See https://mongorepository.codeplex.com/discussions/453750

MockMongoRepositority.cs

using MongoRepository;
using System;
using System.Collections.Generic;
using System.Linq;

public class MockMongoRepositority<T> : IRepository<T>
    where T : IEntity
{
    private IList<T> _collection { get; set; }
    public MockMongoRepositority()
    {
        _collection = new List<T>();
    }

    public MockMongoRepositority(IList<T> basecollection)
    {
        _collection = basecollection;
    }

    public void Add(IEnumerable<T> entities)
    {
        foreach (var e in entities)
            this.Add(e);
    }

    public T Add(T entity)
    {
        entity.Id = Guid.NewGuid().ToString();
        _collection.Add(entity);
        return entity;
    }

    public MongoDB.Driver.MongoCollection<T> Collection
    {
        get { throw new NotImplementedException(); }
    }

    public long Count()
    {
        return _collection.Count();
    }

    public void Delete(System.Linq.Expressions.Expression<Func<T, bool>> criteria)
    {
        var entitiesToDelete = _collection.AsQueryable().Where(criteria);
        foreach (var entity in entitiesToDelete)
            this.Delete(entity);
    }

    public void Delete(T entity)
    {
        this.Delete(entity.Id);
    }

    public void Delete(string id)
    {
        _collection.Remove(this.GetById(id));
    }

    public void DeleteAll()
    {
        _collection.Clear();
    }

    public bool Exists(System.Linq.Expressions.Expression<Func<T, bool>> criteria)
    {
        return _collection.AsQueryable().Any(criteria);
    }

    public T GetById(string id)
    {
        return _collection.First(x => x.Id == id);
    }

    public T GetSingle(System.Linq.Expressions.Expression<Func<T, bool>> criteria)
    {
        return _collection.AsQueryable().FirstOrDefault(criteria);
    }

    public void RequestDone()
    {
        throw new NotImplementedException();
    }

    public IDisposable RequestStart()
    {
        throw new NotImplementedException();
    }

    public void Update(IEnumerable<T> entities)
    {
        foreach (var entity in entities)
        {
            this.Update(entity);
        }
    }

    public T Update(T entity)
    {
        this.Delete(entity.Id);
        _collection.Add(entity);
        return entity;
    }

    public IEnumerator<T> GetEnumerator()
    {
        return this.GetEnumerator();
    }

    System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
    {
        return _collection.GetEnumerator();
    }

    public Type ElementType
    {
        get { return typeof(T); }
    }

    public System.Linq.Expressions.Expression Expression
    {
        get { return _collection.AsQueryable<T>().Expression; }
    }

    public IQueryProvider Provider
    {
        get { return _collection.AsQueryable<T>().Provider; }
    }
}