CodeDOM -- generating an interface to a type

This is a fairly simple process -- reflect over the public properties and methods, and generate a matching interface member, rather like this:

	let GenerateInterface (target:Type) =
	let compileUnit = new CodeCompileUnit()
	let scope = new CodeNamespace("Generated")
	let namespaces = new HashSet<string>()
	compileUnit.Namespaces.Add( scope ) |> ignore
	let wrapper = new CodeTypeDeclaration("I" + target.Name + "Wrapper")
	wrapper.IsInterface <- true
	scope.Types.Add(wrapper) |> ignore
	IncludeProperties wrapper target namespaces
	IncludeMethods wrapper target namespaces
	namespaces
	|> Seq.sort
	|> Seq.iter (fun n-> scope.Imports.Add(new CodeNamespaceImport(n)))
	compileUnit

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which puts the required namespaces into order (TODO: sort System.* ahead of the rest).

The two functions look much the same, so could be factored further:

	let IncludeProperties (klass:CodeTypeDeclaration) (target:Type) (namespaces:HashSet<string>) =
	target.GetProperties(BindingFlags.Instance ||| BindingFlags.Public)
	|> Seq.sortBy (fun p -> p.Name)
	|> Seq.iter (fun p ->
	let m = new CodeMemberProperty()
	m.Name <- p.Name
	namespaces.Add(p.PropertyType.Namespace) |> ignore
	m.Type <- TypeRef(p.PropertyType)
	m.HasGet <- p.CanRead
	m.HasSet <- p.CanWrite
	p.GetIndexParameters()
	|> Seq.iter ( fun t ->
	let px = new CodeParameterDeclarationExpression()
	px.Direction <- FieldDirection.In
	px.Name <- t.Name
	namespaces.Add(t.ParameterType.Namespace) |> ignore
	px.Type <- TypeRef(t.ParameterType)
	m.Parameters.Add(px) |> ignore
	)
	if m.Parameters.Count > 0 then m.Name <- "Item"
	klass.Members.Add( m ) |> ignore
	)

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which special cases the indexer property; and

	let IncludeMethods (klass:CodeTypeDeclaration) (target:Type) (namespaces:HashSet<string>) =
	target.GetMethods(BindingFlags.Instance ||| BindingFlags.Public)
	|> Seq.filter (fun x -> not x.IsSpecialName )
	|> Seq.sortBy (fun p -> p.Name)
	|> Seq.iter (fun p ->
	let m = new CodeMemberMethod()
	m.Name <- p.Name
	m.ReturnType <- TypeRef(p.ReturnType)
	namespaces.Add(p.ReturnType.Namespace) |> ignore
	p.GetParameters()
	|> Seq.iter ( fun (t:ParameterInfo) ->
	let px = new CodeParameterDeclarationExpression()
	px.Direction <- if t.IsIn && t.IsOut then FieldDirection.Ref
	else if t.IsOut then FieldDirection.Out
	else FieldDirection.In
	px.Name <- t.Name
	namespaces.Add(t.ParameterType.Namespace) |> ignore
	px.Type <- TypeRef(t.ParameterType)
	m.Parameters.Add(px) |> ignore
	)
	klass.Members.Add( m ) |> ignore
	)

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(TODO: generics and attributes, which I don't need for my immediate use case)

To make the code look less cluttered, to get type specifications which aren't fully namespace qualified everywhere, we want to specify types with keyword synonyms by type object, and the rest by name (with the namespaces included by using or open). So we have helper functions

	let SysType (t:Type) =
	if t = typeof<System.Object> ||
	t = typeof<System.Char> ||
	t = typeof<System.Boolean> ||
	t = typeof<System.String> ||
	t = typeof<System.Int32> ||
	// also uint, float, etc... ; probably also Nullables
	t = typeof<System.Void> then true
	else false
	let TypeRef (t:Type) =
	if t.IsArray then
	if SysType <| t.GetElementType() then new CodeTypeReference(t)
	else new CodeTypeReference(t.Name)
	else if SysType t then new CodeTypeReference(t)
	else new CodeTypeReference(t.Name)

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where again, TODO handling generic types, especially collections, to get them looking pretty.

So if we use string as the type, and dump to C# we get

	namespace Generated {
	using System;
	using System.Globalization;
	using System.Text;
	public interface IStringWrapper {
	char this[int index] {
	get;
	}
	int Length {
	get;
	}
	object Clone();
	...
	string TrimStart(char[] trimChars);
	}
	}

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and in F#

	namespace Generated
	// Generated by F# CodeDom stuff omitted...
	open System
	open System.Globalization
	open System.Text
	type
	IStringWrapper = interface
	abstract Item : int -> char with get
	abstract Length : int with get
	abstract TrimStart : char[] -> string
	...
	abstract Clone : unit -> obj
	end

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where interestingly, the sorted order of the inserted members is reversed.