Handling Dialogs in WPF with MVVM

Question

In the MVVM pattern for WPF  handling dialogs is one of the more complex operations   As your view model does not know anything about the view  dialog communication can be interesting   I can expose an ICommand that when the view invokes it  a dialog can appear   Does anyone know of a good way to handle results from dialogs  I am speaking about windows dialogs such as MessageBox   One of the ways we did this was have an event on the viewmodel that the view would subscribe to when a dialog was required     public event EventHandler lt MyDeleteArgs gt  RequiresDeleteDialog    This is OK  but it means that the view requires code which is something I would like to stay away from

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Why not just raise an event in the VM and subscribe to the event in the view   This would keep the application logic and the view seperate and still allow you to use a child window for dialogs

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I think the view could have code to handle the event from the view model   Depending on the event scenario  it could also have an event trigger that subscribes to view model events  and one or more actions to invoke in response

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I think the view could have code to handle the event from the view model   Depending on the event scenario  it could also have an event trigger that subscribes to view model events  and one or more actions to invoke in response

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I ve implemented a Behavior that listens to a Message from the ViewModel  It s based on Laurent Bugnion solution  but since it doesn t use code behind and is more reusable  I think it s more elegant   How to make WPF behave as if MVVM is supported out of the box

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You should use a mediator for this  Mediator is a common design pattern also known as Messenger in some of its implementations  It s a paradigm of type Register Notify and enables your ViewModel and Views to communicate through a low-coupled messaging mecanism   You should check out the google WPF Disciples group  and just search for Mediator  You will be much happy with the answers     You can however start with this   http   joshsmithonwpf wordpress com 2009 04 06 a-mediator-prototype-for-wpf-apps   Enjoy    Edit  you can see the answer to this problem with the MVVM Light Toolkit here   http   mvvmlight codeplex com Thread View aspx ThreadId 209338

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I think that the handling of a dialog should be the responsibility of the view  and the view needs to have code to support that   If you change the ViewModel - View interaction to handle dialogs then the ViewModel is dependant on that implementation  The simplest way to deal with this problem is to make the View responsible for performing the task  If that means showing a dialog then fine  but could also be a status message in the status bar etc   My point is that the whole point of the MVVM pattern is to separate business logic from the GUI  so you shouldn t be mixing GUI logic  to display a dialog  in the business layer  the ViewModel

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I ve implemented a Behavior that listens to a Message from the ViewModel  It s based on Laurent Bugnion solution  but since it doesn t use code behind and is more reusable  I think it s more elegant   How to make WPF behave as if MVVM is supported out of the box

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I think that the handling of a dialog should be the responsibility of the view  and the view needs to have code to support that   If you change the ViewModel - View interaction to handle dialogs then the ViewModel is dependant on that implementation  The simplest way to deal with this problem is to make the View responsible for performing the task  If that means showing a dialog then fine  but could also be a status message in the status bar etc   My point is that the whole point of the MVVM pattern is to separate business logic from the GUI  so you shouldn t be mixing GUI logic  to display a dialog  in the business layer  the ViewModel

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Sorry  but I have to chime in   I have been through several of the suggested solutions  before finding the Prism Wpf Interactivity namespace in the Prism project   You can use interaction requests and popup window action to either roll a custom window or for simpler needs there are built in Notification and Confirmation popups   These create true windows and are managed as such   you can pass a context object with any dependencies you need in the dialog   We use this solution at my work since I found it   We have numerous senior devs here and noone has come up with anything better   Our previous solution was the dialog service into an overlay and using a presenter class to make it happen  but you had to have factories for all of the dialog viewmodels  etc     This isn t trivial but it also isn t super complicated   And it is built in to Prism and is therefore best  or better  practice IMHO   My 2 cents

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I rolled my own window loader described in an answer to this question   Managing multiple WPF views in an application

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I use this approach for dialogs with MVVM   All I have to do now is call the following from my view model     var result   this uiDialogService ShowDialog  Dialogwindow title goes here   dialogwindowVM

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I struggled with the same problem  I have come up with a way to intercommunicate between the View and the ViewModel  You can initiate sending a message from the ViewModel to the View to tell it to show a messagebox and it will report back with the result  Then the ViewModel can respond to the result returned from the View   I demonstrate this in my blog

