All CLA requirements met.

Tagging subscribers to this area: @dotnet/ncl
See info in area-owners.md if you want to be subscribed.

This could be a break change.

Consider if users find UriBuilder.Uri.ToString() is buggy so they write a helper ToFullEncodedString as below:

var builder = new UriBuilder("https://www.microsoft.com")
{
    UserName = "mallory@hackers.net",
    Password = "hunter2",
};
Console.WriteLine(builder.ToFullEncodedString());

public static class UriBuilderExtension
{
    public static string ToFullEncodedString(this UriBuilder builder)
    {
        string plainUserName = builder.UserName;
        builder.UserName = System.Web.HttpUtility.UrlEncode(plainUserName);
        string result = builder.Uri.ToString();
        builder.UserName = plainUserName;
        return result;
    }
}

I suppose it's possible.

We can avoid taking that risk by deferring the encoding to the ToString instead of the setters.

And maybe : should also be encoded there.

I would avoid changing : for now. Uri doesn't enforce the exact format of UserInfo.
That is, new Uri("http://name:::password@host/").UserInfo is valid and changing it would be a breaking change.

Thanks for the feedback--I have pushed a change that I believe will address this by moving the EncodeUserInfo call to the ToString() method per MihaZupan's comment, and also added a new unit test for this situation. Please let me know if any other changes should be made, thank you!

Test failure is #74795