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    <p>Ooh, this looks like a <em>fun</em> problem :)</p> <p>So first, let's set up our faux-source, since I don't have your DB handy:</p> <pre><code>// SETUP: fake up a data source var folks = new[]{"Alex", "James", "Jessica"}; var cats = new[]{"C#", "VB.NET", "LINQ"}; var r = new Random(); var entryCount = 100; var entries = from i in Enumerable.Range(0, entryCount) let id = r.Next(0, 999999) let person = folks[r.Next(0, folks.Length)] let category = cats[r.Next(0, cats.Length)] let date = DateTime.Now.AddDays(r.Next(0, 100) - 50) select new Journal() { Id = id, AuthorName = person, Category = category, CreatedAt = date }; </code></pre> <p>Ok, so now we've got a set of data to work with, let's look at what we want...we want something with a "shape" like:</p> <pre><code>public Expression&lt;Func&lt;Journal, ????&gt;&gt; GetThingToGroupByWith( string[] someMagicStringNames, ????) </code></pre> <p>That has roughly the same functionality as (in pseudo code):</p> <pre><code>GroupBy(x =&gt; new { x.magicStringNames }) </code></pre> <p>Let's dissect it one piece at a time. First, how the heck do we do this dynamically?</p> <pre><code>x =&gt; new { ... } </code></pre> <p>The compiler does the magic for us normally - what it does is define a new <code>Type</code>, and we can do the same:</p> <pre><code> var sourceType = typeof(Journal); // define a dynamic type (read: anonymous type) for our needs var dynAsm = AppDomain .CurrentDomain .DefineDynamicAssembly( new AssemblyName(Guid.NewGuid().ToString()), AssemblyBuilderAccess.Run); var dynMod = dynAsm .DefineDynamicModule(Guid.NewGuid().ToString()); var typeBuilder = dynMod .DefineType(Guid.NewGuid().ToString()); var properties = groupByNames .Select(name =&gt; sourceType.GetProperty(name)) .Cast&lt;MemberInfo&gt;(); var fields = groupByNames .Select(name =&gt; sourceType.GetField(name)) .Cast&lt;MemberInfo&gt;(); var propFields = properties .Concat(fields) .Where(pf =&gt; pf != null); foreach (var propField in propFields) { typeBuilder.DefineField( propField.Name, propField.MemberType == MemberTypes.Field ? (propField as FieldInfo).FieldType : (propField as PropertyInfo).PropertyType, FieldAttributes.Public); } var dynamicType = typeBuilder.CreateType(); </code></pre> <p>So what we've done here is define a custom, throwaway type that has one field for each name we pass in, which is the same type as the (either Property or Field) on the source type. Nice!</p> <p>Now how do we give LINQ what it wants?</p> <p>First, let's set up an "input" for the func we'll return:</p> <pre><code>// Create and return an expression that maps T =&gt; dynamic type var sourceItem = Expression.Parameter(sourceType, "item"); </code></pre> <p>We know we'll need to "new up" one of our new dynamic types...</p> <pre><code>Expression.New(dynamicType.GetConstructor(Type.EmptyTypes)) </code></pre> <p>And we'll need to initialize it with the values coming in from that parameter...</p> <pre><code>Expression.MemberInit( Expression.New(dynamicType.GetConstructor(Type.EmptyTypes)), bindings), </code></pre> <p>But what the heck are we going to use for <code>bindings</code>? Hmm...well, we want something that binds to the corresponding properties/fields in the source type, but remaps them to our <code>dynamicType</code> fields...