SharePoint 2007 Blog

Here is the code for creating a custom field type in SharePoint that gives you field level security within your site. I used Visual Studio 2008 and the SharePoint blank template to develop this field control. When you create a SharePoint blank project you can right click on the project in Visual Studio and click Add->New Item, choose SharePoint in the categories section, and choose Field Control. Once you deploy the wsp file to your farm you just create a site column based on this field type and away you go plugging it into your lists and libraries.

fieldtypes_PermissionedField.xml

<?xml version="1.0" encoding="utf-8"?>
<FieldTypes>
  <FieldType>
    <Field Name="TypeName">SecureField</Field>
    <Field Name="TypeDisplayName">Secure Field</Field>
    <Field Name="TypeShortDescription">Permission Controlled Field</Field>
    <Field Name="ParentType">Text</Field>
    <Field Name="UserCreatable">TRUE</Field>
    <Field Name="FieldTypeClass">11dd856b-f0bc-4845-a394-59caf1634b57</Field>
  <PropertySchema>
   <Fields>
    <Field Name="PriviligedUsers" StaticName="PriviligedUsers" DisplayName="Priviliged Users" Type="User" Required="TRUE">
    </Field>
   </Fields>
  </PropertySchema>
  </FieldType>
</FieldTypes>

PermissionedField.FieldControl.cs (I'm using Log4Net for logging. Feel free to remove.)

protected override void CreateChildControls()
   {
    base.CreateChildControls();

    try
    {
     if (this.ControlMode == SPControlMode.Edit)
     {
      //Declare the necessary variables
      SPUser tmpPropUser, tmpUser;
      SPField tmpField = this.Field;
      Boolean tmpHasAccess = false;
      int tmpID;

      //Get a reference to the PriviligedUser property
      //associated with this field control
      SPFieldLookupValue tmpCustomProperty = (SPFieldLookupValue)tmpField.GetCustomProperty("PriviligedUsers");
      SPFieldUserValue tmpUserVal = (SPFieldUserValue)tmpField.GetCustomProperty("PriviligedUsers");

      if (tmpUserVal != null && tmpCustomProperty != null)
      {
       //Get the ID of the user or group that is set in the PriviligedUser propery
       tmpID = tmpCustomProperty.LookupId;

       //Get a reference to the user listed set in the PriviligedUser property
       tmpPropUser = tmpUserVal.User;

       //Get a reference to the current user
       tmpUser = SPContext.Current.Web.CurrentUser;

       if (tmpPropUser != null) //The property is set to a SPUser object
       {
        //Check to see of the user specified in the PriviligedUser property
        //is the same as the current user
        if (tmpPropUser.LoginName == tmpUser.LoginName)
        {
         tmpHasAccess = true;
        }
       }
       else //The property is set to a SPGroup object
       {
        //Determine if the user belongs to the security group set
        //in the PriviligedUsers custom property
        foreach (SPGroup tmpGroup in tmpUser.Groups)
        {
         //Enumerate through each group to see if it
         //matches the group id in the PriviligedUsers property
         if (tmpGroup.ID == tmpID)
         {
          tmpHasAccess = true;
          break;
         }
        }
       }

       //Check the users permissions
       if (!tmpHasAccess)
       {
        //Log information to the log
        //_log.Info("Url:" + base.Web.Url + "; List:" + base.List + "; " + tmpUser.LoginName + " does not have permissions to edit the field " + base.FieldName);

        //Set the Control Mode of the Control to Display instead of Edit
        base.ControlMode = SPControlMode.Display;
        base.ItemContext.FormContext.FormMode = SPControlMode.Display;
       }
      }
     }
    }
    catch (Exception ex)
    {
     //Log the caught exception
     _log.Error(ex);

     //Ensure that the exception is thrown after it is caught
     throw ex;
    }
   }