NullReferenceException being thrown despite debugger showing objects C#

Question

I have defined the following class:

public class PerceptronNetwork : NetworkBase
{
    Neuron perceptron { get; set; }

    public PerceptronTrainer trainingMethod;

    public PerceptronNetwork(Neuron aNeuron)
    {
        this.perceptron = aNeuron;
    }

    public double train(TrainingTemplate trainingTemplate, int extMaxGenerations)
    {
        // This is simple but the ideea is suposed to be that in larger networks here
        // I do a foreach over the neurons
        double error =  this.trainingMethod.trainNetwork(trainingTemplate, perceptron,
                                                         extMaxGenerations);
        return error;
    }
}

Whenever I try to use the train method from my main function I get the error

Object reference not set to an instance of an object.

pointing to the perceptron object.

Despite that when I hover over every object in the function call trainingTemplate, perceptron and extMaxGenerations they all seem to be pointing to proper values.

Did I declare or instantiate them wrong in some way ?

What exactly do you mean by "pointing to the perceptron object"? Is the exception definitely in train, or is it in trainNetwork? (As an aside, it's a good idea to get into the habit of following .NET naming conventions... and keeping your fields private.) — Jon Skeet
– Jon Skeet, Commented Mar 12, 2014 at 20:30
We don't see where aNeuron (and therefore perceptron), trainingMethod, trainNetwork and trainingTemplate are created. They all could be null. — Olivier Jacot-Descombes
– Olivier Jacot-Descombes, Commented Mar 12, 2014 at 20:36
@OlivierJacot-Descombes that would not generate an exception until they were used - and in that case the stack trace would point to trainNetwork (or a lower method). — D Stanley
– D Stanley, Commented Mar 12, 2014 at 20:38

Crono · Accepted Answer · 2014-03-12 20:32:52Z

2

Ensure that this.trainingMethod is instantiated. From your code it doesn't seem to be.

If it is then you will have to show the full stack trace.

edited Mar 12, 2014 at 20:32

answered Mar 12, 2014 at 20:31

Crono

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Comments

D Stanley · Accepted Answer · 2014-03-12 20:33:44Z

1

A NullReferenceException is not thrown when you pass a null parameter, it is thrown when you try and access a member property/method/field on a null reference. In your case it means that this.trainingMethod is null.

If trainNetwork had validation code to verify that the incoming parameters were not null, you would most likely get an ArgumentNullException with the name of the parameter that was null indicated.

If trainNetwork tried to reference an instance member on a null value that you passed in, the stack trace would originate from that method, not from train.

answered Mar 12, 2014 at 20:33

D Stanley

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4 Comments

Crono Over a year ago

Ummm... passing a null parameter could result in a NullReferenceException.

D Stanley Over a year ago

Yes but from the method that uses it, not from the method that passes the parameter.

George Bora Over a year ago

This was it, sorry for making such an obvious error (never code when you're tired from work)

D Stanley Over a year ago

@GeorgeBora Happens to the best of us :)

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