How to add and remove group programmativally from list permissions in sharepoint 2010?

I have the following code to add a group with a permission level to a sharepoint list

SPGroup Contribute = web.SiteGroups["Staff"];
SPRoleDefinition ContributeDefinition = web.RoleDefinitions["Read"];
SPRoleAssignment ContributeRoleAssignment = new SPRoleAssignment(Contribute);
ContributeRoleAssignment.RoleDefinitionBindings.Add(ContributeDefinition);开发者_开发问答
list.RoleAssignments.Add(ContributeRoleAssignment);

which adds the group, but how would I do it if I want to revoke the permission to the list for this group?

thanks in advance

---

If you need to remove whole role assgnment you can do:

SPUser user = ...;
list.RoleAssignments.Remove(user);

If you need to revoke some definition binding you can do:

SPUser user = ...;
list.RoleAssignments.GetAssignmentByPrincipal(user).RoleDefinitionBindings.Remove(..);

If that is not you want, please describe in more details.

UPDATE: Here is the sample code which I've tested and it works. It will remove Contribute role and add Read role to target group. Please note, that I have non-english Sharepoint version, so ensure the names of roles.

static void Main(string[] args)
{
  using (SPSite site = new SPSite("site_url"))
  {
    using (SPWeb web = site.OpenWeb())
    {
      SPList list = web.GetList(web.Url + "/" + "list_url");
      SPGroup group = web.SiteGroups["target_group_name"];
      SPRoleDefinition contributeDef = web.RoleDefinitions["Contribute"];
      SPRoleDefinition readDef = web.RoleDefinitions["Read"];
      SPRoleAssignment contributeRole = new SPRoleAssignment(group);
      contributeRole.RoleDefinitionBindings.Add(contributeDef);
      if (!list.HasUniqueRoleAssignments) // required to make role change
        list.BreakRoleInheritance(true);
      var assignmentForGroup = list.RoleAssignments.GetAssignmentByPrincipal(group);
      assignmentForGroup.RoleDefinitionBindings.Remove(contributeDef);
      assignmentForGroup.RoleDefinitionBindings.Add(readDef);
      assignmentForGroup.Update();
    }
  }
}