CS1069 Compilation Error: Cannot find type 'Match' in 'System.Text.RegularExpressions' using C# expressions in XAML

[xdnuos]

Summary
When you build a workflow using CoreWF and use C# code inside it (like for variables or loops), if that C# code needs the Match type (which comes from Regular Expressions/Regex), the workflow fails to compile and run on .NET 8.

The Error You See:
You get a System.InvalidOperationException, and inside that, many CS1069 errors saying something like: "The type name 'Match' could not be found in the namespace 'System.Text.RegularExpressions'". The error also hints that the Match type has been "forwarded" (moved) to a different library called System.Text.RegularExpressions.dll in .NET 8.

Environment:

• CoreWF version: 6.0.3
• .NET version: 8.0
• OS: Windows 11
• Context: Using CoreWF within a Visual studio community 2022

Steps to Reproduce:

1. Create a custom NativeActivity using C# that returns IEnumerable<System.Text.RegularExpressions.Match> (see Matches code below).
2. Create a XAML workflow using C# expressions.
3. Define the System.Text.RegularExpressions namespace in XAML pointing to assembly=System (as shown in the sample XAML).
4. Declare a variable or argument of type IEnumerablestr:Match.
5. Use the custom activity and assign its output to that variable.
6. Use a ForEachstr:Match loop to iterate over the results.
7. Attempt to load or run the workflow.

Custom Activity Code (Matches.cs):

using System;
using System.Activities;
using System.Collections.Generic;
using System.ComponentModel;
using System.Text.RegularExpressions;

public class Matches : NativeActivity
{
    [RequiredArgument]
    public InArgument<string> Input { get; set; }

    [RequiredArgument]
    public InArgument<string> Pattern { get; set; }

    [Category("RegexOptions")]
    public bool Compiled { get; set; } = true;

    [Category("RegexOptions")]
    public bool IgnoreCase { get; set; } = true;

    public OutArgument<IEnumerable<Match>> Results { get; set; }

    protected override void Execute(NativeActivityContext context)
    {
        var options = RegexOptions.None;
        if (Compiled) options |= RegexOptions.Compiled;
        if (IgnoreCase) options |= RegexOptions.IgnoreCase;

        var result = new List<Match>();
        // Regex.Matches comes from System.Text.RegularExpressions assembly
        var ma = Regex.Matches(Input.Get(context), Pattern.Get(context), options);
        foreach (Match m in ma) result.Add(m);
        Results.Set(context, result);
    }
}

Workflow XAML (Workflow.xaml):

