求一恒真恒假的 lambda Express 表达式
当Value为NULL时，如果返回一个真的表达式  

1 方法如下 :

 public static Expression<Func<T, bool>> ExpressionEqual<T, P>(this T entity, string property, P value) where T:class
  {

  if (value == null)
  {
  //
  }

  ParameterExpression param = Expression.Parameter(typeof(T), "entity");

  Expression left = Expression.Property(param, typeof(T).GetProperty(property));  

  left = Expression.Convert(left, value.GetType());
  Expression right = Expression.Constant(value);
  Expression filter = Expression.Equal(left, right);
  Expression<Func<T, bool>> predicate = Expression.Lambda<Func<T, bool>>(filter, param);
  return predicate;
  }

2 外部调用 
  class aClass 
  {
  public string code {get;set}
  }

  var a = new aClass()
  var queryWhere = a.ExpressionEqual("code",a.code);
 
  var resule = aRespostion.getAll().where(queryWhere).tolist();

 

------解决方案--------------------
if (value == null)
{
return a=> true;
}