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The standard approach  After spending years dealing with this problem in WPF  I finally figured out the standard way of implementing dialogs in WPF  Here are the advantages of this approach    CLEAN Doesn t violate MVVM design pattern ViewModal never references any of the UI libraries  WindowBase  PresentationFramework etc   Perfect for automated testing Dialogs can be replaced easily    So what s the key  It is DI   IoC   Here is how it works  I m using MVVM Light  but this approach may be extended to other frameworks as well    Add a WPF Application project to your solution  Call it App  Add a ViewModal Class Library  Call it VM  App references VM project  VM project doesn t know anything about App  Add NuGet reference to MVVM Light to both projects  I m using MVVM Light Standard these days  but you are okay with the full Framework version too  Add an interface IDialogService to VM project   public interface IDialogService     void ShowMessage string msg  bool isError     bool AskBooleanQuestion string msg     string AskStringQuestion string msg  string default value      string ShowOpen string filter  string initDir       string title          string ShowSave string filter  string initDir       string title       string fileName          string ShowFolder string initDir           bool ShowSettings       Expose a public static property of IDialogService type in your ViewModelLocator  but leave registration part for the View layer to perform  This is the key    public static IDialogService DialogService   gt  SimpleIoc Default GetInstance lt IDialogService gt      Add an implementation of this interface in the App project   public class DialogPresenter   IDialogService       private static OpenFileDialog dlgOpen   new OpenFileDialog        private static SaveFileDialog dlgSave   new SaveFileDialog        private static FolderBrowserDialog dlgFolder   new FolderBrowserDialog              lt summary gt          Displays a simple Information or Error message to the user           lt  summary gt           lt param name  msg  gt String text that is to be displayed in the MessageBox lt  param gt           lt param name  isError  gt If true  Error icon is displayed  If false  Information icon is displayed  lt  param gt      public void ShowMessage string msg  bool isError                    if isError                      System Windows MessageBox Show msg   Your Project Title   MessageBoxButton OK  MessageBoxImage Error               else                     System Windows MessageBox Show msg   Your Project Title   MessageBoxButton OK  MessageBoxImage Information                   lt summary gt          Displays a Yes No MessageBox Returns true if user clicks Yes  otherwise false           lt  summary gt           lt param name  msg  gt  lt  param gt           lt returns gt  lt  returns gt      public bool AskBooleanQuestion string msg                    var Result   System Windows MessageBox Show msg   Your Project Title   MessageBoxButton YesNo  MessageBoxImage Question     MessageBoxResult Yes              return Result                  lt summary gt          Displays Save dialog  User can specify file filter  initial directory and dialog title  Returns full path of the selected file if         user clicks Save button  Returns null if user clicks Cancel button           lt  summary gt           lt param name  filter  gt  lt  param gt           lt param name  initDir  gt  lt  param gt           lt param name  title  gt  lt  param gt           lt param name  fileName  gt  lt  param gt           lt returns gt  lt  returns gt      public string ShowSave string filter  string initDir       string title       string fileName                         if   string IsNullOrEmpty title                       dlgSave Title   title              else                     dlgSave Title    Save                if   string IsNullOrEmpty fileName                       dlgSave FileName   fileName              else                     dlgSave FileName                    dlgSave Filter   filter              if   string IsNullOrEmpty initDir                       dlgSave InitialDirectory   initDir               if  dlgSave ShowDialog      DialogResult OK                      return dlgSave FileName              else                     return null              public string ShowFolder string initDir                         if   string IsNullOrEmpty initDir                       dlgFolder SelectedPath   initDir               if  dlgFolder ShowDialog      DialogResult OK                      return dlgFolder SelectedPath              else                     return null                   lt summary gt          Displays Open dialog  User can specify file filter  initial directory and dialog title  Returns full path of the selected file if         user clicks Open button  Returns null if user clicks Cancel button           lt  summary gt           lt param name  filter  gt  lt  param gt           lt param name  initDir  gt  lt  param gt           lt param name  title  gt  lt  param gt           lt returns gt  lt  returns gt      public string ShowOpen string filter  string initDir       string title                         if   string IsNullOrEmpty title                       dlgOpen Title   title              else                     dlgOpen Title    Open                dlgOpen Multiselect   false              dlgOpen Filter   filter              if   string IsNullOrEmpty initDir                       dlgOpen InitialDirectory   initDir               if  dlgOpen ShowDialog      DialogResult OK                      return dlgOpen FileName              else                     return null                  lt summary gt          Shows Settings dialog           lt  summary gt           lt returns gt true if User clicks OK button  otherwise false  lt  returns gt      public bool ShowSettings                     var w   new SettingsWindow                MakeChild w     Show this dialog as child of Microsoft Word window              var Result   w ShowDialog   Value              return Result                  lt summary gt          Prompts user for a single value input  First parameter specifies the message to be displayed in the dialog          and the second string specifies the default value to be displayed in the input box           lt  summary gt           lt param name  m  gt  lt  param gt      public string AskStringQuestion string msg  string default value                    string Result   null               InputBox w   new InputBox                MakeChild w               if  w ShowDialog msg  default value  Value                      Result   w Value               return Result                  lt summary gt          Sets Word window as parent of the specified window           lt  summary gt           lt param name  w  gt  lt  param gt      private static void MakeChild System Windows Window w                    IntPtr HWND   Process GetCurrentProcess   MainWindowHandle              var helper   new WindowInteropHelper w    Owner   HWND             While some of these functions are generic  ShowMessage  AskBooleanQuestion etc    others are specific to this project and use custom Windows  You can add more custom windows in the same fashion  The key is to keep UI-specific elements in the View layer and just expose the returned data using POCOs in the VM layer  Perform IoC Registration your interface in the View layer using this class  You can do this in your main view s constructor  after InitializeComponent   call    SimpleIoc Default Register lt IDialogService  DialogPresenter gt      There you go  You now have access to all your dialog functionality at both VM and View layers  Your VM layer can call these functions like this   var NoTrump   ViewModelLocator DialogService AskBooleanQuestion  Really stop the trade war            So clean you see  The VM layer doesn t know nothing about how a Yes No question will be presented to the user by the UI layer and can still successfully work with the returned result from the dialog    Other free perks   For writing unit test  you can provide a custom implementation of IDialogService in your Test project and register that class in IoC in the constructor your test class  You ll need to import some namespaces such as Microsoft Win32 to access Open and Save dialogs  I have left them out because there is also a WinForms version of these dialogs available  plus someone might want to create their own version  Also note that some of the identifier used in DialogPresenter are names of my own windows  e g  SettingsWindow   You ll need to either remove them from both the interface and implementation or provide your own windows  If your VM performs multi-threading  call MVVM Light s DispatcherHelper Initialize   early in your application s life cycle  Except for DialogPresenter which is injected in the View layer  other ViewModals should be registered in ViewModelLocator and then a public static property of that type should be exposed for the View layer to consume  Something like this   public static SettingsVM Settings   gt  SimpleIoc Default GetInstance lt SettingsVM gt      For the most part  your dialogs should not have any code-behind for stuff like binding or setting DataContext etc  You shouldn t even pass things as constructor parameters  XAML can do that all for you  like this    lt Window x Class  YourViewNamespace SettingsWindow    xmlns  http   schemas microsoft com winfx 2006 xaml presentation    xmlns local  clr-namespace YourViewProject    xmlns vm  clr-namespace YourVMProject assembly YourVMProject    DataContext   x Static vm ViewModelLocator Settings     d DataContext   d DesignInstance Type vm SettingsVM     gt   Setting DataContext this way gives you all kinds of design-time benefits such as Intellisense and auto-completion    Hope that helps everyone

