Castle Windsor Tips – say NO to private setter

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Now, this is something that has stung me and every single one of my colleagues time and time again..

Imagine you have an interface called IMyConfiguration which has a get-only property:

public interface IMyConfiguration
    int Port { get; }

And the implementing class takes in a parameter called port in the constructor and sets the value of the property using a private setter on the property:

public class MyConfiguration : IMyConfiguration
    public MyConfiguration(int port)
        Port = port;

    public int Port { get; private set; }

All looks fine, you set up your castle config to pass in an integer called port:

<component id="MyConfiguration"
           service="MyAssembly.IMyConfiguration, MyAssembly"
           type="MyAssembly.MyConfiguration, MyAssembly"

And you hit Run, and immediately you run into a Castle exception, the exception message is less than useful as it only says ”Object reference not set to an instance of an object.” and the stack trace (see below) doesn’t reveal anything useful either..


If this sounds familiar to you then you should make a note to self as I have done and remember to not use a private setter on a property defined in a class that’s going to be injected into your application by Castle!

To fix this error, you will need to change your implementation to something like this instead:

public class MyConfiguration : IMyConfiguration
    private readonly int _port;

    public MyConfiguration(int port)
        _port = port;

    public int Port { get { return _port; } }
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