wpf - Why does a format string that works in a Binding not work in a MultiBinding? -

i intrigued question: multibinding stringformat of timespan

if have following binding defined starttime of type timespan:

<textblock text={binding path=starttime, stringformat='{}from {0:hh\\:mm}'}" /> 

the above binding evaluates expected. however, scenario in original question shows, if try use same format string in multibinding, fails formatexception:

<textblock>     <textblock.text>         <multibinding stringformat="{}from {0:hh\\:mm} {1:hh\\:mm}">             <binding path="starttime" />             <binding path="endtime" />         </multibinding>     </textblock.text> </textblock> 

the question is, know why? bug or expected behavior? seems odd me same output in multibinding, have change "\:" ':' in format string (as discovered in answering original question).

this appears bug in wpf 4, if not it's @ least breaking change wpf 3.5. take following code example:

<window x:class="wpfsampletestbed.mainwindow"         xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"         title="mainwindow" height="350" width="525">     <stackpanel>         <textblock text="{binding path=starttime, stringformat='{}from {0:hh\\:mm}'}" />          <textblock x:name="textblock2">             <textblock.text>                 <multibinding stringformat="{}from {0:hh\\:mm} {1:hh\\:mm}">                     <binding path="starttime" />                     <binding path="endtime" />                 </multibinding>             </textblock.text>         </textblock>          <textblock x:name="textblock3" text="three" />         <textblock x:name="textblock4" text="four" />          <textblock>         <textblock.text>             <multibinding stringformat="three = {0}, 4 = {1}">                 <binding elementname="textblock3" path="text" />                 <binding elementname="textblock4" path="text" />             </multibinding>         </textblock.text>         </textblock>     </stackpanel>  </window> 

with code behind like:

using system; using system.windows;  namespace wpfsampletestbed {      public partial class mainwindow : window {         public mainwindow() {             initializecomponent();             this.datacontext = new test() {                 starttime = timespan.fromseconds(90),                 endtime = timespan.fromseconds(100),             };         }     }      public class test {         public timespan starttime { get; set; }         public timespan endtime { get; set; }     }  } 

if compile , run code against .net 3.5, output (i.e. window content) this:

from 00:01:30
00:01:30 00:01:40
3
4
3 = three, 4 = 4

taking exact sample code/project , running against .net 4 get:

from 00:01:30

3
4
3 = three, 4 = 4

i found 1 bug report may related, author never responded microsoft closed issue 'not reproducible'.

so appears depending on how child bindings used, stringformat may or may not work in .net 4.