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    <p>You are not trying to extend <code>IQueryable</code> at all by the look of your example. Remove <code>this</code> from method definition and your example should work fine.</p> <pre><code>public class OrderByCollection { // .. shameless cut .. public IQueryable&lt;T&gt; ExecuteOrderBys&lt;T&gt;(IQueryable&lt;T&gt; source) { // .. we don't need no stinking body .. } } </code></pre> <p>Which will make your example work:</p> <pre><code>var q = db.tblJobHeaders; OrderByCollection OBys = new OrderByCollection(); OBys.AddOrderBy("some field", true); OBys.AddOrderBy("anotherfield", false); OBys.ExecuteOrderBys(q); </code></pre> <p>As a side note I would not use <code>bool</code> to define how the orderby's should be sorted. The code is unclear. Use a enum or different methods instead:</p> <pre><code>OBys.AddOrderBy("some field", Sort.Ascending); </code></pre> <p>or:</p> <pre><code>OBys.AddOrderByDescending("some field"); </code></pre> <h2>Update</h2> <p>Extension methods are used to "plugin" methods to existing classes or interfaces. By writing <code>public IQueryable&lt;T&gt; ExecuteOrderBys&lt;T&gt;(this IQueryable&lt;T&gt; source)</code> you are really saying that the method should be hooked into <code>IQueryable&lt;T&gt;</code>. And it should therefore be accessed like <code>myQuery.ExecuteOrderBys</code>.</p> <p>My guess is that extension methods must be static and be included in static classes to avoid confusion:</p> <p>a) They are really not members of the classes or interfaces and can not access anything else than public fields/properties/methods. </p> <p>b) They can extend any class. Without the restriction you could put a method call <code>DoSomething(this ThatObject instance)</code> in a class called <code>ThatObject</code>. The problem with would be that you can't access anything else than the public interface since it's an extension method. </p> <p>The confusing possibilities would be endless ;)</p>
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    1. POWhat does the error "Extension method must be static" mean?
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    1. COBrilliant thank you! I'm a tad confused by the this keyword in the method parameter (I was copying the method header from another similar function), does it mean it's being extended then?
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    2. COcheck my update.
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