Checking constructor parameter for null before calling base

Question

I usually check constructor arguments for null values in the following manner:

public class SomeClass(SomeArgument someArgument) {      if(someArgument == null) throw new ArgumentNullException("someArgument"); } 

But say I have a class that inherits from another class:

public abstract class TheBase {     public TheBase(int id)     {      } }  public class TheArgument {     public int TheId { get; set; } }  public class TheInheritor : TheBase {     public TheInheritor(TheArgument theArgument) : base(theArgument.TheId)     {      } } 

And someone now constructs an instance of TheInheritor like this:

var theVar = new TheInheritor(null); 

I can't think of a way that to check for null before base is being called (and throwing a NullReferenceException). Short of letting TheBase's constructor accept an instance of TheArgument I can't see how I could have this sanity-check. But what if TheArgument is related only to TheInheritor and there are a lot of other classes inheriting from TheBase?

Any recommendations on how to solve this?

Answer 1

You can do it with something like this:

public TheInheritor(TheArgument theArgument)     : base(ConvertToId(theArgument)) { }  private static int ConvertToId(TheArgument theArgument) {     if (theArgument == null)     {         throw new ArgumentNullException("theArgument");     }     return theArgument.Id; } 

Or more generally, something like this:

public TheInheritor(TheArgument theArgument)     : base(Preconditions.CheckNotNull(theArgument).Id) { } 

where Preconditions is a utility class elsewhere, like this:

public static class Preconditions {     public static T CheckNotNull<T>(T value) where T : class     {         if (value == null)         {             throw new ArgumentNullException();         }         return value;     } } 

(This loses the argument name, of course, but you could pass that in as well if necessary.)