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copied!<p>It is compiled in the following way:</p> <ol> <li><p>First, the LINQ query expression is transformed into method calls:</p> <pre><code>public static void Main() { var query = db.Cars.Select<Car, Car>(c => c); foreach (Car aCar in query) { Console.WriteLine(aCar.Name); } } </code></pre></li> <li><p>If <code>db.Cars</code> is of type <code>IEnumerable<Car></code> (which it is for LINQ-to-Objects), then the lambda expression is turned into a separate method:</p> <pre><code>private Car lambda0(Car c) { return c; } private Func<Car, Car> CachedAnonymousMethodDelegate1; public static void Main() { if (CachedAnonymousMethodDelegate1 == null) CachedAnonymousMethodDelegate1 = new Func<Car, Car>(lambda0); var query = db.Cars.Select<Car, Car>(CachedAnonymousMethodDelegate1); foreach // ... } </code></pre> <p>In reality the method is not called <code>lambda0</code> but something like <code><Main>b__0</code> (where <code>Main</code> is the name of the containing method). Similarly, the cached delegate is actually called <code>CS$<>9__CachedAnonymousMethodDelegate1</code>.</p> <p>If you are using LINQ-to-SQL, then <code>db.Cars</code> will be of type <code>IQueryable<Car></code> and this step is very different. It would instead turn the lambda expression into an expression tree:</p> <pre><code>public static void Main() { var parameter = Expression.Parameter(typeof(Car), "c"); var lambda = Expression.Lambda<Func<Car, Car>>(parameter, new ParameterExpression[] { parameter })); var query = db.Cars.Select<Car, Car>(lambda); foreach // ... } </code></pre></li> <li><p>The <code>foreach</code> loop is transformed into a <code>try/finally</code> block (this is the same for both):</p> <pre><code>IEnumerator<Car> enumerator = null; try { enumerator = query.GetEnumerator(); Car aCar; while (enumerator.MoveNext()) { aCar = enumerator.Current; Console.WriteLine(aCar.Name); } } finally { if (enumerator != null) ((IDisposable)enumerator).Dispose(); } </code></pre></li> <li><p>Finally, this is compiled into IL the expected way. The following is for <code>IEnumerable<Car></code>:</p> <pre><code>// Put db.Cars on the stack L_0016: ldloc.0 L_0017: callvirt instance !0 DatabaseContext::get_Cars() // “if” starts here L_001c: ldsfld Func<Car, Car> Program::CachedAnonymousMethodDelegate1 L_0021: brtrue.s L_0034 L_0023: ldnull L_0024: ldftn Car Program::lambda0(Car) L_002a: newobj instance void Func<Car, Car>::.ctor(object, native int) L_002f: stsfld Func<Car, Car> Program::CachedAnonymousMethodDelegate1 // Put the delegate for “c => c” on the stack L_0034: ldsfld Func<Car, Car> Program::CachedAnonymousMethodDelegate1 // Call to Enumerable.Select() L_0039: call IEnumerable<!!1> Enumerable::Select<Car, Car>(IEnumerable<!!0>, Func<!!0, !!1>) L_003e: stloc.1 // “try” block starts here L_003f: ldloc.1 L_0040: callvirt instance IEnumerator<!0> IEnumerable<Car>::GetEnumerator() L_0045: stloc.3 // “while” inside try block starts here L_0046: br.s L_005a L_0048: ldloc.3 // body of while starts here L_0049: callvirt instance !0 IEnumerator<Car>::get_Current() L_004e: stloc.2 L_004f: ldloc.2 L_0050: ldfld string Car::Name L_0055: call void Console::WriteLine(string) L_005a: ldloc.3 // while condition starts here L_005b: callvirt instance bool IEnumerator::MoveNext() L_0060: brtrue.s L_0048 // end of while L_0062: leave.s L_006e // end of try // “finally” block starts here L_0064: ldloc.3 L_0065: brfalse.s L_006d L_0067: ldloc.3 L_0068: callvirt instance void IDisposable::Dispose() L_006d: endfinally </code></pre> <p>The compiled code for the <code>IQueryable<Car></code> version is also as expected. Here is the important part that is different from the above (the local variables will have different offsets and names now, but let’s disregard that):</p> <pre><code>// typeof(Car) L_0021: ldtoken Car L_0026: call Type Type::GetTypeFromHandle(RuntimeTypeHandle) // Expression.Parameter(typeof(Car), "c") L_002b: ldstr "c" L_0030: call ParameterExpression Expression::Parameter(Type, string) L_0035: stloc.3 // Expression.Lambda(...) L_0036: ldloc.3 L_0037: ldc.i4.1 // var paramArray = new ParameterExpression[1] L_0038: newarr ParameterExpression L_003d: stloc.s paramArray L_003f: ldloc.s paramArray L_0041: ldc.i4.0 // paramArray[0] = parameter; L_0042: ldloc.3 L_0043: stelem.ref L_0044: ldloc.s paramArray L_0046: call Expression<!!0> Expression::Lambda<Func<Car, Car>>(Expression, ParameterExpression[]) // var query = Queryable.Select(...); L_004b: call IQueryable<!!1> Queryable::Select<Car, Car>(IQueryable<!!0>, Expression<Func<!!0, !!1>>) L_0050: stloc.1 </code></pre></li> </ol>

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