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POLinq query with multiple OrderBy statements added in a loop
 

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  1. POLinq query with multiple OrderBy statements added in a loop
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    <p>I have a method in a webservice that has parameter with which users can decide how they want to order their results. This is a <code>List(Of String)</code> with the names of the fields in the order they want to sort them.</p> <p>I know I can normally order on multiple columns by doing the following</p> <pre><code>Dim test = Bars.OrderBy(Function(x) x.Foo) _ .ThenBy(Function(x) x.Bar) _ .ThenBy(Function(x) x.Test) </code></pre> <p>However in this case this won't work since I can't chain the <code>ThenBy</code> function because I'm adding the sorting orders in a loop. To use <code>ThenBy</code> I need an <code>IOrderedQueryable</code> collection. This is how I would want it to work</p> <pre><code>Dim sortColumns = {"Foo", "Bar", "Test"} Dim query = From b in Bars For each column in sortColumns Select Case column Case "Foo" query = query.Orderby(Function(x) x.Foo) Case "Bar" query = query.Orderby(Function(x) x.Bar) Case "Test" query = query.Orderby(Function(x) x.Test) End Select Next Dim result = query.Select(Function(x) x.x).ToList() Return result </code></pre> <p>This of course won't work because <code>OrderBy</code> will replace any previous ordering. The only solution I can think of is ordering the list on some other variable first so I already have an <code>IOrderedQueryable</code> collection but this just seems like the wrong approach.</p> <pre><code>Dim bars As New List(Of Bar) Dim sortColumns = {"Foo", "Bar", "Test"} Dim query = bars.Select(Function(x) New With {.Temp = 1, .x = x}) _ .OrderBy(Function(x) x.Temp) For Each column In sortColumns Select Case column Case "Foo" query = query.ThenBy(Function(x) x.x.Foo) Case "Bar" query = query.ThenBy(Function(x) x.x.Bar) Case "Test" query = query.ThenBy(Function(x) x.x.Test) End Select Next Dim result = query.Select(Function(x) x.x).ToList() Return result </code></pre>
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    1. USFreek Buurman
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      1. POLinq query with multiple OrderBy statements added in a loop
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    1. CO"OrderBy will replace any previous ordering". Untrue. OrderBy preserves the prior orderings... meaning: it is a stable re-ordering. Also meaning: it uses the prior ordering to break ties in the current ordering. So: OrderBy(Foo).ThenBy(Bar).ThenBy(Test) is the same as OrderBy(Test).OrderBy(Bar).OrderBy(Foo)
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      1. USAmy B
 

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