NancyFx超级简单视图引擎-替代Linq Enumerables

Question

I've found that if you expose a public property on your Model in SSVE of type IEnumerable<T> and return a Linq enumeration from that property, SSVE doesn't seem to think there are any values in that collection (checking @If.HasValues , so @Model.Values is the enumerable).

For example, my C# property is:

public IEnumerable<string> Warnings
{
    get
    {
        return WarningGenerators.SelectMany(w => w.GetWarnings());
    }
}

In this case, the following html doesn't show up at all (it's not present in the DOM) in the generated page:

@If.HasWarnings
<div class="row">
    <div class="col-lg-12">
        @Partial['WarningsWidget.sshtml', Model.Warnings]
    </div>
</div>
@EndIf

But if I change my C# to this (with no other changes) the html shows up:

public IEnumerable<string> Warnings
{
    get
    {
        return WarningGenerators.SelectMany(w => w.GetWarnings()).ToList();
    }
}

Clearly the call to ToList() is enumerating the enumerable and the entries are then being picked up by SSVE. I'm surprised SSVE doesn't enumerate the collections passed into it though.

Is this behavior intended? Or have I missed something?

Answer 1

I don't know SSVE or Nancy that well, but I do know that sometimes LINQ and Entity Framework don't like when you only execute the query on the MVC page (which is what your first method is doing, as SelectMany() employs deferred execution ). When your query is executed completely on the backend (as when .ToList() is used which employs immediate execution ), it tends to like that a lot better, as the items then are already loaded into memory.

If that is what's going on, at least you have the comfort of knowing that it happens like this in other places as well.