Question

So, the comparison would be between:

MyClass foo = new MyClass();
foo.Property1 = 4;
foo.Property2 = "garfield";

and

MyClass foo = new MyClass { Property1 = 4, Property2 = "garfield" };

Is it syntactic sugar, or is there actually some kind of performance gain (however minute it’s likely to be?)

Answer 1

It’s actually potentially very, very slightly slower to use an object initializer than to call a constructor and then assign the properties, as it has one extra assignment:

MyClass foo = new MyClass();
foo.Property1 = 4;
foo.Property2 = "garfield";

vs

MyClass tmp = new MyClass();
tmp.Property1 = 4;
tmp.Property2 = "garfield";
MyClass foo = tmp;

The properties are all assigned before the reference is assigned to the variable. This difference is a visible one if it’s reusing a variable:

using System;

public class Test
{
    static Test shared;

    string First { get; set; }

    string Second
    {
        set
        {
            Console.WriteLine("Setting second. shared.First={0}",
                              shared == null ? "n/a" : shared.First);
        }
    }

    static void Main()
    {
        shared = new Test { First = "First 1", Second = "Second 1" };
        shared = new Test { First = "First 2", Second = "Second 2" };        
    }
}

The output shows that in the second line of Main, when Second is being set (after First), the value of shared.First is still “First 1” – i.e. shared hasn’t been assigned the new value yet.

As Marc says though, you’ll almost certainly never actually spot a difference.

Anonymous types are guaranteed to use a constructor – the form is given in section 7.5.10.6 of the C# 3 language specification.