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Using Structs (C# Programming Guide)

Updated: July 2008

The struct type is suitable for representing lightweight objects such as Point, Rectangle, and Color. Although it is just as convenient to represent a point as a classwith Auto-ImplementedProperties, a struct might be more efficient in some scenarios. For example, if you declare an array of 1000 Point objects, you will allocate additional memory for referencing each object; in this case, a struct would be less expensive. Because the .NET Framework contains an object called Point, the struct in this example is named "CoOrds" instead.

public struct CoOrds
{
    public int x, y;

    public CoOrds(int p1, int p2)
    {
        x = p1;
        y = p2;
    }
}

It is an error to define a default (parameterless) constructor for a struct. It is also an error to initialize an instance field in a struct body. You can initialize struct members only by using a parameterized constructor or by accessing the members individually after the struct is declared. Any private or otherwise inaccessible members can be initialized only in a constructor.

When you create a struct object using the new operator, it gets created and the appropriate constructor is called. Unlike classes, structs can be instantiated without using the new operator. In such a case, there is no constructor call, which makes the allocation more efficient. However, the fields will remain unassigned and the object cannot be used until all of the fields are initialized.

When a struct contains a reference type as a member, the default constructor of the member must be invoked explicitly, otherwise the member remains unassigned and the struct cannot be used. (This results in compiler error CS0171.)

There is no inheritance for structs as there is for classes. A struct cannot inherit from another struct or class, and it cannot be the base of a class. Structs, however, inherit from the base class Object. A struct can implement interfaces, and it does that exactly as classes do.

Unlike C++, you cannot declare a class using the keyword struct. In C#, classes and structs are semantically different. A struct is a value type, while a class is a reference type. For more information, see Value Types.

Unless you need reference-type semantics, small classes may be more efficiently handled by the system as a struct.

Description

This example demonstrates struct initialization using both default and parameterized constructors.

Code

public struct CoOrds
{
    public int x, y;

    public CoOrds(int p1, int p2)
    {
        x = p1;
        y = p2;
    }
}
// Declare and initialize struct objects. 
class TestCoOrds
{
    static void Main()
    {
        // Initialize:   
        CoOrds coords1 = new CoOrds();
        CoOrds coords2 = new CoOrds(10, 10);

        // Display results:
        Console.Write("CoOrds 1: ");
        Console.WriteLine("x = {0}, y = {1}", coords1.x, coords1.y);

        Console.Write("CoOrds 2: ");
        Console.WriteLine("x = {0}, y = {1}", coords2.x, coords2.y);

        // Keep the console window open in debug mode.
        Console.WriteLine("Press any key to exit.");
        Console.ReadKey();
    }
}
/* Output:
    CoOrds 1: x = 0, y = 0
    CoOrds 2: x = 10, y = 10
*/

Description

This example demonstrates a feature that is unique to structs. It creates a CoOrds object without using the new operator. If you replace the word struct with the word class, the program will not compile.

Code

public struct CoOrds
{
    public int x, y;

    public CoOrds(int p1, int p2)
    {
        x = p1;
        y = p2;
    }
}
// Declare a struct object without "new." 
class TestCoOrdsNoNew
{
    static void Main()
    {
        // Declare an object:
        CoOrds coords1;

        // Initialize:
        coords1.x = 10;
        coords1.y = 20;

        // Display results:
        Console.Write("CoOrds 1: ");
        Console.WriteLine("x = {0}, y = {1}", coords1.x, coords1.y);

        // Keep the console window open in debug mode.
        Console.WriteLine("Press any key to exit.");
        Console.ReadKey();
    }
}
// Output: CoOrds 1: x = 10, y = 20

Date

History

Reason

July 2008

Added paragraph about CS0171

Content bug fix.

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