[API Proposal]: Add protected factory method for creating Label in ILGenerator, abstract out LocalBuilder

[buyaa-n]

Background and motivation

For adding AssemblyBuilder.Save() equivalent in .NET Core we have abstracted most of the System.Refleciton.Emit types and adding new implementations for them. This abstraction did not cover Label and LocalBuilder types and its constructors are internal, so I could not create an instance from the new ILGenerator implementation.

API Proposal

• For LocalBuilder we need to unseal the LocalBuilder class and make it abstract, make the internal constructor protected, also add a public getter that exposes the method the local belongs to.

  namespace System.Reflection.Emit;
  
  -public sealed class LocalBuilder : System.Reflection.LocalVariableInfo
  +public abstract class LocalBuilder : System.Reflection.LocalVariableInfo
  {
  -   internal LocalBuilder(int localIndex, Type localType, MethodInfo method, bool isPinned) { }
  +   protected LocalBuilder() { }
  +   public abstract MethodInfo LocalMethod { get; }
  }

  Abstract LocalMethod property can be used for validation. Parent LocalVariableInfo has public virtual properties like LocalType, LocalIndex, IsPinned.

• For Label struct we need a protected factory method in the abstract ILGenerator type so that we can create an instance from derived types.

  namespace System.Reflection.Emit;
  
  public abstract class ILGenerator
  {
      protected ILGenerator() { }
      ...
      public abstract Label DefineLabel();
  +   protected static Label CreateLabel(int id) { throw null; }
  }
  
  public readonly partial struct Label : System.IEquatable<System.Reflection.Emit.Label>
  {
      ...
  +   public int Id { get; }
  }

API Usage

internal sealed class ILGeneratorImpl : ILGenerator
{
    public override Label DefineLabel()
    {
        LabelHandle metadataLabel = _il.DefineLabel();
        Label emitLabel = CreateLabel(metadataLabel.Id);
        _labelTable.Add(emitLabel, metadataLabel);
        return emitLabel;
    }
}

Related to #92975

CC @steveharter

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Issue Details

---

Background and motivation

For adding AssemblyBuilder.Save() equivalent in .NET Core we have abstracted most of the System.Refleciton.Emit types and adding new implementations for them. This abstraction did not cover Label, LocalBuilder types and their constructors are internal, so I could not create an instance from the new ILGenerator implementation. We need a protected factory methods that create these instances in the abstract ILGenerator type so that we can call theme derived type.

Related to #92975

CC @steveharter

API Proposal

namespace System.Reflection.Emit;

public abstract class ILGenerator
{
    protected ILGenerator() { }
    ....
+   protected Label CreateLabel(int id) { throw null; }
+   protected LocalBuilder CreateLocal(int localIndex, Type localType, MethodInfo methodBuilder, bool isPinned) { throw null; }
}

API Usage

internal sealed class ILGeneratorImpl : ILGenerator
{
    public override Label DefineLabel()
    {
        LabelHandle metadataLabel = _il.DefineLabel();
        Label emitLabel = CreateLabel(metadataLabel.Id);
        _labelTable.Add(emitLabel, metadataLabel);
        return emitLabel;
    }
}

Alternative Designs

No response

Risks

No response

Author:	buyaa-n
Assignees:	-
Labels:	api-suggestion, area-System.Reflection.Emit
Milestone:	-

[teo-tsirpanis]

How about we add a way to get the ID of a label, either as an Id property of Label, or (preferably) as a protected int GetLabelId(Label label); method on ILGenerator?

[buyaa-n]

How about we add a way to get the ID of a label, either as an Id property of Label, or (preferably) as a protected int GetLabelId(Label label); method on ILGenerator?

That can be useful for handling labels, I think it should be added to the Label struct.