ASP.NET C# LINQ conditional WHERE clause

问题:

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; 

回答1:

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... )

回答2:

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".

回答3:

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.