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Item 14: Minimize Duplicate Initialization Logic

Writing constructors is often a repetitive task. Many developers write the first constructor and then copy and paste the code into other constructors to satisfy the multiple overrides defined in the class interface. Ideally, you’re not one of those. If you are, stop it. Veteran C++ programmers would factor the common algorithms into a private helper method. Stop that, too. When you find that multiple constructors contain the same logic, factor that logic into a common constructor instead. You’ll get the benefits of avoiding code duplication, and constructor initializers generate much more efficient object code. The C# compiler recognizes the constructor initializer as special syntax and removes the duplicated variable initializers and the duplicated base class constructor calls. The result is that your final object executes the minimum amount of code to properly initialize the object. You also write the least amount of code by delegating responsibilities to a common constructor.

Constructor initializers allow one constructor to call another constructor. This example shows a simple usage:

public class MyClass
{
    // collection of data
    private List<ImportantData> coll;
    // Name of the instance:
    private string name;

    public MyClass() :
        this(0, "")
    {
    }

    public MyClass(int initialCount) :
        this(initialCount, string.Empty)
    {
    }

    public MyClass(int initialCount, string name)
    {
        coll = (initialCount > 0) ?
        new List<ImportantData>(initialCount) :
        new List<ImportantData>();
        this.name = name;
    }
}

C# 4.0 added default parameters, which you can use to minimize the duplicated code in constructors. You could replace all the different constructors for MyClass with one constructor that specifies default values for all or many of the values:

public class MyClass
{
    // collection of data
    private List<ImportantData> coll;

    // Name of the instance:
    private string name;

    // Needed to satisfy the new() constraint.
    public MyClass() :
        this(0, string.Empty)
    {
    }

    public MyClass(int initialCount = 0, string name = "")
    {
        coll = (initialCount > 0) ?
        new List<ImportantData>(initialCount) :
        new List<ImportantData>();
        this.name = name;
    }
}

There are tradeoffs in choosing default parameters over using multiple overloads. Default parameters create more options for your users. This version of MyClass specifies the default value for both parameters. Users could specify different values for either or both parameters. Producing all the permutations using overloaded constructors would require four different constructor overloads: a parameterless constructor, one that asks for the initial count, one that asks for the name, and one that asks for both parameters. Add more members to your class, and the number of potential overloads grows as the number of permutations of all the parameters grows. That complexity makes default parameters a very powerful mechanism to minimize the number of potential overloads that you need to create.

Defining default values for all parameters to your type’s constructor means that user code will be valid when you call the new MyClass(). When you intend to support this concept, you should create an explicit parameterless constructor in that type, as shown in the example code above. While most code would default all parameters, generic classes that use the new() constraint will not accept a constructor with parameters that have default values. To satisfy the new() constraint, a class must have an explicit parameterless constructor. Therefore, you should create one so that clients can use your type in generic classes or methods that enforce the new() constraint. That’s not to say that every type needs a parameterless constructor. However, if you support one, make sure to add the code so that the parameterless constructor works in all cases, even when called from a generic class with a new() constraint.

You’ll note that the second constructor specifies “” for the default value on the name parameter, rather than the more customary string.Empty. That’s because string.Empty is not a compile-time constant. It is a static property defined in the string class. Because it is not a compile-time constant, you cannot use it for the default value for a parameter.

However, using default parameters instead of overloads creates tighter coupling between your class and all the clients that use it. In particular, the formal parameter name becomes part of the public interface, as does the current default value. Changing parameter values requires a recompile of all client code in order to pick up those changes. That makes overloaded constructors more resilient in the face of potential future changes. You can add new constructors, or change the default behavior for those constructors that don’t specify values, without breaking client code.

Default parameters are the preferred solution to this problem. However, some APIs use reflection to create objects and rely on a parameterless constructor. A constructor with defaults supplied for all arguments is not the same as a parameterless constructor. You may need to write separate constructors that you support as a separate function. With constructors, that can mean a lot of duplicated code. Use constructor chaining, by having one constructor invoke another constructor declared in the same class, instead of creating a common utility routine. Several inefficiencies are present in this alternative method of factoring out common constructor logic:

public class MyClass
{
    private List<ImportantData> coll;
    private string name;

    public MyClass()
    {
        commonConstructor(0, "");
    }


    public MyClass(int initialCount)
    {
        commonConstructor(initialCount, "");
    }

    public MyClass(int initialCount, string Name)
    {
        commonConstructor(initialCount, Name);
    }

    private void commonConstructor(int count,
    string name)
    {
        coll = (count > 0) ?
        new List<ImportantData>(count) :
        new List<ImportantData>();
        this.name = name;
    }
}

