DelegateCommand Class

A delegate command that implements the ICommand interface and calls your parameterless delegates when Execute and CanExecute logic is invoked on the command.

Namespace: DevExpress.Mvvm

Assembly: DevExpress.WinUI.Mvvm.v23.2.dll

NuGet Package: DevExpress.WinUI

Declaration

C#

public class DelegateCommand :
    DelegateCommand<object>

VB.NET

Public Class DelegateCommand
    Inherits DelegateCommand(Of Object)

Related API Members

The following members return DelegateCommand objects:

• AsyncCommand<T>.CancelCommand

Remarks

Delegate commands implement the ICommand interface and allow you to create commands in a ViewModel. A delegate command calls methods (delegates) that you assign to the command when the command’s Execute and CanExecute logic is invoked.

Run Demo: DelegateCommand Module in the WinUI MVVM Demo

DevExpress WinUI MVVM Framework also includes the DelegateCommand<T> class that is the parameterized version of delegate commands.

Create Delegate Commands

The DelegateCommand<T> and DelegateCommand classes contain constructors that accept Execute delegate and optional CanExecute delegate parameters.

The following code sample creates delegate commands that call the MessageBoxService.ShowAsync method:

C#

public class DelegateCommandsViewModel : ViewModelBase {
    public IMessageBoxService MessageBoxService => GetUIService<IMessageBoxService>();
    public DelegateCommand DelegateCommand1 { get; }
    public DelegateCommand<string> DelegateCommand2 { get; }

    public DelegateCommandsViewModel() {
        DelegateCommand1 = new DelegateCommand(
            execute: () => MessageBoxService.ShowAsync("This is a DelegateCommand"));

        DelegateCommand2 = new DelegateCommand<string>(
            execute: x => MessageBoxService.ShowAsync(x),
            canExecute: x => !string.IsNullOrEmpty(x));
    }
}

VB.NET

Public Class DelegateCommandsViewModel
    Inherits ViewModelBase

    Public ReadOnly Property MessageBoxService() As IMessageBoxService
        Get
            Return GetUIService(Of IMessageBoxService)()
        End Get
    End Property
    Private privateDelegateCommand1 As DelegateCommand
    Public Property DelegateCommand1() As DelegateCommand
        Get
            Return privateDelegateCommand1
        End Get
        Private Set(ByVal value As DelegateCommand)
            privateDelegateCommand1 = value
        End Set
    End Property
    Private privateDelegateCommand2 As DelegateCommand(Of String)
    Public Property DelegateCommand2() As DelegateCommand(Of String)
        Get
            Return privateDelegateCommand2
        End Get
        Private Set(ByVal value As DelegateCommand(Of String))
            privateDelegateCommand2 = value
        End Set
    End Property

    Public Sub New()
        DelegateCommand1 = New DelegateCommand(Function() MessageBoxService.ShowAsync("This is a DelegateCommand"))
        DelegateCommand2 = New DelegateCommand(Of String)(Function(x) MessageBoxService.ShowAsync(x), Function(x) Not String.IsNullOrEmpty(x))
    End Sub
End Class

Convert Command Parameters

The DelegateCommand<T> class converts the command argument to the parameterized type if possible.

The following code snippet converts the CommandParameter‘s String value to the type of the DocumentType:

XAML

<Button Command="{x:Bind ShowDocumentCommand}" CommandParameter="Text"/>

C#

public enum DocumentType { Text, Data }

public class DelegateCommandsViewModel : ViewModelBase {
    public DelegateCommand<DocumentType> ShowDocumentCommand { get; }

    public DelegateCommandsViewModel() {
        ShowDocumentCommand = new DelegateCommand<DocumentType>(ShowDocument);
    }
    void ShowDocument(DocumentType parameter) {
        if(parameter == DocumentType.Text) {
            //... 
        }
        if(parameter == DocumentType.Data) {
            //... 
        }
    }
}

VB.NET

Public Enum DocumentType
    Text
    Data
End Enum

Public Class DelegateCommandsViewModel
    Inherits ViewModelBase

    Public Property ShowDocumentCommand As DelegateCommand(Of DocumentType)

    Public Sub New()
        ShowDocumentCommand = New DelegateCommand(Of DocumentType)(AddressOf ShowDocument)
    End Sub

    Private Sub ShowDocument(ByVal parameter As DocumentType)
        If parameter = DocumentType.Text Then
        End If

        If parameter = DocumentType.Data Then
        End If
    End Sub
End Class

Update Commands

Call the RaiseCanExecuteChanged method to update your commands.

C#

public class DelegateCommandsViewModel : ViewModelBase {
    public DelegateCommand GoBackCommand{ get; }
    public DelegateCommand GoForwardCommand { get; }

    public DelegateCommandsViewModel() {
        GoBackCommand = new DelegateCommand(GoBack, CanGoBack);
        GoForwardCommand = new DelegateCommand(GoForward, CanGoForward);
    }

    void GoBack() {
        //... 
        UpdateCommandsState();
    }
    void GoForward() {
        //... 
        UpdateCommandsState();
    }
    bool CanGoBack() {
        //... 
    }
    bool CanGoForward() {
        //... 
    }
    void UpdateCommandsState() {
        GoBackCommand.RaiseCanExecuteChanged();
        GoForwardCommand.RaiseCanExecuteChanged();
    }
}

VB.NET

Public Class DelegateCommandsViewModel
    Inherits ViewModelBase

    Public Property GoBackCommand As DelegateCommand
    Public Property GoForwardCommand As DelegateCommand

    Public Sub New()
        GoBackCommand = New DelegateCommand(AddressOf GoBack, AddressOf CanGoBack)
        GoForwardCommand = New DelegateCommand(AddressOf GoForward, AddressOf CanGoForward)
    End Sub

    Private Sub GoBack()
        UpdateCommandsState()
    End Sub

    Private Sub GoForward()
        UpdateCommandsState()
    End Sub

    Private Function CanGoBack() As Boolean
    End Function

    Private Function CanGoForward() As Boolean
    End Function

    Private Sub UpdateCommandsState()
        GoBackCommand.RaiseCanExecuteChanged()
        GoForwardCommand.RaiseCanExecuteChanged()
    End Sub
End Class

Implements

ICommand<Object>

IDelegateCommand

Inheritance

Object

CommandBase<Object>

DelegateCommand<Object>

DelegateCommand