Untitled

How to mock a public readonly field
public abstract class DataService
{
    public readonly string DataDirectory; //I need to mock the return value

    protected DataService()
    {
        DataDirectory = "c:temp";
    }
}

public abstract class DataService
{
    public readonly DirectoryInfo DataDirectory;

    protected GitService()
    {
        DataDirectory = new DirectoryInfo("c:temp");
    }

    public virtual object GetRepositoryByName(string name)
    {
         //Locate the repo, build it and return it
         string path = Path.Combine(DataDirectory.FullName, name);
         return new BuildRepository(path);
    }
}