Index: generic/predefined.h
===================================================================
diff -u -rf6be532e62dfbe148ebca8205a67688b751298ad -r120f51260309bbe35ba0f142a25e1b18947e3635
--- generic/predefined.h	(.../predefined.h)	(revision f6be532e62dfbe148ebca8205a67688b751298ad)
+++ generic/predefined.h	(.../predefined.h)	(revision 120f51260309bbe35ba0f142a25e1b18947e3635)
@@ -150,14 +150,8 @@
 "eval next -childof $slotobject $args}\n"
 "::xotcl::MetaSlot create ::xotcl::Slot\n"
 "::xotcl::MetaSlot __invalidateobjectparameter\n"
-"proc ::xotcl::parametersFromSlots {obj} {\n"
-"set parameterdefinitions [list]\n"
-"set slots [::xotcl2::objectInfo slotobjects $obj]\n"
-"foreach slot $slots {\n"
-"if {[::xotcl::is $obj type ::xotcl::Object] &&\n"
-"([$slot name] eq \"mixin\" || [$slot name] eq \"filter\")} continue\n"
-"set name [namespace tail $slot]\n"
-"set parameterdefinition \"-$name\"\n"
+"proc ::xotcl::parameterFromSlot {slot name} {\n"
+"set parameterdefinition $name\n"
 "set opts [list]\n"
 "if {[$slot exists required] && [$slot required]} {\n"
 "lappend opts required}\n"
@@ -178,9 +172,16 @@
 "if {[llength $opts] > 0} {\n"
 "append parameterdefinition :[join $opts ,]}\n"
 "if {[info exists arg]} {\n"
-"lappend parameterdefinition $arg\n"
-"unset arg}\n"
-"lappend parameterdefinitions $parameterdefinition}\n"
+"lappend parameterdefinition $arg}\n"
+"return $parameterdefinition}\n"
+"proc ::xotcl::parametersFromSlots {obj} {\n"
+"set parameterdefinitions [list]\n"
+"set slots [::xotcl2::objectInfo slotobjects $obj]\n"
+"foreach slot $slots {\n"
+"if {[::xotcl::is $obj type ::xotcl::Object] &&\n"
+"([$slot name] eq \"mixin\" || [$slot name] eq \"filter\")} continue\n"
+"set name [namespace tail $slot]\n"
+"lappend parameterdefinitions -[::xotcl::parameterFromSlot $slot $name]}\n"
 "return $parameterdefinitions}\n"
 "::xotcl2::Object protected method objectparameter {} {\n"
 "set parameterdefinitions [::xotcl::parametersFromSlots [self]]\n"
@@ -259,8 +260,9 @@
 "set :methodname ${:name}}\n"
 "${:domain} __invalidateobjectparameter\n"
 "set cl [expr {${:per-object} ? \"Object\" : \"Class\"}]\n"
-"::xotcl::dispatch ${:domain} ::xotcl::classes::xotcl2::${cl}::forward \\\n"
-"${:name} ${:manager} [list %1 [${:manager} defaultmethods]] %self \\\n"
+"::xotcl::forward ${:domain} ${:name} \\\n"
+"${:manager} \\\n"
+"[list %1 [${:manager} defaultmethods]] %self \\\n"
 "${:methodname}}}\n"
 "::xotcl::MetaSlot create ::xotcl::InfoSlot\n"
 "createBootstrapAttributeSlots ::xotcl::InfoSlot {\n"
@@ -350,6 +352,7 @@
 ":check_single_value -keep_old_value false $value $predicate $type $obj $var}\n"
 "::xotcl::setinstvar $obj __oldvalue($var) $value}\n"
 "::xotcl::Attribute method mk_type_checker {} {\n"
+"puts stderr \"[self] [self proc]\"\n"
 "set __initcmd \"\"\n"
 "if {[:exists type]} {\n"
 "if {[::xotcl::is ${:type} class]} {\n"
@@ -371,6 +374,15 @@
 "\\[list [::xotcl::self] __default_from_cmd \\[::xotcl::self\\] [list [set :initcmd]]\\]\\n\"} elseif [:exists valuecmd] {\n"
 "append __initcmd \":trace add variable [list ${:name}] read \\\n"
 "\\[list [::xotcl::self] __value_from_cmd \\[::xotcl::self\\] [list [set :valuecmd]]\\]\"}\n"
+"set valueParam [::xotcl::parameterFromSlot [self] \"value\"]\n"
+"if {$valueParam ne \"value\"} {\n"
+"if {[set :multivalued]} {\n"
+":method check_single_value [list $valueParam] {return 1}\n"
+":method check_multiple_values list {foreach a $list {:check_single_value $a}}\n"
+":method assign [list obj var value] {\n"
+":check_multiple_values $value\n"
+"::xotcl::setinstvar $obj $var $value}} else {\n"
+":method assign [list obj var $valueParam] {::xotcl::setinstvar $obj $var $value}}}\n"
 "if {[:exists valuechangedcmd]} {\n"
 "append __initcmd \":trace add variable [list ${:name}] write \\\n"
 "\\[list [::xotcl::self] __value_changed_cmd \\[::xotcl::self\\] [list [set :valuechangedcmd]]\\]\"}\n"
Index: generic/predefined.xotcl
