after.fs

// This was the idea. Unfortunately, it doesn't work. The expression expects every successive item to be
// the same type the first expression, ProvidedProperty. I can upcast them all to MemberInfo, but then they
// don't work as arguments to the next items in the expression.
let createType name (parameters:obj[]) =
    let providedAsmPath = Path.Combine(config.TemporaryFolder, "FixedLengthTypes.dll")
    let providedAsm = ProvidedAssembly(providedAsmPath)
    let length = parameters.[0] :?> int
    let ty = ProvidedTypeDefinition(asm, "FixedLengthTypes", name, Some typeof<obj>, IsErased = false)
    ty.AddMembers(list {
        yield ProvidedProperty("Length", typeof<int>, IsStatic = true, 
                               GetterCode = fun args -> <@@ length @@>)
        let! valueField = ProvidedField("value", typeof<string>)
        yield ProvidedProperty("Value", typeof<string>,
                               GetterCode = fun args -> Expr.FieldGet(args.[0], valueField))
        let! ctor = ProvidedConstructor([ProvidedParameter("value", typeof<string>)],
                                        InvokeCode = fun args -> Expr.FieldSet(args.[0], valueField, args.[1]))
        yield ProvidedMethod("Create", [ProvidedParameter("value", typeof<string>)], ty,
                             InvokeCode = fun args -> Expr.NewObject(ctor, [args.[0]]))
    })

before.fs

let createType name (parameters:obj[]) =
    let providedAsmPath = Path.Combine(config.TemporaryFolder, "FixedLengthTypes.dll")
    let providedAsm = ProvidedAssembly(providedAsmPath)
    let length = parameters.[0] :?> int
    let ty = ProvidedTypeDefinition(asm, "FixedLengthTypes", name, Some typeof<obj>, IsErased = false)
 
    let lengthProp = ProvidedProperty("Length", typeof<int>, IsStatic = true, 
                                      GetterCode = fun args -> <@@ length @@>)
    ty.AddMember(lengthProp)
 
    let valueField = ProvidedField("value", typeof<string>)
    ty.AddMember(valueField)
 
    let valueProp = ProvidedProperty("Value", typeof<string>,
                                     GetterCode = fun args -> Expr.FieldGet(args.[0], valueField))
    ty.AddMember(valueProp)
 
    let ctor = ProvidedConstructor([ProvidedParameter("value", typeof<string>)],
                                   InvokeCode = fun args -> Expr.FieldSet(args.[0], valueField, args.[1]))
    ty.AddMember(ctor)

 
    let createMeth = ProvidedMethod("Create", [ProvidedParameter("value", typeof<string>)], ty,
                                    InvokeCode = fun args -> Expr.NewObject(ctor, [args.[0]]))
    ty.AddMember(createMeth)
 
    providedAsm.AddTypes([ty])
    ty

The list computed expression really starts to shine in this example.