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EDIT  yes I agree this is not a correct MVVM approach and I am now using something similar to what is suggested by blindmeis   One of the way you could to this is  In your Main View Model  where you open the modal    void OpenModal         ModalWindowViewModel mwvm   new ModalWindowViewModel        Window mw   new Window        mw content   mwvm      mw ShowDialog       if mw DialogResult    true                    Your Code  you can access property in mwvm if you need            And in your Modal Window View ViewModel   XAML    lt Button Name  okButton  Command   Binding OkCommand   CommandParameter   Binding RelativeSource  RelativeSource FindAncestor  AncestorType  x Type Window     gt OK lt  Button gt   lt Button Margin  2  VerticalAlignment  Center  Name  cancelButton  IsCancel  True  gt Cancel lt  Button gt    ViewModel   public ICommand OkCommand       get               if   okCommand    null                         okCommand   new ActionCommand lt Window gt  DoOk  CanDoOk                     return  okCommand            void DoOk Window win         lt  --Your Code-- gt      win DialogResult   true      win Close       bool CanDoOk Window win    return true      or similar to what is posted here WPF MVVM  How to close a window

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I rolled my own window loader described in an answer to this question   Managing multiple WPF views in an application

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Sorry  but I have to chime in   I have been through several of the suggested solutions  before finding the Prism Wpf Interactivity namespace in the Prism project   You can use interaction requests and popup window action to either roll a custom window or for simpler needs there are built in Notification and Confirmation popups   These create true windows and are managed as such   you can pass a context object with any dependencies you need in the dialog   We use this solution at my work since I found it   We have numerous senior devs here and noone has come up with anything better   Our previous solution was the dialog service into an overlay and using a presenter class to make it happen  but you had to have factories for all of the dialog viewmodels  etc     This isn t trivial but it also isn t super complicated   And it is built in to Prism and is therefore best  or better  practice IMHO   My 2 cents

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An interesting alternative is to use Controllers which are responsible to show the views  dialogs    How this works is shown by the WPF Application Framework  WAF

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Use a freezable command   lt Grid gt           lt Grid DataContext gt               lt WpfApplication1 ViewModel   gt           lt  Grid DataContext gt             lt Button Content  Text  gt               lt Button Command gt                   lt WpfApplication1 MessageBoxCommand YesCommand   Binding MyViewModelCommand     gt               lt  Button Command gt           lt  Button gt    lt  Grid gt    public class MessageBoxCommand   Freezable  ICommand       public static readonly DependencyProperty YesCommandProperty   DependencyProperty Register           YesCommand           typeof  ICommand           typeof  MessageBoxCommand           new FrameworkPropertyMetadata null                   public static readonly DependencyProperty OKCommandProperty   DependencyProperty Register           OKCommand           typeof  ICommand           typeof  MessageBoxCommand           new FrameworkPropertyMetadata null                   public static readonly DependencyProperty CancelCommandProperty   DependencyProperty Register           CancelCommand           typeof  ICommand           typeof  MessageBoxCommand           new FrameworkPropertyMetadata null                   public static readonly DependencyProperty NoCommandProperty   DependencyProperty Register           NoCommand           typeof  ICommand           typeof  MessageBoxCommand           new FrameworkPropertyMetadata null                   public static readonly DependencyProperty MessageProperty   DependencyProperty Register           Message           typeof  string           typeof  MessageBoxCommand           new FrameworkPropertyMetadata                     public static readonly DependencyProperty MessageBoxButtonsProperty   DependencyProperty Register           MessageBoxButtons           typeof MessageBoxButton           typeof MessageBoxCommand           new FrameworkPropertyMetadata MessageBoxButton OKCancel                  public ICommand YesCommand               get   return  ICommand  GetValue YesCommandProperty             set   SetValue YesCommandProperty  value                public ICommand OKCommand               get   return  ICommand  GetValue OKCommandProperty             set   SetValue OKCommandProperty  value                public ICommand CancelCommand               get   return  ICommand  GetValue CancelCommandProperty             set   SetValue CancelCommandProperty  value                public ICommand NoCommand               get   return  ICommand  GetValue NoCommandProperty             set   SetValue NoCommandProperty  value                public MessageBoxButton MessageBoxButtons               get   return  MessageBoxButton GetValue MessageBoxButtonsProperty             set   SetValue MessageBoxButtonsProperty  value                public string Message               get   return  string  GetValue MessageProperty             set   SetValue MessageProperty  value                public void Execute object parameter                var messageBoxResult   MessageBox Show Message           switch  messageBoxResult                        case MessageBoxResult OK                  OKCommand Execute null                   break              case MessageBoxResult Yes                  YesCommand Execute null                   break              case MessageBoxResult No                  NoCommand Execute null                   break              case MessageBoxResult Cancel                  if  CancelCommand    null  CancelCommand Execute null     Cancel usually means do nothing  so can be null                 break                        public bool CanExecute object parameter                return true             public event EventHandler CanExecuteChanged        protected override Freezable CreateInstanceCore                 throw new NotImplementedException

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An interesting alternative is to use Controllers which are responsible to show the views  dialogs    How this works is shown by the WPF Application Framework  WAF

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You should use a mediator for this  Mediator is a common design pattern also known as Messenger in some of its implementations  It s a paradigm of type Register Notify and enables your ViewModel and Views to communicate through a low-coupled messaging mecanism   You should check out the google WPF Disciples group  and just search for Mediator  You will be much happy with the answers     You can however start with this   http   joshsmithonwpf wordpress com 2009 04 06 a-mediator-prototype-for-wpf-apps   Enjoy    Edit  you can see the answer to this problem with the MVVM Light Toolkit here   http   mvvmlight codeplex com Thread View aspx ThreadId 209338

