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copied!<p>The second will possibly be <em>almost certainly insignificantly</em> faster, because there's one fewer assignment involved, logically. In the first case, the code is actually equivalent to:</p> <pre><code>MyClass tmp = new MyClass() tmp.Id = 1; tmp.Code = "Test"; MyClass c = tmp; </code></pre> <p>It's very possible that the JIT compiler will elide these as you're declaring a <em>new</em> variable - it wouldn't be able to do so if you were assigning to an existing variable with an object initializer.</p> <p>EDIT: I've just tried compiling with and without optimizations turned on, and in this "new variable" case the C# compiler elides the two <em>if</em> it's optimizing. It doesn't otherwise (but the JIT still could). In the "reassignment" case it could make an observable difference, so I wouldn't expect the same optimization. I haven't checked though.</p> <p>I would be very surprised to see a situation where it actually made a significant difference though, so I'd go with the more readable option, which IMO is the first.</p> <p>EDIT: I thought folks might be interested in a benchmark showing it making a difference. This is deliberately horrible code to make the hidden extra assignment slow - I've created a big, mutable struct. Urgh. Anyway...</p> <pre><code>using System; using System.Diagnostics; struct BigStruct { public int value; #pragma warning disable 0169 decimal a1, a2, a3, a4, a5, a6, a7, a8; decimal b1, b2, b3, b4, b5, b6, b7, b8; decimal c1, c2, c3, c4, c5, c6, c7, c8; decimal d1, d2, d3, d4, d5, d6, d7, d8; #pragma warning restore 0169 } class Test { const int Iterations = 10000000; static void Main() { Time(NewVariableObjectInitializer); Time(ExistingVariableObjectInitializer); Time(NewVariableDirectSetting); Time(ExistingVariableDirectSetting); } static void Time(Func<int> action) { Stopwatch stopwatch = Stopwatch.StartNew(); action(); stopwatch.Stop(); Console.WriteLine("{0}: {1}ms", action.Method.Name, stopwatch.ElapsedMilliseconds); } static int NewVariableObjectInitializer() { int total = 0; for (int i = 0; i < Iterations; i++) { BigStruct b = new BigStruct { value = i }; total += b.value; } return total; } static int ExistingVariableObjectInitializer() { int total = 0; BigStruct b; for (int i = 0; i < Iterations; i++) { b = new BigStruct { value = i }; total += b.value; } return total; } static int NewVariableDirectSetting() { int total = 0; for (int i = 0; i < Iterations; i++) { BigStruct b = new BigStruct(); b.value = i; total += b.value; } return total; } static int ExistingVariableDirectSetting() { int total = 0; BigStruct b; for (int i = 0; i < Iterations; i++) { b = new BigStruct(); b.value = i; total += b.value; } return total; } } </code></pre> <p>Results (with /o+ /debug-):</p> <pre><code>NewVariableObjectInitializer: 3328ms ExistingVariableObjectInitializer: 3300ms NewVariableDirectSetting: 1464ms ExistingVariableDirectSetting: 1491ms </code></pre> <p>I'm somewhat surprised that the NewVariableObjectInitializer version is slower than the direct setting ones... it looks like the C# compiler doesn't optimize this case in the way that it does for reference types. I suspect there's some subtlety around value types that prevents it.</p>

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