/// <summary>
/// 验证谓词，无效时返回null
/// </summary>
/// <typeparam name="T">实体类型</typeparam>
/// <param name="predicate">谓词</param>
/// <returns></returns>
public static Expression <Func <T, bool> > ValidatePredicate <T>(Expression <Func <T, bool> > predicate)
{
predicate.CheckNull("predicate");
if (LambdaHelper.GetCriteriaCount(predicate) > 1)
{
throw new InvalidOperationException(string.Format("仅允许添加一个条件，条件：{0}", predicate));
}
if (predicate.Value() == null)
{
return(null);
}
if (string.IsNullOrWhiteSpace(predicate.Value().ToString()))
{
return(null);
}
return(predicate);
}