Object extension method to peek at a private field. Useful when testing an against an object which has state but will not actually execute anything. See example using NSubstitute.

Example.cs

repository.Query(Arg.Is<FindUserByEmailQuery>(x => x.PeekPrivateField("emailAddress") == "foo@bar.com")).Returns(user);

FindUserByEmailQuery.cs

public class FindUserByEmailQuery : MyQUeryBaseClass<User>
{
    private string emailAddress;
    
    public FindUserByEmailQuery(string emailAddress)
    {
        this.emailAddress = emailAddress;
    }
    
    public User DoQUery(IQueryable<User> queryable)
    {
        // performs db query which we don't want for unit tests.
    }
}

PrivateFieldExtensions.cs

public static class PrivateFieldExtensions
{
    public static object PeekPrivateField(this object subject, string field)
    {
        var privateFields = subject.GetType().GetFields(BindingFlags.NonPublic | BindingFlags.Instance);
        var fieldInfo = privateFields.FirstOrDefault(x => x.Name == field);
        if (fieldInfo == null)
        {
            return null;
        }

        return fieldInfo.GetValue(subject);
    }
}