How to open a web page from my application?

For desktop versions of .NET:

System.Diagnostics.Process.Start("http://www.webpage.com");

For .NET Core, the default for ProcessStartInfo.UseShellExecute has changed from true to false, and so you have to explicitly set it to true for this to work:

System.Diagnostics.Process.Start(new ProcessStartInfo
    {
        FileName = "http://www.webpage.com",
        UseShellExecute = true
    });

To further complicate matters, this property cannot be set to true for UWP apps (so none of these solutions are usable for UWP).

Accepted answer no longer works on .NET Core 3. To make it work, use the following method:

var psi = new ProcessStartInfo
{
    FileName = url,
    UseShellExecute = true
};
Process.Start (psi);

I've been using this line to launch the default browser:

System.Diagnostics.Process.Start("http://www.google.com"); 

While a good answer has been given (using Process.Start), it is safer to encapsulate it in a function that checks that the passed string is indeed a URI, to avoid accidentally starting random processes on the machine.

public static bool IsValidUri(string uri)
{
    if (!Uri.IsWellFormedUriString(uri, UriKind.Absolute))
        return false;
    Uri tmp;
    if (!Uri.TryCreate(uri, UriKind.Absolute, out tmp))
        return false;
    return tmp.Scheme == Uri.UriSchemeHttp || tmp.Scheme == Uri.UriSchemeHttps;
}

public static bool OpenUri(string uri) 
{
    if (!IsValidUri(uri))
        return false;
     System.Diagnostics.Process.Start(uri);
     return true;
}

You cannot launch a web page from an elevated application. This will raise a 0x800004005 exception, probably because explorer.exe and the browser are running non-elevated.

To launch a web page from an elevated application in a non-elevated web browser, use the code made by Mike Feng. I tried to pass the URL to lpApplicationName but that didn't work. Also not when I use CreateProcessWithTokenW with lpApplicationName = "explorer.exe" (or iexplore.exe) and lpCommandLine = url.

The following workaround does work: Create a small EXE-project that has one task: Process.Start(url), use CreateProcessWithTokenW to run this .EXE. On my Windows 8 RC this works fine and opens the web page in Google Chrome.