Change SharePoint group permissions from code

Just “quick and dirty”

public static void ChangePermissions(SPSecurableObject subject, SPPrincipal principal, SPRoleType roleType)
{
    SPRoleDefinition roleDefinition = null;
    SPRoleAssignment roleAssignment = subject.RoleAssignments.GetAssignmentByPrincipal(principal);

    if (roleAssignment != null && subject != null)
    {
        switch (subject.GetType().Name)
        {
            case "SPList":
                if (!((SPList)subject).HasUniqueRoleAssignments)
                    ((SPList)subject).BreakRoleInheritance(true,false);
                roleDefinition = ((SPList)subject).ParentWeb.RoleDefinitions.GetByType(roleType);
                break;
            case "SPWeb":
                if (!((SPWeb)subject).HasUniqueRoleAssignments)
                    ((SPWeb)subject).BreakRoleInheritance(true, false);
                roleDefinition = ((SPWeb)subject).RoleDefinitions.GetByType(roleType);
                break;
            case "SPItem":
                if (!((SPItem)subject).HasUniqueRoleAssignments)
                    ((SPItem)subject).BreakRoleInheritance(true, false);
                roleDefinition = ((SPItem)subject).Fields.List.ParentWeb.RoleDefinitions.GetByType(roleType);
                break;
            case "SPListItem":
                if (!((SPListItem)subject).HasUniqueRoleAssignments)
                    ((SPListItem)subject).BreakRoleInheritance(true, false);
                roleDefinition = ((SPListItem)subject).Fields.List.ParentWeb.RoleDefinitions.GetByType(roleType);
                break;
            default:
                break;
        }

        if (roleDefinition != null)
        {
            roleAssignment.RoleDefinitionBindings.RemoveAll();
            roleAssignment.RoleDefinitionBindings.Add(roleDefinition);
            roleAssignment.Update();
        }
    }
}

Implementation:

static void Main(string[] args)
{
    using(SPSite site = new SPSite("http://devsp"))
    {
        using(SPWeb web = site.RootWeb)
        {
            SPList list = web.Lists.TryGetList("TestList");
            SPListItem item = list.Items[0]; //grab first item

            string groupName = "MyGroup";
            ChangePermissions((SPSecurableObject)web, web.SiteGroups.GetByName("groupName"), SPRoleType.Contributor);
            ChangePermissions((SPSecurableObject)list, web.SiteGroups.GetByName("groupName"), SPRoleType.Contributor);
            ChangePermissions((SPSecurableObject)item, web.SiteGroups.GetByName("groupName"), SPRoleType.Contributor);
        }
    }
}

Originally posted on my blog