How to: Initialize Objects by Using an Object Initializer (C# Programming Guide)

You can use object initializers to initialize type objects in a declarative manner without explicitly invoking a constructor for the type.

The following examples show how to use object initializers with named objects. The compiler processes object initializers by first accessing the default instance constructor and then processing the member initializations. Therefore, if the parameterless constructor is declared as private in the class, object initializers that require public access will fail.

You must use an object initializer if you're defining an anonymous type. For more information, see How to: Return Subsets of Element Properties in a Query.

Example

The following example shows how to initialize a new StudentName type by using object initializers. This example sets properties in the StudentName type:

public class HowToObjectInitializers
{
    public static void Main()
    {
        // Declare a StudentName by using the constructor that has two parameters.
        StudentName student1 = new StudentName("Craig", "Playstead");

        // Make the same declaration by using an object initializer and sending 
        // arguments for the first and last names. The default constructor is 
        // invoked in processing this declaration, not the constructor that has
        // two parameters.
        StudentName student2 = new StudentName
        {
            FirstName = "Craig",
            LastName = "Playstead",
        };

        // Declare a StudentName by using an object initializer and sending 
        // an argument for only the ID property. No corresponding constructor is
        // necessary. Only the default constructor is used to process object 
        // initializers.
        StudentName student3 = new StudentName
        {
            ID = 183
        };

        // Declare a StudentName by using an object initializer and sending
        // arguments for all three properties. No corresponding constructor is 
        // defined in the class.
        StudentName student4 = new StudentName
        {
            FirstName = "Craig",
            LastName = "Playstead",
            ID = 116
        };

        Console.WriteLine(student1.ToString());
        Console.WriteLine(student2.ToString());
        Console.WriteLine(student3.ToString());
        Console.WriteLine(student4.ToString());
    }
    // Output:
    // Craig  0
    // Craig  0
    //   183
    // Craig  116

    public class StudentName
    {
        // The default constructor has no parameters. The default constructor 
        // is invoked in the processing of object initializers. 
        // You can test this by changing the access modifier from public to 
        // private. The declarations in Main that use object initializers will 
        // fail.
        public StudentName() { }

        // The following constructor has parameters for two of the three 
        // properties. 
        public StudentName(string first, string last)
        {
            FirstName = first;
            LastName = last;
        }

        // Properties.
        public string FirstName { get; set; }
        public string LastName { get; set; }
        public int ID { get; set; }

        public override string ToString() => FirstName + "  " + ID;
    }
}

Object initializers can be used to set indexers in an object. The following example defines a BaseballTeam class that uses an indexer to get and set players at different positions. The initializer can assign players, based on the abbreviation for the position, or the number used for each position baseball scorecards:

public class HowToIndexInitializer
{
    public class BaseballTeam
    {
        private string[] players = new string[9];
        private readonly List<string> positionAbbreviations = new List<string>
        {
            "P", "C", "1B", "2B", "3B", "SS", "LF", "CF", "RF"
        };

        public string this[int position]
        {
            // Baseball positions are 1 - 9.
            get { return players[position-1]; }
            set { players[position-1] = value; }
        }
        public string this[string position]
        {
            get { return players[positionAbbreviations.IndexOf(position)]; }
            set { players[positionAbbreviations.IndexOf(position)] = value; }
        }
    }

    public static void Main()
    {
        var team = new BaseballTeam
        {
            ["RF"] = "Mookie Betts",
            [4] = "Jose Altuve",
            ["CF"] = "Mike Trout"
        };

        Console.WriteLine(team["2B"]);
    }
}

See also