Well it is just a simple copy of the umbraco internals that are not public. You have to be careful regarding future version. No one knows what will change ;-)

/// <summary>
/// Helper class to render the content of an umbraco HTML field.
/// </summary>
public class FieldRenderer
{
/// <summary>
/// Renders the content of an umbraco docment type property and returns the result
/// as a HTML string.
/// </summary>
/// <param name="alias">The alias of the property to render.</param>
/// <param name="contentId">The ID of the content element containing the property.</param>
/// <returns>The content of an umbraco document type property as HTML string.</returns>
public HtmlString GetContent(string alias, int contentId)
{
if (string.IsNullOrEmpty(alias))
{
throw new ArgumentNullException("alias");
}
 
var item = new Item
{
Field = alias,
TextIfEmpty = string.Empty,
LegacyAttributes = GetAttributeCollectionAdapter(alias),
NodeId = contentId.ToString(CultureInfo.CurrentCulture)
};

var containerPage = new FormlessPage();
containerPage.Controls.Add(item);

using (var output = new StringWriter(CultureInfo.CurrentCulture))
using (var htmlWriter = new HtmlTextWriter(output))
{
ItemRenderer.Instance.Init(item);
ItemRenderer.Instance.Load(item);
ItemRenderer.Instance.Render(item, htmlWriter);
return new HtmlString(output.ToString());
}
}

/// <summary>
/// Creates a new <see cref="AttributeCollectionAdapter"/> with the required
/// attributes to render a field.
/// </summary>
/// <param name="field">The alias of the field to create the adapter for.</param>
/// <returns>The created <see cref="AttributeCollectionAdapter"/> object.</returns>
private static AttributeCollectionAdapter GetAttributeCollectionAdapter(string field)
{
var attributes = new Dictionary<string, string>
{
{ "field", field },
{ "recursive", "false" },
{ "useifempty", string.Empty },
{ "textifempty", string.Empty },
{ "stripparagraph", "false" },
{ "case", string.Empty },
{ "inserttextbefore", string.Empty },
{ "inserttextafter", string.Empty },
{ "convertlinebreaks", "false" },
{ "formatasdate", "false" },
{ "formatasdatewithtime", "false" },
{ "formatasdatewithtimeseparator", string.Empty }
};

var attributesForItem = new AttributeCollectionAdapter(
new AttributeCollection(
new StateBag()));

foreach (var i in attributes)
{
attributesForItem.Add(i.Key, i.Value);
}

return attributesForItem;
        }
}

/// <summary>
/// Represents a formless .aspx file to be able to render a control outside the current page context
/// without verification of the server form.
/// </summary>
internal class FormlessPage : Page
{
/// <summary>
/// Handles the server form verification. Stays empty to avoid the
/// verification.
/// </summary>
/// <param name="control">The <see cref="Control"/> object to verify.</param>
public override void VerifyRenderingInServerForm(Control control)
{
// Empty to avoid the verification
}
}