===================================================================
diff -u -rf6be532e62dfbe148ebca8205a67688b751298ad -r120f51260309bbe35ba0f142a25e1b18947e3635
--- generic/predefined.xotcl	(.../predefined.xotcl)	(revision f6be532e62dfbe148ebca8205a67688b751298ad)
+++ generic/predefined.xotcl	(.../predefined.xotcl)	(revision 120f51260309bbe35ba0f142a25e1b18947e3635)
@@ -317,6 +317,43 @@
 
 # Provide the a slot based mechanism for building an object
 # configuration interface from slot definitions
+proc ::xotcl::parameterFromSlot {slot name} {
+  set parameterdefinition $name
+  set opts [list]
+  if {[$slot exists required] && [$slot required]} {
+    lappend opts required
+  }
+  if {[$slot exists type]} {
+    lappend opts [$slot type]
+  }
+  if {[$slot exists arg]} {
+    lappend opts arg=[$slot arg]
+  }
+  if {[$slot exists default]} {
+    set arg [::xotcl::setinstvar $slot default]
+    # deactivated for now: || [string first {$} $arg] > -1
+    if {[string match {*\[*\]*} $arg]} {
+      lappend opts substdefault
+    }
+  } elseif {[$slot exists initcmd]} {
+    set arg [::xotcl::setinstvar $slot initcmd]
+    lappend opts initcmd
+  } 
+  if {[$slot exists methodname]} {
+    set methodname [::xotcl::setinstvar $slot methodname]
+    if {$methodname ne $name} {
+      lappend opts arg=$methodname
+    }
+  }
+  if {[llength $opts] > 0} {
+    append parameterdefinition :[join $opts ,]
+  }
+  if {[info exists arg]} {
+    lappend parameterdefinition $arg
+  }
+  return $parameterdefinition
+}
+
 proc ::xotcl::parametersFromSlots {obj} {
   set parameterdefinitions [list]
   set slots [::xotcl2::objectInfo slotobjects $obj]
@@ -326,41 +363,7 @@
         ([$slot name] eq "mixin" || [$slot name] eq "filter")
       } continue
     set name [namespace tail $slot]
-    set parameterdefinition "-$name"
-    set opts [list]
-    if {[$slot exists required] && [$slot required]} {
-      lappend opts required
-    }
-    if {[$slot exists type]} {
-      lappend opts [$slot type]
-    }
-    if {[$slot exists arg]} {
-      lappend opts arg=[$slot arg]
-    }
-    if {[$slot exists default]} {
-      set arg [::xotcl::setinstvar $slot default]
-      # deactivated for now: || [string first {$} $arg] > -1
-      if {[string match {*\[*\]*} $arg]} {
-	lappend opts substdefault
-      }
-    } elseif {[$slot exists initcmd]} {
-      set arg [::xotcl::setinstvar $slot initcmd]
-      lappend opts initcmd
-    } 
-    if {[$slot exists methodname]} {
-      set methodname [::xotcl::setinstvar $slot methodname]
-      if {$methodname ne $name} {
-        lappend opts arg=$methodname
-      }
-    }
-    if {[llength $opts] > 0} {
-      append parameterdefinition :[join $opts ,]
-    }
-    if {[info exists arg]} {
-      lappend parameterdefinition $arg
-      unset arg
-    }
-    lappend parameterdefinitions $parameterdefinition
+    lappend parameterdefinitions -[::xotcl::parameterFromSlot $slot $name]
   }
   return $parameterdefinitions
 }
@@ -502,9 +505,10 @@
     ${:domain} __invalidateobjectparameter
     set cl [expr {${:per-object} ? "Object" : "Class"}]
     # since the domain object might be xotcl1 or xotcl2, use dispatch
-    ::xotcl::dispatch ${:domain} ::xotcl::classes::xotcl2::${cl}::forward \
-        ${:name} ${:manager} [list %1 [${:manager} defaultmethods]] %self \
-       ${:methodname}
+    ::xotcl::forward ${:domain} ${:name} \
+        ${:manager} \
+        [list %1 [${:manager} defaultmethods]] %self \
+        ${:methodname}
   }
 }
 
@@ -669,6 +673,7 @@
   ::xotcl::setinstvar $obj __oldvalue($var) $value
 }
 ::xotcl::Attribute method mk_type_checker {} {
+  puts stderr "[self] [self proc]"
   set __initcmd ""
   if {[:exists type]} {
     if {[::xotcl::is ${:type} class]} {
@@ -705,6 +710,21 @@
     append __initcmd ":trace add variable [list ${:name}] read \
 	\[list [::xotcl::self] __value_from_cmd \[::xotcl::self\] [list [set :valuecmd]]\]"
