<SharePointPermissionAttribute(SecurityAction.InheritanceDemand, ObjectModel:=True)> _
<SharePointPermissionAttribute(SecurityAction.LinkDemand, ObjectModel:=True)> _
Public Class LinkFieldValue
    Inherits HtmlTagValue

Dim instance As LinkFieldValue

[SharePointPermissionAttribute(SecurityAction.InheritanceDemand, ObjectModel=true)] 
[SharePointPermissionAttribute(SecurityAction.LinkDemand, ObjectModel=true)] 
public class LinkFieldValue : HtmlTagValue

using SPListItem = Microsoft.SharePoint.SPListItem;
using LinkFieldValue = Microsoft.SharePoint.Publishing.Fields.LinkFieldValue;

namespace Microsoft.SDK.SharePointServer.Samples
{
public static class LinkFieldValueSamples
  {
  // You can change any of the following data that will be set into the LinkFieldValue.
  private const string NewNavigateUrl = "/Documents/SampleFile.docx";
  private const bool NewUseDefaultIcon = false;
  private const string NewTarget = "_blank";
  private const string NewToolTip = "Sample tool tip for the link";
  private const string NewTextPrefix = "Open [";
  private const string NewTextSuffix = "] in a new window";

  // You can change the following default LinkFieldValue HTML markup.
  private const string DefaultLinkFieldValueHtml = "<A href=\"/Documents/SampleFile.docx\" target=\"_blank\"><IMG src=\"/_layouts/images/icdoc.gif\" alt=\"\">Open [SampleFile.docx] in a new window</a>";

  public static string GetAndSetLinkFieldValue(
  SPListItem listItemWithLinkField,
  string linkFieldName)
    {
    if (null == listItemWithLinkField)
      {
      throw new System.ArgumentNullException("listItemWithLinkField", "The listItemWithLinkField argument must be a valid SPListItem");
      }

    if (string.IsNullOrEmpty(linkFieldName))
    {
    throw new System.ArgumentNullException("linkFieldName", "The linkFieldName argument must be a valid link field name on the SPListItem");
    }
    // Retrieve the current value from an SPListItem object
    // with a column of the LinkField type with the
    // name linkFieldName.
    LinkFieldValue currentFieldValue = 
    listItemWithLinkField[linkFieldName] as LinkFieldValue;

    // If there is no current value, construct a new empty value.
    if (null == currentFieldValue)
    {
      currentFieldValue = new LinkFieldValue();
    }

    // If this property is not set to a value, the ToString() for the
    //  LinkFieldValue returns String.Empty and nothing is stored in
    //  the field value when it is set back into the SPListItem.
    currentFieldValue.NavigateUrl = NewNavigateUrl;

    // These properties are optional for a LinkFieldValue.
     currentFieldValue.Target = NewTarget;
     currentFieldValue.ToolTip = NewToolTip;
     currentFieldValue.UseDefaultIcon = NewUseDefaultIcon;

    // Use the static GetDefaultIconUrl method to find the correct
    //  icon URL in the given Web site for the file extension in NewNavigateUrl.
    currentFieldValue.IconUrl =
    LinkFieldValue.GetDefaultIconUrl(NewNavigateUrl, listItemWithLinkField.Web);

     // Use the static GetDefaultDisplayText method to find the default
     // link text for the given NewNavigateUrl.
     currentFieldValue.Text = NewTextPrefix +
     LinkFieldValue.GetDefaultDisplayText(NewNavigateUrl) +
     NewTextSuffix;

    // The changes to the properties of the LinkFieldValue object are
    // local to this specific object instance and not stored in the SPListItem.
    // To store the changes, the value must be set back to the SPListItem field value.
    listItemWithLinkField[linkFieldName] = currentFieldValue;
    listItemWithLinkField.Update();

    // The ToString() returns the HTML markup that reflects the new LinkFieldValue properties.
    return currentFieldValue.ToString();
    }
  }
}