Accessing UI (Main) Thread safely in WPF

The best way to go about it would be to get a SynchronizationContext from the UI thread and use it. This class abstracts marshalling calls to other threads, and makes testing easier (in contrast to using WPF's Dispatcher directly). For example:

class MyViewModel
{
    private readonly SynchronizationContext _syncContext;

    public MyViewModel()
    {
        // we assume this ctor is called from the UI thread!
        _syncContext = SynchronizationContext.Current;
    }

    // ...

    private void watcher_Changed(object sender, FileSystemEventArgs e)
    {
         _syncContext.Post(o => DGAddRow(crp.Protocol, ft), null);
    }
}

Use [Dispatcher.Invoke(DispatcherPriority, Delegate)] to change the UI from another thread or from background.

Step 1. Use the following namespaces

using System.Windows;
using System.Threading;
using System.Windows.Threading;

Step 2. Put the following line where you need to update UI

Application.Current.Dispatcher.Invoke(DispatcherPriority.Background, new ThreadStart(delegate
{
    //Update UI here
}));

Syntax

[BrowsableAttribute(false)]
public object Invoke(
  DispatcherPriority priority,
  Delegate method
)

Parameters

priority

Type: System.Windows.Threading.DispatcherPriority

The priority, relative to the other pending operations in the Dispatcher event queue, the specified method is invoked.

method

Type: System.Delegate

A delegate to a method that takes no arguments, which is pushed onto the Dispatcher event queue.

Return Value

Type: System.Object

The return value from the delegate being invoked or null if the delegate has no return value.

Version Information

Available since .NET Framework 3.0

You can use

Dispatcher.Invoke(Delegate, object[])

on the Application's (or any UIElement's) dispatcher.

You can use it for example like this:

Application.Current.Dispatcher.Invoke(new Action(() => { /* Your code here */ }));

or

someControl.Dispatcher.Invoke(new Action(() => { /* Your code here */ }));