Auto-Implemented Properties (C# Programming Guide)

In C# 3.0 and later, auto-implemented properties make property-declaration more concise when no additional logic is required in the property accessors. They also enable client code to create objects. When you declare a property as shown in the following example, the compiler creates a private, anonymous backing field that can only be accessed through the property's get and set accessors. In C# 9 and later, init accessors can also be declared as auto-implemented properties.

Example

The following example shows a simple class that has some auto-implemented properties:

// This class is mutable. Its data can be modified from
// outside the class.
class Customer
{
    // Auto-implemented properties for trivial get and set
    public double TotalPurchases { get; set; }
    public string Name { get; set; }
    public int CustomerId { get; set; }

    // Constructor
    public Customer(double purchases, string name, int id)
    {
        TotalPurchases = purchases;
        Name = name;
        CustomerId = id;
    }

    // Methods
    public string GetContactInfo() { return "ContactInfo"; }
    public string GetTransactionHistory() { return "History"; }

    // .. Additional methods, events, etc.
}

class Program
{
    static void Main()
    {
        // Intialize a new object.
        Customer cust1 = new Customer(4987.63, "Northwind", 90108);

        // Modify a property.
        cust1.TotalPurchases += 499.99;
    }
}

You can't declare auto-implemented properties in interfaces. Auto-implemented properties declare a private instance backing field, and interfaces may not declare instance fields. Declaring a property in an interface without defining a body declares a property with accessors that must be implemented by each type that implements that interface.

In C# 6 and later, you can initialize auto-implemented properties similarly to fields:

public string FirstName { get; set; } = "Jane";  

The class that is shown in the previous example is mutable. Client code can change the values in objects after creation. In complex classes that contain significant behavior (methods) as well as data, it's often necessary to have public properties. However, for small classes or structs that just encapsulate a set of values (data) and have little or no behaviors, you should use one of the following options for making the objects immutable:

  • Declare only a get accessor (immutable everywhere except the constructor).
  • Declare a get accessor and an init accessor (immutable everywhere except during object construction).
  • Declare the set accessor as private (immutable to consumers).

For more information, see How to implement a lightweight class with auto-implemented properties.

See also