<Activity mc:Ignorable="sap sap2010 sads" x:Class="WorkflowTest.Workflow1"
 xmlns="http://schemas.microsoft.com/netfx/2009/xaml/activities"
 xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
 xmlns:mca="clr-namespace:Microsoft.CSharp.Activities;assembly=System.Activities"
 xmlns:sads="http://schemas.microsoft.com/netfx/2010/xaml/activities/debugger"
 xmlns:sap="http://schemas.microsoft.com/netfx/2009/xaml/activities/presentation"
 xmlns:sap2010="http://schemas.microsoft.com/netfx/2010/xaml/activities/presentation"
 xmlns:scg="clr-namespace:System.Collections.Generic;assembly=mscorlib"
 xmlns:sco="clr-namespace:System.Collections.ObjectModel;assembly=mscorlib"
 xmlns:str="clr-namespace:System.Text.RegularExpressions;assembly=System"
 xmlns:u="clr-namespace:UtilitiesActivity;assembly=UtilitiesActivity"
 xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
  <x:Members>
    <x:Property Name="res" Type="OutArgument(scg:IEnumerable(str:Match))" />
  </x:Members>
  <sap2010:ExpressionActivityEditor.ExpressionActivityEditor>C#</sap2010:ExpressionActivityEditor.ExpressionActivityEditor>
  <sap2010:WorkflowViewState.IdRef>WorkflowTest.Workflow1_1</sap2010:WorkflowViewState.IdRef>
  <TextExpression.NamespacesForImplementation>
    <sco:Collection x:TypeArguments="x:String">
      <x:String>System</x:String>
      <x:String>System.Collections.Generic</x:String>
      <x:String>System.Data</x:String>
      <x:String>System.Linq</x:String>
      <x:String>System.Text</x:String>
    </sco:Collection>
  </TextExpression.NamespacesForImplementation>
  <TextExpression.ReferencesForImplementation>
    <sco:Collection x:TypeArguments="AssemblyReference">
      <AssemblyReference>AFusion.Rpa.Activity.Http</AssemblyReference>
      <AssemblyReference>Microsoft.CSharp</AssemblyReference>
      <AssemblyReference>System</AssemblyReference>
      <AssemblyReference>System.Activities</AssemblyReference>
      <AssemblyReference>System.Configuration</AssemblyReference>
      <AssemblyReference>System.Core</AssemblyReference>
      <AssemblyReference>System.Data</AssemblyReference>
      <AssemblyReference>System.Runtime.Serialization</AssemblyReference>
      <AssemblyReference>System.ServiceModel</AssemblyReference>
      <AssemblyReference>System.ServiceModel.Activities</AssemblyReference>
      <AssemblyReference>System.Xaml</AssemblyReference>
      <AssemblyReference>System.Xml</AssemblyReference>
      <AssemblyReference>System.Xml.Linq</AssemblyReference>
      <AssemblyReference>WindowsBase</AssemblyReference>
      <AssemblyReference>PioneerActivity</AssemblyReference>
      <AssemblyReference>UtilitiesActivity</AssemblyReference>
      <AssemblyReference>mscorlib</AssemblyReference>
      <AssemblyReference>System.Core</AssemblyReference>
      <AssemblyReference>WorkflowTest</AssemblyReference>
    </sco:Collection>
  </TextExpression.ReferencesForImplementation>
  <Sequence sap2010:WorkflowViewState.IdRef="Sequence_1">
    <Sequence.Variables>
      <Variable x:TypeArguments="x:String" Name="export" />
    </Sequence.Variables>
    <u:Matches Compiled="True" sap2010:WorkflowViewState.IdRef="Matches_3" IgnoreCase="True" Input="hello" Pattern="%[a-z]">
      <u:Matches.Results>
        <OutArgument x:TypeArguments="scg:IEnumerable(str:Match)">
          <mca:CSharpReference x:TypeArguments="scg:IEnumerable(str:Match)">res</mca:CSharpReference>
        </OutArgument>
      </u:Matches.Results>
    </u:Matches>
    <ForEach x:TypeArguments="str:Match" DisplayName="ForEach&lt;Match&gt;" sap2010:WorkflowViewState.IdRef="ForEach`1_2">
      <ForEach.Values>
        <InArgument x:TypeArguments="scg:IEnumerable(str:Match)">
          <mca:CSharpValue x:TypeArguments="scg:IEnumerable(str:Match)">res</mca:CSharpValue>
        </InArgument>
      </ForEach.Values>
      <ActivityAction x:TypeArguments="str:Match">
        <ActivityAction.Argument>
          <DelegateInArgument x:TypeArguments="str:Match" Name="item" />
        </ActivityAction.Argument>
        <WriteLine sap2010:WorkflowViewState.IdRef="WriteLine_1">
          <InArgument x:TypeArguments="x:String">
            <mca:CSharpValue x:TypeArguments="x:String">item.Value</mca:CSharpValue>
          </InArgument>
        </WriteLine>
      </ActivityAction>
    </ForEach>
    <Delay Duration="00:00:10" sap2010:WorkflowViewState.IdRef="Delay_1" />
    <sads:DebugSymbol.Symbol>d0VDOlxVc2Vyc1xodW5naG5cRG9jdW1lbnRzXFJQQVxXb3JrZmxvd1Rlc3RcV29ya2Zsb3dUZXN0XFdvcmtmbG93LnhhbWwLMgNQDgIBATYFPBECAQ09BU0PAgEETgVOTAIBAjZkNmsCARI5CzlmAgEPNnQ2fAIBDkALQF4CAQpHCUsVAgEFThVOHwIBA0kNSVUCAQY=</sads:DebugSymbol.Symbol>
  </Sequence>
  <sap2010:WorkflowViewState.ViewStateManager>
    <sap2010:ViewStateManager>
      <sap2010:ViewStateData Id="Matches_3" sap:VirtualizedContainerService.HintSize="287,22" />
      <sap2010:ViewStateData Id="WriteLine_1" sap:VirtualizedContainerService.HintSize="210,62" />
      <sap2010:ViewStateData Id="ForEach`1_2" sap:VirtualizedContainerService.HintSize="287,212" />
      <sap2010:ViewStateData Id="Delay_1" sap:VirtualizedContainerService.HintSize="287,22" />
      <sap2010:ViewStateData Id="Sequence_1" sap:VirtualizedContainerService.HintSize="309,460">
        <sap:WorkflowViewStateService.ViewState>
          <scg:Dictionary x:TypeArguments="x:String, x:Object">
            <x:Boolean x:Key="IsExpanded">True</x:Boolean>
          </scg:Dictionary>
        </sap:WorkflowViewStateService.ViewState>
      </sap2010:ViewStateData>
      <sap2010:ViewStateData Id="WorkflowTest.Workflow1_1" sap:VirtualizedContainerService.HintSize="349,540" />
    </sap2010:ViewStateManager>
  </sap2010:WorkflowViewState.ViewStateManager>
</Activity>

Full Error Log:

System.InvalidOperationException: 'Compilation failures occurred:
 Line 259: (260,89): error CS1069: The type name 'Match' could not be found in the namespace 'System.Text.RegularExpressions'. This type has been forwarded to assembly 'System.Text.RegularExpressions, Version=8.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' Consider adding a reference to that assembly.
 Line 321: (322,89): error CS1069: The type name 'Match' could not be found in the namespace 'System.Text.RegularExpressions'. This type has been forwarded to assembly 'System.Text.RegularExpressions, Version=8.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' Consider adding a reference to that assembly.
 ... (rest of the CS1069 errors) ...
 Line 0: warning CS1701: Assuming assembly reference 'System.Linq.Expressions, Version=6.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' used by 'System.Activities' matches identity 'System.Linq.Expressions, Version=8.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' of 'System.Linq.Expressions', you may need to supply runtime policy
 ... (other CS1701 warnings) ...
 Line 0: Unable to load assembly 'System.ServiceModel'.
 Line 0: Unable to load assembly 'System.ServiceModel.Activities'.
 Line 0: Unable to load assembly 'WorkflowTest'.
Complete results are contained in the Data property of this exception.  Please correct the errors in the source and retry the Load.'