ASP.NET C# LINQ conditional WHERE clause

Question

I'm trying to get values from numerous dropdown lists and build a where statement depending on the options selected. If they are not slected then they should be excluded from the select statement.

This is how I would have done it but I gather that it can not be done in this way with linq.

 IEnumerable<IGrouping<string, Forest>> treeQuery =

        from trees in Forest
        if (ddlType1.SelectedValue!=null)
        {
            string strWhere += trees.Type1 == ddlType1.SelectedValue
        }
        else if (ddlType2.SelectedValue!=null)
        {
            string strWhere += trees.Type2 == ddlType2.SelectedValue
        }
        where strWhere
        orderby trees.Nuts
        group trees by trees.TrunkColour;

Any help would be greatly appreciated.

This is the code before I added the example in...

 IEnumerable<IGrouping<string, Forest>> treeQuery =

        from trees in Forest
        where trees.Type1 == "oak"
        orderby trees.Nuts
        group trees by trees.TrunkColour;

your second example is making an assignment where I think you want to test for equality i.e. you have = when I think you meant == — Russ Cam
– Russ Cam, Commented Nov 1, 2011 at 20:17
if a user selects both a value from ddlType1 and ddlType2 should the ddlType2 be ignored as per your example? — Rune FS
– Rune FS, Commented Nov 1, 2011 at 20:33

Jon Skeet · Accepted Answer · 2011-11-01 20:22:44Z

10

In this situation you can use the compositional nature of queries, but you don't want to use query expressions. So:

// Or IEnumerable<Forest>, depending on the type involved...
IQueryable<Forest> query = Forest;
if (ddlType1.SelectedValue!=null)
{
    query = query.Where(trees => trees.Type1 == ddlType1.SelectedValue);
}
else if (ddlType2.SelectedValue!=null)
{
    query = query.Where(trees => trees.Type2 == ddlType2.SelectedValue);
}
var finalQuery = query.OrderBy(trees => tree.Nuts)
                      .GroupBy(trees => trees.TrunkColour);

This won't actually execute the query until you start using the results - so you can add filtering, ordering etc bit by bit until you're "ready to go".

answered Nov 1, 2011 at 20:22

Jon Skeet

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Comments

sq33G · Accepted Answer · 2011-11-01 20:43:33Z

2

For each one of your dropdowns, you can add a clause to your where:

where (ddlType1.SelectedValue == "" || trees.Type1 == ddlType1.SelectedValue)
    && (ddlType2.SelectedValue == "" || trees.Type2 == ddlType2.SelectedValue)
    // && ( type 3... )

answered Nov 1, 2011 at 20:43

sq33G

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1 Comment

user1024416 Over a year ago

Many thanks for all of the replies. This is the solution I ended up using.

Johannes Kommer · Accepted Answer · 2011-11-01 20:27:02Z

1

As this only involves two conditions you could do it in a single LINQ query as such:

Forest.Where(tree => 
        (
            (ddlType1.SelectedValue == null || tree.Type1 == ddlType1.SelectedValue) &&
            (ddlType2.SelectedValue == null || tree.Type2 == ddlType2.SelectedValue)
        ))
        .OrderBy(tree => tree.Nuts)
        .GroupBy(tree => tree.TrunkColour);

Although as the number of conditions increases, it's worth splitting out the query into seperate parts for improved readability as this format can potentially get unreadable fast with multiple variables.

edited Nov 1, 2011 at 20:27

answered Nov 1, 2011 at 20:20

Johannes Kommer

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