Code to give one role (Read, Contribute, etc) to an SPPrincipal

GivePrincipalRoleToItem.cs

public static void GivePrincipalRoleToItem(SPWeb web, SPListItem item, SPPrincipal principal, string roleDefinitionName)
{
    if (null == principal)
    {
        throw new ArgumentException(string.Format("Unable to give role '{0}'; passed principal is null.", roleDefinitionName));
    }

    bool exists = false;
    SPRoleAssignment roleAssignment = null;
    try
    {
        roleAssignment = item.RoleAssignments.GetAssignmentByPrincipal(principal);
        exists = true;
    }
    catch (Exception)
    {
        // not found in role assignment
    }

    if (null == roleAssignment)
    {
        roleAssignment = new SPRoleAssignment(principal);
    }

    if (exists)
    {
        // the group should only have the role that was specified
        // if we don't remove them all, it will do an "append"
        // ex: if it has Read and we are adding Contribute, will have
        // Read AND Contribute
        roleAssignment.RoleDefinitionBindings.RemoveAll();
    }

    SPRoleDefinition roleDefinition = web.RoleDefinitions[roleDefinitionName];
    roleAssignment.RoleDefinitionBindings.Add(roleDefinition);

    if (exists)
    {
        roleAssignment.Update();
    }
    else
    {
        item.RoleAssignments.Add(roleAssignment);
    }
}