Management.Automation.ScriptBlock.Create(Parser parser, String fileName, String fileContents)
   at System.Management.Automation.ExternalScriptInfo.ParseScriptContents(Parser parser, String fileName, String
fileContents, Nullable`1 definingLanguageMode)
   at System.Management.Automation.ExternalScriptInfo.get_ScriptBlock()
   at System.Management.Automation.CommandDiscovery.CreateCommandProcessorForScript(ExternalScriptInfo scriptInfo,
ExecutionContext context, Boolean useNewScope, SessionStateInternal sessionState)
   at System.Management.Automation.CommandDiscovery.LookupCommandProcessor(CommandInfo commandInfo, CommandOrigin
commandOrigin, Nullable`1 useLocalScope, SessionStateInternal sessionState)
   at System.Management.Automation.ExecutionContext.CreateCommand(String command, Boolean dotSource)
   at System.Management.Automation.PipelineOps.AddCommand(PipelineProcessor pipe, CommandParameterInternal[]
commandElements, CommandBaseAst commandBaseAst, CommandRedirection[] redirections, ExecutionContext context)
   at System.Management.Automation.PipelineOps.InvokePipeline(Object input, Boolean ignoreInput,
CommandParameterInternal[][] pipeElements, CommandBaseAst[] pipeElementAsts, CommandRedirection[][]
commandRedirections, FunctionContext funcContext)
   at System.Management.Automation.Interpreter.ActionCallInstruction`6.Run(InterpretedFrame frame)
   at System.Management.Automation.Interpreter.EnterTryCatchFinallyInstruction.Run(InterpretedFrame frame)
CategoryInfo          : ParserError: (:) [], ParseException
FullyQualifiedErrorId : TypeNotFound
InvocationInfo        :
    ScriptLineNumber : 5
    OffsetInLine     : 16
    HistoryId        : 2
    ScriptName       : C:\myform.ps1
    Line             : class MyForm : Form {

    Statement        : Form
    PositionMessage  : At C:\myform.ps1:5 char:16
                       + class MyForm : Form {
                       +                ~~~~
    PSScriptRoot     : C:\
    PSCommandPath    : C:\myform.ps1
    CommandOrigin    : Internal
ScriptStackTrace      : at <ScriptBlock>, <No file>: line 1

Environment data

PSVersion                      7.4.1
PSEdition                      Core
GitCommitId                    7.4.1
OS                             Microsoft Windows 10.0.23619
Platform                       Win32NT
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0…}
PSRemotingProtocolVersion      2.3
SerializationVersion           1.1.0.1
WSManStackVersion              3.0

Visuals

No response

Reproduce the issue with the myform.ps1 example, then trace parsing from ScriptBlock.Create through ExternalScriptInfo.ParseScriptContents as shown in the stack trace. Inspect how using assembly, using namespace, and Add-Type affect type resolution before a class declaration; done means the script parses and opens the empty form without a prior command.