public class StringConcatTests : Linq3IntegrationTest
{
    [Fact]
    public void String_concat()
    {
        var collection = CreateCollection();

        var query = collection.AsQueryable()
            .Where(i => string.Concat(i.A, i.B) == "A1B1");

        var results = query.ToList();

        results.Should().OnlyContain(i => (i.A + i.B) == "A1B1");
    }

    private IMongoCollection<Data> CreateCollection()
    {
        var collection = GetCollection<Data>("test");
        CreateCollection(
            collection,
            new Data { A = "A1", B = "B1" },
            new Data { A = "A2", B = "B2" });
        return collection;
    }

    private class Data
    {
        public string A { get; set; }

        public string B { get; set; }
    }
}