Linq查询语法与方法链：返回类型

Question

Reading here and here , I understand query syntax and method extensions are pretty much a syntactic difference. But now, I've a data structure containing measurement data and want determine percentile. I write:

var ds = (from device in dataSet.devices
          where !device.paramValues[i].Equals(double.NaN)
          select device.paramValues[i]).OrderBy(val => val);
double median = percentile(ds as IOrderedEnumerable<double>, 0.5);

and all works fine. ds is of type System.Linq.OrderedEnumerable<double,double>

What confuses me is to write the whole thing in query syntax:

var ds = (from device in dataSet.devices
          where !device.paramValues[i].Equals(double.NaN)
          orderby device.paramValues[i]
          select device.paramValues[i]);
double median = percentile(ds as IOrderedEnumerable<double>, 0.5);

Now, ds is of type System.Linq.Enumerable.WhereSelectEnumerableIterator<Dataset.DeviceData,double> and the call to the percentile function fails.

Not sure yet what I'm missing here... - there is no praticular reason why I'd prefer the second syntax, but I'd like to understand the difference ... Thanks for your help!

Answer 1

from device in dataSet.devices
where !device.paramValues[i].Equals(double.NaN)
select device.paramValues[i]

is transformed into methods as follows:

dataSet.devices
       .Where(device => !device.paramValues[i].Equals(double.NaN))
       .Select(device => device.paramValues[i]);

Adding your OrderBy call you get

dataSet.devices
       .Where(device => !device.paramValues[i].Equals(double.NaN))
       .Select(device => device.paramValues[i])
       .OrderBy(val => val);

The other query

from device in dataSet.devices
where !device.paramValues[i].Equals(double.NaN)
orderby device.paramValues[i]
select device.paramValues[i];

is transformed to

dataSet.devices
       .Where(device => !device.paramValues[i].Equals(double.NaN))
       .OrderBy(device => device.paramValues[i])
       .Select(device => device.paramValues[i]);

As you can see, it's not exactly the same method chain, and that's because you get different object as a result. Content is the same, but the type returned is not.

Answer 2

Look at the other answer first. Here is how you do the same with query syntax:

from device in dataSet.devices
where !device.paramValues[i].Equals(double.NaN)
select device.paramValues[i] into x //into syntax
orderby x
select x;