   }
+  set valueParam [::xotcl::parameterFromSlot [self] "value"]
+  if {$valueParam ne "value"} {
+    if {[set :multivalued]} {
+      :method check_single_value [list $valueParam] {return 1}
+      :method check_multiple_values list {foreach a $list {:check_single_value $a}}
+      #puts stderr "adding multiple assignmethod for [self] with $valueParam"
+      :method assign [list obj var value] {
+        :check_multiple_values $value
+        ::xotcl::setinstvar $obj $var $value
+      }
+    } else {
+      #puts stderr "adding single assignmethod for [self] with $valueParam"
+      :method assign [list obj var $valueParam] {::xotcl::setinstvar $obj $var $value}
+    }
+  }
   #append __initcmd [:mk_type_checker]
   if {[:exists valuechangedcmd]} {
     append __initcmd ":trace add variable [list ${:name}] write \
@@ -727,12 +747,13 @@
   :method forward args {::xotcl::next; :optimize}
   :method init args    {::xotcl::next; :optimize}
   :public method optimize {} {
+    #puts stderr OPTIMIZER
     if {[set :multivalued]} return
     if {[set :defaultmethods] ne {get assign}} return
     #puts stderr assign=[:info callable -which assign]
     if {[:info callable -which assign] ne "::xotcl::Slot alias assign ::xotcl::setinstvar"} return
     if {[:info callable -which get]    ne "::xotcl::Slot alias get ::xotcl::setinstvar"} return
-    #puts stderr "**** optimizing       ${:domain} $forwarder ${:name}"
+    #puts stderr "**** optimizing       [${:domain} info method definition ${:name}]"
     ::xotcl::setter ${:domain} {*}[expr {${:per-object} ? "-per-object" : ""}] ${:name}
   }
 }
Index: tests/parameters.xotcl
===================================================================
diff -u -rf6be532e62dfbe148ebca8205a67688b751298ad -r120f51260309bbe35ba0f142a25e1b18947e3635
--- tests/parameters.xotcl	(.../parameters.xotcl)	(revision f6be532e62dfbe148ebca8205a67688b751298ad)
+++ tests/parameters.xotcl	(.../parameters.xotcl)	(revision 120f51260309bbe35ba0f142a25e1b18947e3635)
@@ -474,15 +474,47 @@
     "Value 'o' of mix has not mixin M" \
     "does not have mixin M"
 
-# TODO: error messages for failed conversions not consistent
-# TODO: setter should perform parameter checking:
-#       (a) simple approach: make scripted setter methods
+# TODO: naming "type" and "mixin" not perfect.
+#       maybe "type" => "hastype"
+#       maybe "mixin" => "hasmixin"
+#       => effects as well ::xotcl::is
+#
+# TODO: It looks, as if we need multivalues as well on object
+#       parameters. If a slot has multivalued set, objectparameter
+#       must honor it. This would allow general checking of e.g.  list
+#       of integers, list of objects, etc. Therefore, we would not
+#       need to duplicate this functionality on the slots.
+#
+# TODO (optimization): optimizer can improve parameter checking:
+#       (a) simple approach: make scripted setter methods on domain
 #       (b) maybe nicer: provide arguments to c-setter to 
 #           pass parameter definition
+#
+# TODO: error messages for failed conversions are not consistent
+#       should happen, when all kind of parameters finally settled
 # 
-# The following test fails currently:
-#?? {p o xxx} "Invalid argument: cannot convert 'xxx' to object"
+? {p o o} \
+    "o" \
+    "value is an object"
+?? {p o xxx} \
+    "Invalid argument: cannot convert 'xxx' to object" \
+    "value is not an object"
 
+ParamTest slots {
+  ::xotcl::Attribute create os -type object -multivalued true
+}
+
+? {p os o} \
+    "o" \
+    "value is a list of objects (1 element)"
+? {p os {o c1 d1}} \
+    "o c1 d1" \
+    "value is a list of objects (multiple elements)"
+
+?? {p os {o xxx d1}} \
+    "Invalid argument: cannot convert 'xxx' to object" \
+    "list with invalid object"
+
 ## TODO regression test for type checking, parameter options (initcmd,
 ## substdefault, combinations with defaults, ...), etc.
 puts stderr =====END