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New in C# 3.0: Create and Initialize Collection Objects in One Step

May 19, 2006

To recap a previous article, the new object initializer feature in C# 3.0 is a simple syntax that eases the construction and initialization of objects. Suppose you have a class, Student, that looks like this:

public class Student
   public string firstName;
   public string lastName;

You can create an object of this class by using object initializers as follows:

var student1 = new Student{firstName = "Bruce", lastName = "Willis"};

C# 3.0's new collection initializers operate in a similar manner. Any object that implements System.Collections.Generic.ICollection<T> can have its values initialized with a collection initializer.

A collection initializer is made up of the following:

  • A sequence of object initializers, enclosed by "{" and "}" tokens and separated by commas.
  • Element initializers, each of which specifies an element to be added to the collection object being added. (Element initializers cannot be assignment expressions in a collection initializer.)

How does it work? A collection initializer must observe the following rules:

  • The collection object to which a collection initializer is applied must be of a type that implements System.Collections.Generic.ICollection<T> for exactly one T.
  • An implicit conversion from the type of each element initializer to T must exist. A collection initializer invokes the ICollection<T>.Add(T) method for each specified element in order.

As an example, the following collection initializer creates and initializes a new collection of strings with three members "Alice", "Bob", and "Chris":

List<string> names = new List<string> { "Alice", "Bob", "Chris" };
Note: All the initialization values are of type string. Otherwise, you'd get a compiler error.

Collection Initializers in Action

Suppose you want to represent a class and its enrolled students. You can do this through collection initializers by using C# 3.0 code such as the following:

using System;
using System.Collections.Generic;
using System.Text;
using System.Query;
using System.Xml.XLinq;
using System.Data.DLinq;
namespace CollectionInitializer
   class Program
      public class MyClass
         public string nameofClass;
         public List<string> studentNames = new List<string>();
      static void Main(string[] args)
         var classes = new List<MyClass>
            new MyClass
               nameofClass = "Science",
               studentNames = {"Laura", "George"}
            new MyClass
               nameofClass = "Commerce",
               studentNames = {"Bill", "Hillary"}

If you have Visual Studio 2005 (any flavor) and the LINQ Preview installed, you can compile the above code in the IDE.

If you do not have VS 2005 but have the LINQ Preview installed, you can use the following command to compile the code from the command line:

C:\Program Files\LINQ Preview\Bin\Csc.exe
/reference:"C:\Program Files\LINQ Preview\Bin\System.Data.DLinq.dll"
/reference:"C:\Program Files\LINQ Preview\Bin\System.Query.dll"
/reference:"C:\Program Files\LINQ Preview\Bin\System.Xml.XLinq.dll" Program.cs

Code Internals

Take a closer look at this snippet from the preceding C# 3.0 code:

var classes = new List<MyClass>
   new MyClass
      nameofClass = "Science",
      studentNames = {"Laura", "George"}
   new MyClass
      nameofClass = "Commerce",
      studentNames = {"Bill", "Hillary"}

To the compiler, it has the same effect as the following:

var classes = new List<MyClass>();
var __c1 = new MyClass ();
__c1.nameofClass = "Science";
var __c2 = new MyClass();
__c2.nameofClass = "Commerce";

If you fire up ILDASM and open the compiled binary, you will see something similar to Figure 1.

Click here for a larger image.

Figure 1: Compiled Binary of Sample Code Snippet

If you double-click the Main node in ILDASM, you will see the following code:

.method private hidebysig static void  Main(string[] args) cil managed
   // Code size       138 (0x8a)
   .maxstack  3
   .locals init ([0] class [mscorlib]System.Collections.Generic.List`1
                     <class CollectionInitializer.Program/MyClass>
                 [1] class [mscorlib]System.Collections.Generic.List`1
                     <class CollectionInitializer.Program/MyClass>
                 [2] class CollectionInitializer.Program/MyClass
                 [3] class CollectionInitializer.Program/MyClass
   IL_0000:  nop
   IL_0001:  nop
   IL_0002:  newobj     instance void class [mscorlib]System.Collections.
                        Generic.List`1<class CollectionInitializer.
   IL_0007:  stloc.1
   IL_0008:  ldloc.1
   IL_0009:  nop
   IL_000a:  newobj     instance void CollectionInitializer.
   IL_000f:  stloc.2
   IL_0010:  ldloc.2
   IL_0011:  ldstr      "Science"
   IL_0016:  stfld      string CollectionInitializer.
   IL_001b:  nop
   IL_001c:  ldloc.2
   IL_001d:  ldfld      class [mscorlib]System.Collections.Generic.List`1
                        <string> CollectionInitializer.
   IL_0022:  ldstr      "Laura"
   IL_0027:  callvirt   instance void class [mscorlib]System.
   IL_002c:  nop
   IL_002d:  ldloc.2
   IL_002e:  ldfld      class [mscorlib]System.Collections.Generic.List`1
                        <string> CollectionInitializer.
   IL_0033:  ldstr      "George"
   IL_0038:  callvirt   instance void class [mscorlib]System.Collections.
   IL_003d:  nop
   IL_003e:  nop
   IL_003f:  ldloc.2
   IL_0040:  nop
   IL_0041:  callvirt   instance void class [mscorlib]System.Collections.
                        Generic.List`1<class CollectionInitializer.
   IL_0046:  nop
   IL_0047:  ldloc.1
   IL_0048:  nop
   IL_0049:  newobj     instance void CollectionInitializer.
   IL_004e:  stloc.3
   IL_004f:  ldloc.3
   IL_0050:  ldstr      "Commerce"
   IL_0055:  stfld      string CollectionInitializer.
   IL_005a:  nop
   IL_005b:  ldloc.3
   IL_005c:  ldfld      class [mscorlib]System.Collections.Generic.List`1
                        <string> CollectionInitializer.
   IL_0061:  ldstr      "Bill"
   IL_0066:  callvirt   instance void class [mscorlib]System.Collections.
   IL_006b:  nop
   IL_006c:  ldloc.3
   IL_006d:  ldfld      class [mscorlib]System.Collections.Generic.List`1
                        <string> CollectionInitializer.
   IL_0072:  ldstr      "Hillary"
   IL_0077:  callvirt   instance void class [mscorlib]System.Collections.
   IL_007c:  nop
   IL_007d:  nop
   IL_007e:  ldloc.3
   IL_007f:  nop
   IL_0080:  callvirt   instance void class [mscorlib]System.Collections.
                        Generic.List`1<class CollectionInitializer.
   IL_0085:  nop
   IL_0086:  ldloc.1
   IL_0087:  nop
   IL_0088:  stloc.0
   IL_0089:  ret
}    // end of method Program::Main

As you can see from the two snippets prior to the above, C# 3.0 presents a significant improvement in language syntax.

One-Step Creation and Initialization for Object Collections

Collection initializers, one of the new features of C# 3.0, provide a new syntax to initialize collection objects. This simple syntax combines collection object creation and initialization into a single step.

Download the Code

You download the source code for this article here.

About the Author

Vipul Patel was a Microsoft MVP for two years. He specializes in C# and deployment issues. You can reach him at

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