c# - 声明一个空的 Queryable？

Question

如何声明这样的变量？

            var rData = from nc in ctx.NEWSLETTER_CLIENTS
                        join ni in ctx.NEWSLETTER_INDICES on nc.INDEX_NUM 
                                                          equals ni.INDEX_NUM
                        select new
                        {
                            ClientID = nc.CLIENT_ID,
                            Email = nc.CLIENT_EMAIL_ADDRESS,
                            Index = nc.INDEX_NUM,
                            MainClass = ni.MAIN_CLASS,
                            SubClass = ni.SUB_CLASS,
                            App1 = ni.VALUE_1,
                            App2 = ni.VALUE_2,
                            App3 = ni.VALUE_3,
                            App4 = ni.VALUE_4
                        };

        // Now I need to declare on a variable named fData under the function scope,
        // so I can later use it:

        var fData = ...; //What do I declare here?

        if(x)
            fData = fData.Concat(rData.Where(u => ...));
        if(y)
            fData = fData.Concat(rData.Where(u => ...));
        // etc

Answer 1

IQueryable<type of p> fData = null;

如果你想稍后使用查询(if 之后的 iow):

var fData = Enumerable.Empty<type of p>().AsQueryable();

更新:

现在用于匿名类型:

IQueryable<T> RestOfMethod<T>(IQueryable<T> rData)
{
  var fData = Enumerable.Empty<T>().AsQueryable(); // or = rData;

  if(x)
    fData = fData.Concat(rData.Where(u => ...));
  if(y)
    fData = fData.Concat(rData.Where(u => ...));

  return fData;
}

// original code location
var rData = some query;
var fData = RestOfMethod(rData);

更新 2:

正如所指出的，上面的代码实际上不起作用，因为 Where 的谓词不知道类型。您可以进一步重构它以在参数中包含谓词，例如:

IQueryable<T> RestOfMethod<T>(IQueryable<T> rData, 
  Expression<Func<T,bool>> pred1,
  Expression<Func<T,bool>> pred2) 
{ ... }

更新 3:(可能是 hacky)

var fData = rData.Take(0); // should be cheap.