DotNetBrowser

Up to version 1.11.1:

The user-agent string can be modified only once, before you create any Browser instances. You can provide your own user-agent string using the BrowserPreferences.SetUserAgent(String userAgent) method:

BrowserPreferences.SetUserAgent("My User Agent String");

From version 1.12:

The user-agent string can be modified during runtime. You can set your own user-agent string or get it using the Browser.UserAgent property:

Browser browser = BrowserFactory.Create();
browser.UserAgent = "Modified User Agent String";
string currentUserAgentString = browser.UserAgent;

Example

MainWindow.xaml

<Window x:Class="WPF.UserAgent.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <Grid Name="mainLayout">
        
    </Grid>
</Window>

MainWindow.xaml.cs

using System;
using System.IO;
using DotNetBrowser;
using DotNetBrowser.WPF;
using System.Windows;

namespace WPF.UserAgent
{
    public partial class MainWindow : Window
    {
        Browser browser;
        WPFBrowserView browserView;

        public MainWindow()
        {
            BrowserPreferences.SetUserAgent("My User Agent String");
            InitializeComponent();

            browser = BrowserFactory.Create();
            browserView = new WPFBrowserView(browser);
            mainLayout.Children.Add(browserView);

            browser.UserAgent = "Modified User Agent String";

            browserView.Browser.LoadURL("http://whatsmyuseragent.com/");
        }
    }
}