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I think that the handling of a dialog should be the responsibility of the view  and the view needs to have code to support that   If you change the ViewModel - View interaction to handle dialogs then the ViewModel is dependant on that implementation  The simplest way to deal with this problem is to make the View responsible for performing the task  If that means showing a dialog then fine  but could also be a status message in the status bar etc   My point is that the whole point of the MVVM pattern is to separate business logic from the GUI  so you shouldn t be mixing GUI logic  to display a dialog  in the business layer  the ViewModel

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I really struggled with this concept for a while when learning  still learning  MVVM   What I decided  and what I think others already decided but which wasn t clear to me is this   My original thought was that a ViewModel should not be allowed to call a dialog box directly as it has no business deciding how a dialog should appear  Beacause of this I started thinking about how I could pass messages much like I would have in MVP  i e  View ShowSaveFileDialog      However  I think this is the wrong approach   It is OK for a ViewModel to call a dialog directly   However  when you are testing a ViewModel   that means that the dialog will either pop up during your test  or fail all together  never really tried this      So  what needs to happen is while testing is to use a  test  version of your dialog   This means that for ever dialog you have  you need to create an Interface and either mock out the dialog response or create a testing mock that will have a default behaviour   You should already be using some sort of Service Locator or IoC that you can configure to provide you the correct version depending on the context   Using this approach  your ViewModel is still testable and depending on how you mock out your dialogs  you can control the behaviour   Hope this helps

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I was pondering a similar problem when asking how the view model for a task or dialog should look like   My current solution looks like this     public class SelectionTaskModel lt TChoosable gt    ViewModel     where TChoosable   ViewModel       public SelectionTaskModel ICollection lt TChoosable gt  choices       public ReadOnlyCollection lt TChoosable gt  Choices   get        public void Choose TChoosable choosen       public void Abort        When the view model decides that user input is required  it pulls up a instance of SelectionTaskModel with the possible choices for the user  The infrastructure takes care of bringing up the corresponding view  which in proper time will call the Choose   function with the user s choice

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I ve written a fairly comprehensive article about this very topic and also developed a pop-in library for MVVM Dialogs  Strict adherence to MVVM is not only possible but very clean when implemented properly  and it can be easily extended to third-party libraries that don t adhere to it themselves   https   www codeproject com Articles 820324 Implementing-Dialog-Boxes-in-MVVM

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I think that the handling of a dialog should be the responsibility of the view  and the view needs to have code to support that   If you change the ViewModel - View interaction to handle dialogs then the ViewModel is dependant on that implementation  The simplest way to deal with this problem is to make the View responsible for performing the task  If that means showing a dialog then fine  but could also be a status message in the status bar etc   My point is that the whole point of the MVVM pattern is to separate business logic from the GUI  so you shouldn t be mixing GUI logic  to display a dialog  in the business layer  the ViewModel

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I really struggled with this concept for a while when learning  still learning  MVVM   What I decided  and what I think others already decided but which wasn t clear to me is this   My original thought was that a ViewModel should not be allowed to call a dialog box directly as it has no business deciding how a dialog should appear  Beacause of this I started thinking about how I could pass messages much like I would have in MVP  i e  View ShowSaveFileDialog      However  I think this is the wrong approach   It is OK for a ViewModel to call a dialog directly   However  when you are testing a ViewModel   that means that the dialog will either pop up during your test  or fail all together  never really tried this      So  what needs to happen is while testing is to use a  test  version of your dialog   This means that for ever dialog you have  you need to create an Interface and either mock out the dialog response or create a testing mock that will have a default behaviour   You should already be using some sort of Service Locator or IoC that you can configure to provide you the correct version depending on the context   Using this approach  your ViewModel is still testable and depending on how you mock out your dialogs  you can control the behaviour   Hope this helps

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I ve written a fairly comprehensive article about this very topic and also developed a pop-in library for MVVM Dialogs  Strict adherence to MVVM is not only possible but very clean when implemented properly  and it can be easily extended to third-party libraries that don t adhere to it themselves   https   www codeproject com Articles 820324 Implementing-Dialog-Boxes-in-MVVM

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I was pondering a similar problem when asking how the view model for a task or dialog should look like   My current solution looks like this     public class SelectionTaskModel lt TChoosable gt    ViewModel     where TChoosable   ViewModel       public SelectionTaskModel ICollection lt TChoosable gt  choices       public ReadOnlyCollection lt TChoosable gt  Choices   get        public void Choose TChoosable choosen       public void Abort        When the view model decides that user input is required  it pulls up a instance of SelectionTaskModel with the possible choices for the user  The infrastructure takes care of bringing up the corresponding view  which in proper time will call the Choose   function with the user s choice

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Karl Shifflett has created a sample application for showing dialog boxes using service approach and Prism InteractionRequest approach   I like the service approach - It s less flexible so users are less likely to break something    It s also consistent with the WinForms part of my application  MessageBox Show  But if you plan to show a lot of different dialogs  then InteractionRequest is a better way to go   http   karlshifflett wordpress com 2010 11 07 in-the-box-ndash-mvvm-training

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I was pondering a similar problem when asking how the view model for a task or dialog should look like   My current solution looks like this     public class SelectionTaskModel lt TChoosable gt    ViewModel     where TChoosable   ViewModel       public SelectionTaskModel ICollection lt TChoosable gt  choices       public ReadOnlyCollection lt TChoosable gt  Choices   get        public void Choose TChoosable choosen       public void Abort        When the view model decides that user input is required  it pulls up a instance of SelectionTaskModel with the possible choices for the user  The infrastructure takes care of bringing up the corresponding view  which in proper time will call the Choose   function with the user s choice

