Deep Null Coalescing in C# with Dis.OrDat<T>

10 January 2011 Tyler-Jensen Code

The other day, a friend and I were reviewing some code like this and decided we deeply disliked it.

// var line1 = person.Address.Lines.Line1 ?? string.Empty;
// throws NullReferenceException: 
//    {"Object reference not set to an instance of an object."}

// The ugly alternative

var line1 = person.Address == null
        ? "n/a"
        : person.Address.Lines == null
        ? "n/a"
        : person.Address.Lines.Line1;

If you have ever written code like that or even worse, with the if (obj != null) statements, then you can certainly relate to our lack of love for the constructs required to avoid the dratted NullReferenceException.

So we experimented with a solution and nearly got there. Tonight I finished that up and the new Dis class is born with the OrDat<T> method so you can now write this instead.

var line2 = Dis.OrDat<string>(() => person.Address.Lines.Line2, "n/a");

Here’s the full example and if you’re patient enough to scroll down, you’ll also find the full implementation of the Dis.OrDat class and method. 

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace DeepNullCoalescence
{
  class Program
  {
    static void Main(string[] args)
    {
      var person = new Person();

      // var line1 = person.Address.Lines.Line1 ?? string.Empty;
      // throws NullReferenceException: 
      //    {"Object reference not set to an instance of an object."}
      
      // The ugly alternative
      var line1 = person.Address == null
              ? "n/a"
              : person.Address.Lines == null
              ? "n/a"
              : person.Address.Lines.Line1;

      // A cooler alternative
      var line2 = Dis.OrDat<string>(() => person.Address.Lines.Line2, "n/a");

      Console.WriteLine(line1);
      Console.WriteLine(line2);
      Console.ReadLine();
    }
  }

  internal class Person
  {
    public Address Address { get; set; }
  }

  internal class Address
  {
    public AddressLines Lines { get; set; }
    public string State { get; set; }
  }

  internal class AddressLines
  {
    public string Line1 { get; set; }
    public string Line2 { get; set; }
  }
}

And for the patient reader, here is the actual magic.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Linq.Expressions;

namespace DeepNullCoalescence
{
  public static class Dis
  {
    public static T OrDat<T>(Expression<Func<T>> expr, T dat)
    {
      try
      {
        var func = expr.Compile();
        var result = func.Invoke();
        return result ?? dat; //now we can coalesce
      }
      catch (NullReferenceException)
      {
        return dat;
      }
    }
  }
}
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