C# Language Specification

Automatically implemented properties

When a property is specified as an automatically implemented property, a hidden backing field is automatically available for the property, and the accessors are implemented to read from and write to that backing field.

The following example:

public class Point {
public int X { get; set; } // automatically implemented
public int Y { get; set; } // automatically implemented
}

is equivalent to the following declaration:

public class Point {
private int x;
private int y;
public int X { get { return x; } set { x = value; } }
public int Y { get { return y; } set { y = value; } }
}

Because the backing field is inaccessible, it can be read and written only through the property accessors, even within the containing type. This means that automatically implemented read-only or write-only properties do not make sense, and are disallowed. It is however possible to set the access level of each accessor differently. Thus, the effect of a read-only property with a private backing field can be mimicked like this:

public class ReadOnlyPoint {
public int X { get; private set; }
public int Y { get; private set; }
public ReadOnlyPoint(int x, int y) { X = x; Y = y; }
}

This restriction also means that definite assignment of struct types with auto-implemented properties can only be achieved using the standard constructor of the struct, since assigning to the property itself requires the struct to be definitely assigned. This means that user-defined constructors must call the default constructor.

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