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Code refactoring - NULL check

Part 1
Part 2
Part 3

Let’s look over the following code:
public class Foo
{
  SomeFoo _some;
  
  ...

  public int GetA()
  {
    if(_some == null)
    {
      return 0;
    }

    return _some.A;
  }

  public string GetB()
  {
    if(_some == null)
    {
      return null;
    }

    return _some.B;
  }

  public int GetTotalX()
  {
    if(_some == null)
    {
      return -1;
    }

    return _some.A + _some.C;
  }

}
We can see that the IF checks appears in more than one place. Even if is only a simple check, there can be a lot of places where it appears.
If we manage to extract this check in a generic method, than the code would be more legible. Because we don’t return each time the default value of an object we need to be able to return custom “default” value for the case when or object is null.
We can image a method that accept as parameter the object that we check if is null or not and another two parameters that represent the default value that need to be return and a generic function that return the expected value.
private TReturn ExecuteFuncOverObj<TReturn>(object obj, TReturn defaultValue, Func<TReturn> func)
{
  if( obj == null)
  {
    return defaultValue;
  }

  return func.Invoke();
}
In this way our code would look like this:

public class Foo
{
  SomeFoo _some;
  
  ...

  public int GetA()
  {
    return ExecuteFuncOverObj(_some, () => _some.A, 0);
  }

  public string GetB()
  {
    return ExecuteFuncOverObj(_some, () => _some.B, null);
  }

  public int GetTotalX()
  {

    return ExecuteFuncOverObj(_some, () => _some.A + _some.C, -1);
  }

}
If we need this behavior in other methods we could define a helper function or an extension method of Object.
Enjoy!
Part 1
Part 2
Part 3

Comments

  1. In Java it is recommended (and I would imagine C# is quite similar) that you don't use null, rather a "neutral" object. For example empty maps / lists / sets, 0, empty string etc. (depending on the context in which the object will be used).

    ReplyDelete
    Replies
    1. Indeed, another name for this would be the "null object pattern" or "special case pattern" (http://martinfowler.com/eaaCatalog/specialCase.html).
      Anyway, this would require to wrap all external classes if they are subject to null checks.

      Delete
  2. So you suggest that in order to avoid having an if, we create a new function call and we pass it a lambda function? I fail to see the benefit here. It's 1) counterintuitive 2) requires that a new developer using that code understand a new layer of indirection and 3) it's wasted resources. Of course, the approach can be optimized by the compiler, but the strategy explains why typical and advanced high-level languages users produce bloated solutions.

    ReplyDelete

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