# Command Descriptors

**RadPdfViewer**'s command descriptors are wrappers of its predefined set of commands. They were introduced as a way to easily enable/disable the commands based on the current document state. This comes in handy in the creation of UI for the viewer such as a toolbar, context menu, etc.

The **CommandDescriptor** class contains 2 public properties:

* **IsEnabled**(Boolean): Gets or sets if the command is enabled.

* **Command**(ICommand): The command to be executed.

## Creating Custom Command Descriptors

The following section demonstrates how you can create a custom command along with the respective command descriptor.

This example creates a custom command by inheriting from the **FixedDocumentViewerCommandBase** class.

```C#
public class FitToWidthCommand : FixedDocumentViewerCommandBase
{
    private const double PageMargin = 20;

    public FitToWidthCommand(FixedDocumentViewerBase fixedDocumentViewerBase)
        : base(fixedDocumentViewerBase)
    {
    }

    public override void Execute(object parameter)
    {
        double width = this.Viewer.ActualWidth - 2 * PageMargin;
        double pageWidth = this.Viewer.CurrentPage.ActualWidth;
        this.Viewer.ScaleFactor = width / pageWidth;
    }
}
```

#### **VB**

```VB.NET
	    Public Class FitToWidthCommand
	        Inherits FixedDocumentViewerCommandBase
	        Private Const PageMargin As Double = 20
	
	        Public Sub New(fixedDocumentViewerBase As FixedDocumentViewerBase)
	            MyBase.New(fixedDocumentViewerBase)
	        End Sub
	
	        Public Overrides Sub Execute(parameter As Object)
	            Dim width As Double = Me.Viewer.ActualWidth - 2 * PageMargin
	            Dim pageWidth As Double = Me.Viewer.CurrentPage.ActualWidth
	            Me.Viewer.ScaleFactor = width / pageWidth
	        End Sub
	    End Class
```

Further, you can create a respective command descriptor with the custom command and use it for the purposes of your application. This can be achieved in one of the following ways:

* Create a class inheriting from the **DefaultCommandDescriptors** class which implements the default command descriptors and add a property of type **CommandDescriptor** to it.

* Create a class inheriting from the **CommandDescriptors** class which does not implement the default command descriptors and implement them yourself.

The below snippet demosntrates how you can use the first of the mentioned methods of creating a command descriptor.

```C#
public class CustomCommandDescriptors : DefaultCommandDescriptors
{
    private readonly CommandDescriptor fitToWidthCommandDescriptor;

    public CommandDescriptor FitToWidthCommandDescriptor
    {
        get
        {
            return this.fitToWidthCommandDescriptor;
        }
    }

    public CustomCommandDescriptors(FixedDocumentViewerBase fixedDocumentViewerBase)
        : base(fixedDocumentViewerBase)
    {
        this.fitToWidthCommandDescriptor = new CommandDescriptor(new FitToWidthCommand(fixedDocumentViewerBase));
    }
}
```

#### **VB**

```VB.NET
	    Public Class FitToWidthCommand
	        Inherits FixedDocumentViewerCommandBase
	        Private Const PageMargin As Double = 20
	
	        Public Sub New(fixedDocumentViewerBase As FixedDocumentViewerBase)
	            MyBase.New(fixedDocumentViewerBase)
	        End Sub
	
	        Public Overrides Sub Execute(parameter As Object)
	            Dim width As Double = Me.Viewer.ActualWidth - 2 * PageMargin
	            Dim pageWidth As Double = Me.Viewer.CurrentPage.ActualWidth
	            Me.Viewer.ScaleFactor = width / pageWidth
	        End Sub
	    End Class
```

Once the custom command descriptors are implemented you just need to set **RadPdfViewer**'s **CommandDescriptors** property.

```C#
this.customCommandDescriptors = new CustomCommandDescriptors(this.viewer);
```

#### **VB**

```VB.NET
	    Me.customCommandDescriptors = New CustomCommandDescriptors(Me.viewer)
```

> tip Find a runnable project of the previous example in the [WPF Samples GitHub repository](https://github.com/telerik/xaml-sdk/tree/master/PdfViewer/CustomCommandDescriptor).