That version looks the same, but it generates far-less-efficient object code. The compiler adds code to perform several functions on your behalf in constructors. It adds statements for all variable initializers (see Item 12 earlier in this chapter). It calls the base class constructor. When you write your own common utility function, the compiler cannot factor out this duplicated code. The IL for the second version is the same as if you’d written this:

public class MyClass
{
    private List<ImportantData> coll;
    private string name;

    public MyClass()
    {
        // Instance Initializers would go here.
        object(); // Not legal, illustrative only.
        commonConstructor(0, "");
    }


    public MyClass(int initialCount)
    {
        // Instance Initializers would go here.
        object(); // Not legal, illustrative only.
        commonConstructor(initialCount, "");
    }

    public MyClass(int initialCount, string Name)
    {
        // Instance Initializers would go here.
        object(); // Not legal, illustrative only.
        commonConstructor(initialCount, Name);
    }

    private void commonConstructor(int count,
    string name)
    {
        coll = (count > 0) ?
        new List<ImportantData>(count) :
        new List<ImportantData>();
        this.name = name;
    }
}

If you could write the construction code for the first version the way the compiler sees it, you’d write this:

// Not legal, illustrates IL generated:
public class MyClass
{
    private List<ImportantData> coll;
    private string name;

    public MyClass()
    {
        // No variable initializers here.
        // Call the third constructor, shown below.
        this(0, ""); // Not legal, illustrative only.
    }


    public MyClass(int initialCount)
    {
        // No variable initializers here.
        // Call the third constructor, shown below.
        this(initialCount, "");
    }

    public MyClass(int initialCount, string Name)
    {
        // Instance Initializers would go here.
        //object(); // Not legal, illustrative only.
        coll = (initialCount > 0) ?
        new List<ImportantData>(initialCount) :
        new List<ImportantData>();
        name = Name;
    }
}

The difference is that the compiler does not generate multiple calls to the base class constructor, nor does it copy the instance variable initializers into each constructor body. The fact that the base class constructor is called only from the last constructor is also significant: You cannot include more than one constructor initializer in a constructor definition. You can delegate to another constructor in this class using this(), or you can call a base class constructor using base(). You cannot do both.

Still don’t buy the case for constructor initializers? Then think about read-only constants. In this example, the name of the object should not change during its lifetime. This means that you should make it read-only. That causes the common utility function to generate compiler errors:

public class MyClass
{
    // collection of data
    private List<ImportantData> coll;
    // Number for this instance
    private int counter;
    // Name of the instance:
    private readonly string name;


    public MyClass()
    {
        commonConstructor(0, string.Empty);
    }

    public MyClass(int initialCount)
    {
        commonConstructor(initialCount, string.Empty);
    }

    public MyClass(int initialCount, string Name)
    {
        commonConstructor(initialCount, Name);
    }

    private void commonConstructor(int count,
    string name)
    {
        coll = (count > 0) ?
        new List<ImportantData>(count) :
        new List<ImportantData>();
        // ERROR changing the name outside of a constructor.
        //this.name = name;
    }
}

The compiler enforces the read-only nature of this.name and will not allow any code not in a constructor to modify it. C#’s constructor initializers provide the alternative. All but the most trivial classes contain more than one constructor. Their job is to initialize all the members of an object. By their very nature, these functions have similar or, ideally, shared logic. Use the C# constructor initializer to factor out those common algorithms so that you write them once and they execute once.

Both default parameters and overloads have their place. In general, you should prefer default values to overloaded constructors. After all, if you are letting client developers specify parameter values at all, your constructor must be capable of handling any values that users specify. Your original default values should always be reasonable and shouldn’t generate exceptions. Therefore, even though changing the default parameter values is technically a breaking change, it shouldn’t be observable to your clients. Their code will still use the original values, and those original values should still produce reasonable behavior. That minimizes the potential hazards of using default values.

This is the last item about object initialization in C#. That makes it a good time to review the entire sequence of events for constructing an instance of a type. You should understand both the order of operations and the default initialization of an object. You should strive to initialize every member variable exactly once during construction. The best way for you to accomplish this is to initialize values as early as possible. Here is the order of operations for constructing the first instance of a type:

  1. Static variable storage is set to 0.

  2. Static variable initializers execute.

  3. Static constructors for the base class execute.

  4. The static constructor executes.

  5. Instance variable storage is set to 0.

  6. Instance variable initializers execute.

  7. The appropriate base class instance constructor executes.

  8. The instance constructor executes.

Subsequent instances of the same type start at step 5 because the class initializers execute only once. Also, steps 6 and 7 are optimized so that constructor initializers cause the compiler to remove duplicate instructions.

The C# language compiler guarantees that everything gets initialized in some way when an object is created. At a minimum, you are guaranteed that all memory your object uses has been set to 0 when an instance is created. This is true for both static members and instance members. Your goal is to make sure that you initialize all the values the way you want and execute that initialization code only once. Use initializers to initialize simple resources. Use constructors to initialize members that require more sophisticated logic. Also factor calls to other constructors to minimize duplication.

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