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I use this approach for dialogs with MVVM   All I have to do now is call the following from my view model     var result   this uiDialogService ShowDialog  Dialogwindow title goes here   dialogwindowVM

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I was pondering a similar problem when asking how the view model for a task or dialog should look like   My current solution looks like this     public class SelectionTaskModel lt TChoosable gt    ViewModel     where TChoosable   ViewModel       public SelectionTaskModel ICollection lt TChoosable gt  choices       public ReadOnlyCollection lt TChoosable gt  Choices   get        public void Choose TChoosable choosen       public void Abort        When the view model decides that user input is required  it pulls up a instance of SelectionTaskModel with the possible choices for the user  The infrastructure takes care of bringing up the corresponding view  which in proper time will call the Choose   function with the user s choice

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A good MVVM dialog should    Be declared with only XAML  Get all of it s behavior from databinding    Unfortunately  WPF doesn t provide these features  Showing a dialog requires a code-behind call to ShowDialog    The Window class  which supports dialogs  can t be declared in XAML so it can t easily be databound to the DataContext   To solve this  I wrote a XAML stub control that sits in the logical tree and relays databinding to a Window and handles showing and hiding the dialog  You can find it here  http   www codeproject com KB WPF XAMLDialog aspx  It s really simply to use and doesn t require any strange changes to your ViewModel and doesn t require events or messages  The basic call looks like this     lt dialog Dialog Content   Binding Path DialogViewModel   Showing  True    gt    You probably want to add a style that sets Showing  I explain it in my article  I hope this helps you

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There are two good ways to do this  1  a dialog service  easy  clean   and 2  view assisted   View assisted provides some neat features  but is usually not worth it   DIALOG SERVICE  a  a dialog service interface like via constructor or some dependency container   interface IDialogService       Task ShowDialogAsync DialogViewModel dlgVm      b  Your implementation of IDialogService should open a window  or inject some control into the active window   create a view corresponding to the name of the given dlgVm type  use container registration or convention or a ContentPresenter with type associated DataTemplates    ShowDialogAsync should create a TaskCompletionSource and return its  Task proptery   The DialogViewModel class itself needs an event you can invoke in the derived class when you want to close  and watch in the dialog view to actually close hide the dialog and complete the TaskCompletionSource     b  To use  simply call await this DialogService ShowDialog myDlgVm  on your instance of some DialogViewModel-derived class   After await returns  look at properties you ve added on your dialog VM to determine what happened  you don t even need a callback   VIEW ASSISTED  This has your view listening to an event on the viewmodel   This could all be wrapped up into a Blend Behavior to avoid code behind and resource usage if you re so inclined  FMI  subclass the  Behavior  class to see a sort of Blendable attached property on steroids    For now  we ll do this manually on each view   a  Create an OpenXXXXXDialogEvent with a custom payload  a DialogViewModel derived class    b  Have the view subscribe to the event in its OnDataContextChanged event   Be sure to hide and unsubscribe if the old value    null and in the Window s Unloaded event   c  When the event fires  have the view open your view  which might be in a resource on your page  or you could locate it by convention elsewhere  like in the the dialog service approach    This approach is more flexible  but requires more work to use   I don t use it much   The one nice advantage are the ability to place the view physically inside a tab  for example   I have used an algorithm to place it in the current user control s bounds  or if not big enough  traverse up the visual tree until a big enough container is found     This allows dialogs to be close to the place they re actually used  only dim the part of the app related to the current activity  and let the user move around within the app without having to manually push dialogs away  even have multiple quasi-modal dialogs open on different tabs or sub-views

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Why not just raise an event in the VM and subscribe to the event in the view   This would keep the application logic and the view seperate and still allow you to use a child window for dialogs

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I had the same situation and wrapped up the MessageBox into a designer invisible control  The details are in my blog  http   geekswithblogs net mukapu archive 2010 03 12 user-prompts-messagebox-with-mvvm aspx  The same can be extended to any modal dialogs  file browse control etc

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I had the same situation and wrapped up the MessageBox into a designer invisible control  The details are in my blog  http   geekswithblogs net mukapu archive 2010 03 12 user-prompts-messagebox-with-mvvm aspx  The same can be extended to any modal dialogs  file browse control etc

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I suggest forgoing the 1990 s modal dialogs and instead implementing a control as an overlay  canvas absolute positioning  with visibility tied to a boolean back in the VM  Closer to an ajax type control   This is very useful    lt BooleanToVisibilityConverter x Key  booltoVis    gt    as in    lt my ErrorControl Visibility   Binding Path ThereWasAnError  Mode TwoWay  Converter  StaticResource booltoVis   UpdateSourceTrigger PropertyChanged    gt    Here s how I have one implemented as a user control  Clicking on the  x  closes the control in a line of code in the usercontrol s code behind   Since I have my Views in an  exe and ViewModels in a dll  I don t feel bad about code that manipulates UI

