/// <summary>
 /// 当前表达式是否能与指定表达式合并
 /// </summary>
 /// <param name="expression"></param>
 /// <returns></returns>
 public virtual bool CanCombine(ExpressionBase expression)
 {
     return(expression == null);
 }
 public override bool CanCombine(ExpressionBase expression)
 {
     return(expression == null || expression is TextConstantExpression);
 }