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  • Dan Smith 13 posts 93 karma points
    Aug 11, 2016 @ 22:47
    Dan Smith
    0

    Error when filtering on multi-select dropdown menu

    I'm encountering a null reference error when I'm attempting to perform a filter based on values stored in a page's multi-select dropdown. I have this code in my template:

    var selection = Model.Content.Children.Where("Visible").ToList();
var filterCategories = new List<string>();
var filterProjectTypes = new List<string>();
if (Request.QueryString["category"] != null)
{
    filterCategories = Request.QueryString["category"].Split(',').ToList();
}

if (Request.QueryString["projectType"] != null)
{
    filterProjectTypes = Request.QueryString["projectType"].Split(',').ToList();
}

if (filterCategories.Any())
{
    selection = selection.Where(x => x.GetPropertyValue<string>("category").Split(',').Any(y => filterCategories.Contains(y))).ToList();
}

if (filterProjectTypes.Any())
{
    selection = selection.Where(x => x.GetPropertyValue<string>("projectType").Split(',').Any(y => filterProjectTypes.Contains(y))).ToList();
}

    So what's happening here is the page is pulling querystring values into lists and comparing them to the values stored in different multi-select dropdowns stored on each node. The filter on the "category" property works fine, but when it hits the exact same operation for the "projectTypes" property it gets a null reference error.

    I did some further testing and found that it's the .Split(',') operation that's causing it. For some reason it's not seeing the result of x.GetPropertyValue<string>("projectType") as an actual string. It seems to be...nothing. When I pull the value individually and attempt to perform any object operations on it, even a GetType(), I get the same error.

    It's odd that it doesn't have this issue with the "category" property but any other similar properties will give me the same error.

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  • Dan Smith 13 posts 93 karma points
    Aug 12, 2016 @ 14:56
    Dan Smith
    0

    Just to add to the mystery, I can make the page display these properties by grabbing them and looping through them, so I know I have the alias names right. As long as I'm not performing any other operations on them they show up on the page just fine.

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  • Nicholas Westby 2054 posts 7103 karma points c-trib
    Aug 12, 2016 @ 15:20
    Nicholas Westby
    100

    Try this:

    selection = selection.Select(x => new
{
    Value = x.GetPropertyValue<string>("projectType"),
    Content = x
})
.Where(x => !string.IsNullOrWhiteSpace(x.Value))
.Where(x => x.Value.Split(',').Any(y => filterProjectTypes.Contains(y)))
.Select(x => x.Content)
.ToList();
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  • Dan Smith 13 posts 93 karma points
    Aug 12, 2016 @ 15:31
    Dan Smith
    0

    Are you a wizard? That worked like a charm. Thanks! That's been bugging me for weeks.

    I'm still curious about why it's only seeing the category property as a string where the other properties seem to be type-less.

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  • Nicholas Westby 2054 posts 7103 karma points c-trib
    Aug 12, 2016 @ 16:41
    Nicholas Westby
    0

    why it's only seeing the category property as a string where the other properties seem to be type-less.

    I'm not sure I follow. What properties seem to be type-less? What observation leads you to believe they are type-less? What do you mean by "type-less" (e.g., dynamic, anonymous, casted as object, etc.)?

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  • Dan Smith 13 posts 93 karma points
    Aug 12, 2016 @ 17:11
    Dan Smith
    0

    When I select the property into a List<string> then loop through it and display it on the page, it displays fine until I attempt to do any type-specific actions on it.

    So if I do this:

    var thingy = selection.Select(x => x.GetPropertyValue<string>("projectType")).ToList();

    Then in the HTML of the page, do this:

    @foreach(var thing in thingy)
    {
        <p>@thing</p>
    }

    It lists the comma separated list of the projectType properties of each child node no problem. But if I do this:

    @foreach(var thing in thingy)
    {
        <p>@thing.ToString()</p>
    }

    I get the null reference error. I can't even do thing.GetType().

    However, if I change thingy to be a List<string> pulled from the category properties of each node, I can do ToString(), Split(), Substring(), etc.

    Both projectType and category properties are being stored in a "Dropdown list multiple", which should return a string of comma separated values, but only the category property is treated as if it's a string.

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  • Nicholas Westby 2054 posts 7103 karma points c-trib
    Aug 12, 2016 @ 17:18
    Nicholas Westby
    0

    Ah. I don't think it has anything to do with the type. In both instances, it is a string.

    The reason you are getting the null reference exception is because one of the nodes contains no projectType value. Because it is a null value, calling thing.ToString() produces a null reference exception, as you are attempting to call an instance function on a null value.

    In other words, it is a string; it's just a null string. That is why my suggestion worked; because it filters out any null values. It may not have been apparent because you wouldn't be able to see the null value visibly with this code:

    <p>@thing</p>

    It would be more apparent if you did this instead:

    <p>@(thing == null ? "null value" : thing)</p>

    If it is null, you will see "null value" rather than nothing at all.

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  • Dan Smith 13 posts 93 karma points
    Aug 12, 2016 @ 17:27
    Dan Smith
    0

    That makes total sense. Thanks again!

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