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There are two good ways to do this  1  a dialog service  easy  clean   and 2  view assisted   View assisted provides some neat features  but is usually not worth it   DIALOG SERVICE  a  a dialog service interface like via constructor or some dependency container   interface IDialogService       Task ShowDialogAsync DialogViewModel dlgVm      b  Your implementation of IDialogService should open a window  or inject some control into the active window   create a view corresponding to the name of the given dlgVm type  use container registration or convention or a ContentPresenter with type associated DataTemplates    ShowDialogAsync should create a TaskCompletionSource and return its  Task proptery   The DialogViewModel class itself needs an event you can invoke in the derived class when you want to close  and watch in the dialog view to actually close hide the dialog and complete the TaskCompletionSource     b  To use  simply call await this DialogService ShowDialog myDlgVm  on your instance of some DialogViewModel-derived class   After await returns  look at properties you ve added on your dialog VM to determine what happened  you don t even need a callback   VIEW ASSISTED  This has your view listening to an event on the viewmodel   This could all be wrapped up into a Blend Behavior to avoid code behind and resource usage if you re so inclined  FMI  subclass the  Behavior  class to see a sort of Blendable attached property on steroids    For now  we ll do this manually on each view   a  Create an OpenXXXXXDialogEvent with a custom payload  a DialogViewModel derived class    b  Have the view subscribe to the event in its OnDataContextChanged event   Be sure to hide and unsubscribe if the old value    null and in the Window s Unloaded event   c  When the event fires  have the view open your view  which might be in a resource on your page  or you could locate it by convention elsewhere  like in the the dialog service approach    This approach is more flexible  but requires more work to use   I don t use it much   The one nice advantage are the ability to place the view physically inside a tab  for example   I have used an algorithm to place it in the current user control s bounds  or if not big enough  traverse up the visual tree until a big enough container is found     This allows dialogs to be close to the place they re actually used  only dim the part of the app related to the current activity  and let the user move around within the app without having to manually push dialogs away  even have multiple quasi-modal dialogs open on different tabs or sub-views

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I struggled with the same problem  I have come up with a way to intercommunicate between the View and the ViewModel  You can initiate sending a message from the ViewModel to the View to tell it to show a messagebox and it will report back with the result  Then the ViewModel can respond to the result returned from the View   I demonstrate this in my blog

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I know it s an old question  but when I  did this search  I find a lot of related question  but I did not find a really clear response  So I make my own implementation of a dialogbox messagebox popin  and I share it  I think it is  MVVM proof   and I try to make it simple and proper  but I am new to WPF  so feel free to comment  or even make pull request   https   github com Plasma-Paris Plasma WpfUtils  You can use it like this   public  RelayCommand  YesNoMessageBoxCommand     get   private  set     async  void  YesNoMessageBox             var  result     await   Service ShowMessage  This  is  the  content  of  the  message  box     This  is  the  title    System Windows MessageBoxButton YesNo           if   result      System Windows MessageBoxResult Yes                               Or like this if you want more sophisticated popin    var  result     await   Service ShowCustomMessageBox new  MyMessageBoxViewModel         What  you  want            And it is showing things like this

