How to mock/ override readonly fields for unit testing

Question

I am using nj4x and it has a class that has readonly fields as follows.

   public class MarketInformation
    {
    public readonly double ASK;
    public readonly double BID;
    public readonly double DIGITS;
    public readonly double EXPIRATION;
    public readonly double FREEZELEVEL;
    }

I am writing unit tests for classes and methods writing these classes. For isolation/ mocking I am using Nsubstitute and MicrosoftFakes (shims mainly). I want to override these fields so when they are called in my methods I get predictable results for testing. I generated shim for this class but all it provides me is a constructor, now allowing me to initialize it still. Is there any way to set these fields from outside this class?

Answer 1

It isn't pretty, but if you have access to the instance that needs to be modified, you can use the SetField method of the PrivateObject class . The PrivateObject class is part of the MS unit testing framework.

For example, consider the class:

public class ClassWithReadOnly
{
    public readonly string Foo;

    public ClassWithReadOnly()
    {
        Foo = "bar";
    }
}

You can set the read-only field like so:

var test = new ClassWithReadOnly();
Console.WriteLine("Before Foo == '{0}'", test.Foo);
var po = new PrivateObject(test);
po.SetField("Foo", "oof");
Console.WriteLine("After Foo == '{0}'", test.Foo);

The output will be:

Before Foo == 'bar'

After Foo == 'oof'

Answer 2

You'll probably need to create a wrapper class with your own get properties to stub or shim it out. You could create your own interface if you wanted to use stubs (not shown).

public class MarketInformationWrapper : MarketInformation
{
    ...
    public double Ask
    {
        get { return ASK; }
    }
    ...
}