</p> <pre><code> var bindings = dynamicType .GetFields() .Select(p =&gt; Expression.Bind( p, Expression.PropertyOrField( sourceItem, p.Name))) .OfType&lt;MemberBinding&gt;() .ToArray(); </code></pre> <p>Oof...nasty looking, but we're still not done - so we need to declare a return type for the <code>Func</code> we're creating via Expression trees...when in doubt, use <code>object</code>! </p> <pre><code>Expression.Convert( expr, typeof(object)) </code></pre> <p>And finally, we'll bind this to our "input parameter" via <code>Lambda</code>, making the whole stack:</p> <pre><code> // Create and return an expression that maps T =&gt; dynamic type var sourceItem = Expression.Parameter(sourceType, "item"); var bindings = dynamicType .GetFields() .Select(p =&gt; Expression.Bind(p, Expression.PropertyOrField(sourceItem, p.Name))) .OfType&lt;MemberBinding&gt;() .ToArray(); var fetcher = Expression.Lambda&lt;Func&lt;T, object&gt;&gt;( Expression.Convert( Expression.MemberInit( Expression.New(dynamicType.GetConstructor(Type.EmptyTypes)), bindings), typeof(object)), sourceItem); </code></pre> <p>For ease of use, let's wrap the whole mess up as an extension method, so now we've got:</p> <pre><code>public static class Ext { // Science Fact: the "Grouper" (as in the Fish) is classified as: // Perciformes Serranidae Epinephelinae public static Expression&lt;Func&lt;T, object&gt;&gt; Epinephelinae&lt;T&gt;( this IEnumerable&lt;T&gt; source, string [] groupByNames) { var sourceType = typeof(T); // define a dynamic type (read: anonymous type) for our needs var dynAsm = AppDomain .CurrentDomain .DefineDynamicAssembly( new AssemblyName(Guid.NewGuid().ToString()), AssemblyBuilderAccess.Run); var dynMod = dynAsm .DefineDynamicModule(Guid.NewGuid().ToString()); var typeBuilder = dynMod .DefineType(Guid.NewGuid().ToString()); var properties = groupByNames .Select(name =&gt; sourceType.GetProperty(name)) .Cast&lt;MemberInfo&gt;(); var fields = groupByNames .Select(name =&gt; sourceType.GetField(name)) .Cast&lt;MemberInfo&gt;(); var propFields = properties .Concat(fields) .Where(pf =&gt; pf != null); foreach (var propField in propFields) { typeBuilder.DefineField( propField.Name, propField.MemberType == MemberTypes.Field ? (propField as FieldInfo).FieldType : (propField as PropertyInfo).PropertyType, FieldAttributes.Public); } var dynamicType = typeBuilder.CreateType(); // Create and return an expression that maps T =&gt; dynamic type var sourceItem = Expression.Parameter(sourceType, "item"); var bindings = dynamicType .GetFields() .Select(p =&gt; Expression.Bind( p, Expression.PropertyOrField(sourceItem, p.Name))) .OfType&lt;MemberBinding&gt;() .ToArray(); var fetcher = Expression.Lambda&lt;Func&lt;T, object&gt;&gt;( Expression.Convert( Expression.MemberInit( Expression.New(dynamicType.GetConstructor(Type.EmptyTypes)), bindings), typeof(object)), sourceItem); return fetcher; } } </code></pre> <p>Now, to use it:</p> <pre><code>// What you had originally (hand-tooled query) var db = entries.AsQueryable(); var query = db.GroupBy(x =&gt; new { Year = x.CreatedAt.Year, Month = x.CreatedAt.Month }, prj =&gt; prj.AuthorName) .Select(data =&gt; new { Key = data.Key.Year * 100 + data.Key.Month, // very ugly code, I know Details = data.GroupBy(y =&gt; y).Select(z =&gt; new { z.Key, Count = z.Count() }) }); var func = db.Epinephelinae(new[]{"CreatedAt", "AuthorName"}); var dquery = db.GroupBy(func, prj =&gt; prj.AuthorName); </code></pre> <p>This solution lacks the flexibility of "nested statements", like "CreatedDate.Month", but with a bit of imagination, you could possibly extend this idea to work with any freeform query.</p>
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    1. CO+1 for a thorough explanation + hardwork.
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    3. COthanks @JerKimball, very good & interesting explanation.
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