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The standard approach  After spending years dealing with this problem in WPF  I finally figured out the standard way of implementing dialogs in WPF  Here are the advantages of this approach    CLEAN Doesn t violate MVVM design pattern ViewModal never references any of the UI libraries  WindowBase  PresentationFramework etc   Perfect for automated testing Dialogs can be replaced easily    So what s the key  It is DI   IoC   Here is how it works  I m using MVVM Light  but this approach may be extended to other frameworks as well    Add a WPF Application project to your solution  Call it App  Add a ViewModal Class Library  Call it VM  App references VM project  VM project doesn t know anything about App  Add NuGet reference to MVVM Light to both projects  I m using MVVM Light Standard these days  but you are okay with the full Framework version too  Add an interface IDialogService to VM project   public interface IDialogService     void ShowMessage string msg  bool isError     bool AskBooleanQuestion string msg     string AskStringQuestion string msg  string default value      string ShowOpen string filter  string initDir       string title          string ShowSave string filter  string initDir       string title       string fileName          string ShowFolder string initDir           bool ShowSettings       Expose a public static property of IDialogService type in your ViewModelLocator  but leave registration part for the View layer to perform  This is the key    public static IDialogService DialogService   gt  SimpleIoc Default GetInstance lt IDialogService gt      Add an implementation of this interface in the App project   public class DialogPresenter   IDialogService       private static OpenFileDialog dlgOpen   new OpenFileDialog        private static SaveFileDialog dlgSave   new SaveFileDialog        private static FolderBrowserDialog dlgFolder   new FolderBrowserDialog              lt summary gt          Displays a simple Information or Error message to the user           lt  summary gt           lt param name  msg  gt String text that is to be displayed in the MessageBox lt  param gt           lt param name  isError  gt If true  Error icon is displayed  If false  Information icon is displayed  lt  param gt      public void ShowMessage string msg  bool isError                    if isError                      System Windows MessageBox Show msg   Your Project Title   MessageBoxButton OK  MessageBoxImage Error               else                     System Windows MessageBox Show msg   Your Project Title   MessageBoxButton OK  MessageBoxImage Information                   lt summary gt          Displays a Yes No MessageBox Returns true if user clicks Yes  otherwise false           lt  summary gt           lt param name  msg  gt  lt  param gt           lt returns gt  lt  returns gt      public bool AskBooleanQuestion string msg                    var Result   System Windows MessageBox Show msg   Your Project Title   MessageBoxButton YesNo  MessageBoxImage Question     MessageBoxResult Yes              return Result                  lt summary gt          Displays Save dialog  User can specify file filter  initial directory and dialog title  Returns full path of the selected file if         user clicks Save button  Returns null if user clicks Cancel button           lt  summary gt           lt param name  filter  gt  lt  param gt           lt param name  initDir  gt  lt  param gt           lt param name  title  gt  lt  param gt           lt param name  fileName  gt  lt  param gt           lt returns gt  lt  returns gt      public string ShowSave string filter  string initDir       string title       string fileName                         if   string IsNullOrEmpty title                       dlgSave Title   title              else                     dlgSave Title    Save                if   string IsNullOrEmpty fileName                       dlgSave FileName   fileName              else                     dlgSave FileName                    dlgSave Filter   filter              if   string IsNullOrEmpty initDir                       dlgSave InitialDirectory   initDir               if  dlgSave ShowDialog      DialogResult OK                      return dlgSave FileName              else                     return null              public string ShowFolder string initDir                         if   string IsNullOrEmpty initDir                       dlgFolder SelectedPath   initDir               if  dlgFolder ShowDialog      DialogResult OK                      return dlgFolder SelectedPath              else                     return null                   lt summary gt          Displays Open dialog  User can specify file filter  initial directory and dialog title  Returns full path of the selected file if         user clicks Open button  Returns null if user clicks Cancel button           lt  summary gt           lt param name  filter  gt  lt  param gt           lt param name  initDir  gt  lt  param gt           lt param name  title  gt  lt  param gt           lt returns gt  lt  returns gt      public string ShowOpen string filter  string initDir       string title                         if   string IsNullOrEmpty title                       dlgOpen Title   title              else                     dlgOpen Title    Open                dlgOpen Multiselect   false              dlgOpen Filter   filter              if   string IsNullOrEmpty initDir                       dlgOpen InitialDirectory   initDir               if  dlgOpen ShowDialog      DialogResult OK                      return dlgOpen FileName              else                     return null                  lt summary gt          Shows Settings dialog           lt  summary gt           lt returns gt true if User clicks OK button  otherwise false  lt  returns gt      public bool ShowSettings                     var w   new SettingsWindow                MakeChild w     Show this dialog as child of Microsoft Word window              var Result   w ShowDialog   Value              return Result                  lt summary gt          Prompts user for a single value input  First parameter specifies the message to be displayed in the dialog          and the second string specifies the default value to be displayed in the input box           lt  summary gt           lt param name  m  gt  lt  param gt      public string AskStringQuestion string msg  string default value                    string Result   null               InputBox w   new InputBox                MakeChild w               if  w ShowDialog msg  default value  Value                      Result   w Value               return Result                  lt summary gt          Sets Word window as parent of the specified window           lt  summary gt           lt param name  w  gt  lt  param gt      private static void MakeChild System Windows Window w                    IntPtr HWND   Process GetCurrentProcess   MainWindowHandle              var helper   new WindowInteropHelper w    Owner   HWND             While some of these functions are generic  ShowMessage  AskBooleanQuestion etc    others are specific to this project and use custom Windows  You can add more custom windows in the same fashion  The key is to keep UI-specific elements in the View layer and just expose the returned data using POCOs in the VM layer  Perform IoC Registration your interface in the View layer using this class  You can do this in your main view s constructor  after InitializeComponent   call    SimpleIoc Default Register lt IDialogService  DialogPresenter gt      There you go  You now have access to all your dialog functionality at both VM and View layers  Your VM layer can call these functions like this   var NoTrump   ViewModelLocator DialogService AskBooleanQuestion  Really stop the trade war            So clean you see  The VM layer doesn t know nothing about how a Yes No question will be presented to the user by the UI layer and can still successfully work with the returned result from the dialog    Other free perks   For writing unit test  you can provide a custom implementation of IDialogService in your Test project and register that class in IoC in the constructor your test class  You ll need to import some namespaces such as Microsoft Win32 to access Open and Save dialogs  I have left them out because there is also a WinForms version of these dialogs available  plus someone might want to create their own version  Also note that some of the identifier used in DialogPresenter are names of my own windows  e g  SettingsWindow   You ll need to either remove them from both the interface and implementation or provide your own windows  If your VM performs multi-threading  call MVVM Light s DispatcherHelper Initialize   early in your application s life cycle  Except for DialogPresenter which is injected in the View layer  other ViewModals should be registered in ViewModelLocator and then a public static property of that type should be exposed for the View layer to consume  Something like this   public static SettingsVM Settings   gt  SimpleIoc Default GetInstance lt SettingsVM gt      For the most part  your dialogs should not have any code-behind for stuff like binding or setting DataContext etc  You shouldn t even pass things as constructor parameters  XAML can do that all for you  like this    lt Window x Class  YourViewNamespace SettingsWindow    xmlns  http   schemas microsoft com winfx 2006 xaml presentation    xmlns local  clr-namespace YourViewProject    xmlns vm  clr-namespace YourVMProject assembly YourVMProject    DataContext   x Static vm ViewModelLocator Settings     d DataContext   d DesignInstance Type vm SettingsVM     gt   Setting DataContext this way gives you all kinds of design-time benefits such as Intellisense and auto-completion    Hope that helps everyone

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My current solution solves most of the issues you mentioned yet its completely abstracted from platform specific things and can be reused  Also i used no code-behind only binding with DelegateCommands that implement ICommand   Dialog is basically a View - a separate control that has its own ViewModel and it is shown from the ViewModel of the main screen but triggered from the UI via DelagateCommand binding   See full Silverlight 4 solution here Modal dialogs with MVVM and Silverlight 4

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Karl Shifflett has created a sample application for showing dialog boxes using service approach and Prism InteractionRequest approach   I like the service approach - It s less flexible so users are less likely to break something    It s also consistent with the WinForms part of my application  MessageBox Show  But if you plan to show a lot of different dialogs  then InteractionRequest is a better way to go   http   karlshifflett wordpress com 2010 11 07 in-the-box-ndash-mvvm-training

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A good MVVM dialog should    Be declared with only XAML  Get all of it s behavior from databinding    Unfortunately  WPF doesn t provide these features  Showing a dialog requires a code-behind call to ShowDialog    The Window class  which supports dialogs  can t be declared in XAML so it can t easily be databound to the DataContext   To solve this  I wrote a XAML stub control that sits in the logical tree and relays databinding to a Window and handles showing and hiding the dialog  You can find it here  http   www codeproject com KB WPF XAMLDialog aspx  It s really simply to use and doesn t require any strange changes to your ViewModel and doesn t require events or messages  The basic call looks like this     lt dialog Dialog Content   Binding Path DialogViewModel   Showing  True    gt    You probably want to add a style that sets Showing  I explain it in my article  I hope this helps you

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My current solution solves most of the issues you mentioned yet its completely abstracted from platform specific things and can be reused  Also i used no code-behind only binding with DelegateCommands that implement ICommand   Dialog is basically a View - a separate control that has its own ViewModel and it is shown from the ViewModel of the main screen but triggered from the UI via DelagateCommand binding   See full Silverlight 4 solution here Modal dialogs with MVVM and Silverlight 4

