An easy to implement DragDropBehavior for incorporating drag and drop in your silverlight 4 application; also includes a FileDragDropBehavior for easy file drag and drop

DragDropBehaviours.cs

using System.IO;
using System.Windows;
using System.Windows.Input;
using System.Windows.Interactivity;

/// <summary>
/// A simple file drag and drop behavior.
/// </summary>
public class FileDragDropBehavior : DragDropBehavior<FileInfo[]>
{
    /// <summary>
    /// The <see cref="FilesDroppedCommand" /> dependency property's name.
    /// </summary>
    public const string FilesDroppedCommandPropertyName = "FilesDroppedCommand";

    /// <summary>
    /// Gets or sets the value of the <see cref="FilesDroppedCommand" />
    /// property. This is a dependency property.
    /// </summary>
    public ICommand FilesDroppedCommand
    {
        get
        {
            return (ICommand)GetValue(FilesDroppedCommandProperty);
        }
        set
        {
            SetValue(FilesDroppedCommandProperty, value);
        }
    }

    /// <summary>
    /// Identifies the <see cref="FilesDroppedCommand" /> dependency property.
    /// </summary>
    public static readonly DependencyProperty FilesDroppedCommandProperty = DependencyProperty.Register(
        FilesDroppedCommandPropertyName,
        typeof(ICommand),
        typeof(FileDragDropBehavior),
        new PropertyMetadata(null));
        
    /// <summary>
    /// Determines if this drop event should run.
    /// </summary>
    /// <param name="sender">The sender.</param>
    /// <param name="e">The <see cref="System.Windows.DragEventArgs"/> instance containing the event data.</param>
    /// <returns>True if the event should run, otherwise false</returns>
    protected override bool ShouldDropEventRun(object sender, DragEventArgs e)
    {
        return this.FilesDroppedCommand != null;
    }

    /// <summary>
    /// Called when [dropped data].
    /// </summary>
    /// <param name="files">The files.</param>
    protected override void OnDroppedData(FileInfo[] files)
    {
        if (files == null)
            return;
            
        if (this.FilesDroppedCommand.CanExecute(files))
            this.FilesDroppedCommand.Execute(files);
            
    }

    /// <summary>
    /// Gets the data string.
    /// </summary>
    /// <returns></returns>
    protected override string GetDataString()
    {
        return DataFormats.FileDrop;
    }
}

public abstract class DragDropBehavior<TData> : Behavior<FrameworkElement>
{


    protected override void OnAttached()
    {
        AssociatedObject.AllowDrop = true;
        AssociatedObject.Drop += AssociatedObject_Drop;

        base.OnAttached();
    }

    protected override void OnDetaching()
    {
        AssociatedObject.Drop -= AssociatedObject_Drop;

        base.OnDetaching();
    }

    protected void AssociatedObject_Drop(object sender, DragEventArgs e)
    {
        if(!ShouldDropEventRun(sender, e))
            return;
            
        var dataName = GetDataString();
        if(!e.Data.GetDataPresent(dataName))
            return;
                
        var data = (TData)e.Data.GetData(dataName);

        OnDroppedData(data);
    }

    /// <summary>
    /// Called when [dropped data].
    /// </summary>
    /// <param name="data">The data.</param>
    protected abstract void OnDroppedData(TData data);

    /// <summary>
    /// Determines if a drop event should run; use this to check for null values and what-not.
    /// </summary>
    /// <param name="sender">The sender.</param>
    /// <param name="e">The <see cref="System.Windows.DragEventArgs"/> instance containing the event data.</param>
    /// <returns>True if should run, otherwise false</returns>
    protected virtual bool ShouldDropEventRun(object sender, DragEventArgs e)
    {
        return true;
    }

    /// <summary>
    /// When overriden, gets the data string to call e.Data.GetData(...) with.
    /// </summary>
    /// <returns>The data string to call GetData(...) with.</returns>
    protected abstract string GetDataString();
}

DragDropExampleXaml.xaml

<Border
    Width="300"
    Height="200"
    Margin="0 20 0 20"
    Background="LightGray"
    x:Name="DropArea">
    <i:Interaction.Behaviors>
        <con:FileDragDropBehavior
            FilesDroppedCommand="{Binding FilesDroppedCmd}" />
    </i:Interaction.Behaviors>
    <TextBlock
        FontWeight="Bold" FontSize="20"
        HorizontalAlignment="Center"
        VerticalAlignment="Center"
        Text="Drag Files Here" />
</Border>