What's the difference between an object initializer and a constructor?

Object Initializers were something added to C# 3, in order to simplify construction of objects when you're using an object.

Constructors run, given 0 or more parameters, and are used to create and initialize an object before the calling method gets the handle to the created object. For example:

MyObject myObjectInstance = new MyObject(param1, param2);

In this case, the constructor of MyObject will be run with the values param1 and param2. These are both used to create the new MyObject in memory. The created object (which is setup using those parameters) gets returned, and set to myObjectInstance.

In general, it's considered good practice to have a constructor require the parameters needed in order to completely setup an object, so that it's impossible to create an object in an invalid state.

However, there are often "extra" properties that could be set, but are not required. This could be handled through overloaded constructors, but leads to having lots of constructors that aren't necessarily useful in the majority of circumstances.

This leads to object initializers - An Object Initializer lets you set properties or fields on your object after it's been constructed, but before you can use it by anything else. For example:

MyObject myObjectInstance = new MyObject(param1, param2)
{
    MyProperty = someUsefulValue
};

This will behave about the same as if you do this:

MyObject myObjectInstance = new MyObject(param1, param2);
myObjectInstance.MyProperty = someUsefulValue;

However, in multi-threaded environments the atomicity of the object initializer may be beneficial, since it prevents the object from being in a not-fully initialized state (see this answer for more details) - it's either null or initialized like you intended.

Also, object initializers are simpler to read (especially when you set multiple values), so they give you the same benefit as many overloads on the constructor, without the need to have many overloads complicating the API for that class.

A constructor is a defined method on a type which takes a specified number of parameters and is used to create and initialize an object.

An object initializer is code that runs on an object after a constructor and can be used to succinctly set any number of fields on the object to specified values. The setting of these fields occurs after the constructor is called.

You would use a constructor without the help of an object initializer if the constructor sufficiently set the initial state of the object. An object initializer however must be used in conjunction with a constructor. The syntax requires the explicit or implicit use (VB.Net and C#) of a constructor to create the initial object. You would use an object initializer when the constructor does not sufficiently initialize the object to your use and a few simple field and/or property sets would.

When you do

Person p = new Person { Name = "a", Age = 23 };

this is what an object initializer essentially does:

Person tmp = new Person(); //creates temp object calling default constructor
tmp.Name = "a";
tmp.Age = 23;
p = tmp;

Now this facilitates behaviour like this. Knowing how object initializers work is important.