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Use a freezable command   lt Grid gt           lt Grid DataContext gt               lt WpfApplication1 ViewModel   gt           lt  Grid DataContext gt             lt Button Content  Text  gt               lt Button Command gt                   lt WpfApplication1 MessageBoxCommand YesCommand   Binding MyViewModelCommand     gt               lt  Button Command gt           lt  Button gt    lt  Grid gt    public class MessageBoxCommand   Freezable  ICommand       public static readonly DependencyProperty YesCommandProperty   DependencyProperty Register           YesCommand           typeof  ICommand           typeof  MessageBoxCommand           new FrameworkPropertyMetadata null                   public static readonly DependencyProperty OKCommandProperty   DependencyProperty Register           OKCommand           typeof  ICommand           typeof  MessageBoxCommand           new FrameworkPropertyMetadata null                   public static readonly DependencyProperty CancelCommandProperty   DependencyProperty Register           CancelCommand           typeof  ICommand           typeof  MessageBoxCommand           new FrameworkPropertyMetadata null                   public static readonly DependencyProperty NoCommandProperty   DependencyProperty Register           NoCommand           typeof  ICommand           typeof  MessageBoxCommand           new FrameworkPropertyMetadata null                   public static readonly DependencyProperty MessageProperty   DependencyProperty Register           Message           typeof  string           typeof  MessageBoxCommand           new FrameworkPropertyMetadata                     public static readonly DependencyProperty MessageBoxButtonsProperty   DependencyProperty Register           MessageBoxButtons           typeof MessageBoxButton           typeof MessageBoxCommand           new FrameworkPropertyMetadata MessageBoxButton OKCancel                  public ICommand YesCommand               get   return  ICommand  GetValue YesCommandProperty             set   SetValue YesCommandProperty  value                public ICommand OKCommand               get   return  ICommand  GetValue OKCommandProperty             set   SetValue OKCommandProperty  value                public ICommand CancelCommand               get   return  ICommand  GetValue CancelCommandProperty             set   SetValue CancelCommandProperty  value                public ICommand NoCommand               get   return  ICommand  GetValue NoCommandProperty             set   SetValue NoCommandProperty  value                public MessageBoxButton MessageBoxButtons               get   return  MessageBoxButton GetValue MessageBoxButtonsProperty             set   SetValue MessageBoxButtonsProperty  value                public string Message               get   return  string  GetValue MessageProperty             set   SetValue MessageProperty  value                public void Execute object parameter                var messageBoxResult   MessageBox Show Message           switch  messageBoxResult                        case MessageBoxResult OK                  OKCommand Execute null                   break              case MessageBoxResult Yes                  YesCommand Execute null                   break              case MessageBoxResult No                  NoCommand Execute null                   break              case MessageBoxResult Cancel                  if  CancelCommand    null  CancelCommand Execute null     Cancel usually means do nothing  so can be null                 break                        public bool CanExecute object parameter                return true             public event EventHandler CanExecuteChanged        protected override Freezable CreateInstanceCore                 throw new NotImplementedException

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EDIT  yes I agree this is not a correct MVVM approach and I am now using something similar to what is suggested by blindmeis   One of the way you could to this is  In your Main View Model  where you open the modal    void OpenModal         ModalWindowViewModel mwvm   new ModalWindowViewModel        Window mw   new Window        mw content   mwvm      mw ShowDialog       if mw DialogResult    true                    Your Code  you can access property in mwvm if you need            And in your Modal Window View ViewModel   XAML    lt Button Name  okButton  Command   Binding OkCommand   CommandParameter   Binding RelativeSource  RelativeSource FindAncestor  AncestorType  x Type Window     gt OK lt  Button gt   lt Button Margin  2  VerticalAlignment  Center  Name  cancelButton  IsCancel  True  gt Cancel lt  Button gt    ViewModel   public ICommand OkCommand       get               if   okCommand    null                         okCommand   new ActionCommand lt Window gt  DoOk  CanDoOk                     return  okCommand            void DoOk Window win         lt  --Your Code-- gt      win DialogResult   true      win Close       bool CanDoOk Window win    return true      or similar to what is posted here WPF MVVM  How to close a window

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I suggest forgoing the 1990 s modal dialogs and instead implementing a control as an overlay  canvas absolute positioning  with visibility tied to a boolean back in the VM  Closer to an ajax type control   This is very useful    lt BooleanToVisibilityConverter x Key  booltoVis    gt    as in    lt my ErrorControl Visibility   Binding Path ThereWasAnError  Mode TwoWay  Converter  StaticResource booltoVis   UpdateSourceTrigger PropertyChanged    gt    Here s how I have one implemented as a user control  Clicking on the  x  closes the control in a line of code in the usercontrol s code behind   Since I have my Views in an  exe and ViewModels in a dll  I don t feel bad about code that manipulates UI

User · Answer

I know it s an old question  but when I  did this search  I find a lot of related question  but I did not find a really clear response  So I make my own implementation of a dialogbox messagebox popin  and I share it  I think it is  MVVM proof   and I try to make it simple and proper  but I am new to WPF  so feel free to comment  or even make pull request   https   github com Plasma-Paris Plasma WpfUtils  You can use it like this   public  RelayCommand  YesNoMessageBoxCommand     get   private  set     async  void  YesNoMessageBox             var  result     await   Service ShowMessage  This  is  the  content  of  the  message  box     This  is  the  title    System Windows MessageBoxButton YesNo           if   result      System Windows MessageBoxResult Yes                               Or like this if you want more sophisticated popin    var  result     await   Service ShowCustomMessageBox new  MyMessageBoxViewModel         What  you  want            And it is showing things like this

[.net] Handling Dialogs in WPF with MVVM

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