Question

Consider a factory method that I may or may not control, passed to another class as a Func<T>:

// this.FactoryMethod is an external dependency passed into this class
// through constructor or property injection assume that it is not null
// but there is no guarantee that it will return a non-null reference.
Func<IModel> FactoryMethod;

public IModel GetPopulatedModel(int state, FileInfo someFile)
{       
   // Argument validation omitted for brevity...

   // This operation could return null.
   var model = this.FactoryMethod()
   if (model == null)
   {
      throw new SomeException("Factory method failed to produce a value.");
   }

   // Safe to assign to properties at this point.
   model.Priority = state;
   model.File = someFile;
   return model;
}

I would like to throw something that indicates the operation failed.

ArgumentNullException would be misleading because there is nothing wrong with the arguments:

The exception that is thrown when a null reference (Nothing in Visual
Basic) is passed to a method that does not accept it as a valid
argument.

InvalidOperationException doesn’t really seem to be on point:

The exception that is thrown when a method call is invalid for the
object’s current state.

It seems weird to consider a local variable to be part of the object's current state.

What would be good exception to throw? I’m surprised there is no OperationFailedException in System. Should I write my own exception, or is there a good one out there?

Answer 1

Two examples from the .NET Framework:

• The SecurityTokenParameters.Clone Method throws an InvalidOperationException if the SecurityTokenParameters.CloneCore Method returns null.

  InvalidOperationException Class

  The exception that is thrown when a method call is invalid for the object’s current state.

• The ClientCredentials.Clone Method throws a NotImplementedException if the ClientCredentials.CloneCore Method returns null.

  NotImplementedException Class

  The exception that is thrown when a requested method or operation is not implemented.

CloneCore is essentially a factory method for clones of the current object.

I would use a NotImplementedException, because the factory method does not implement what is requested from it.