Keep NULL rows last on Dynamic Linq Order By

Question

I am using this snippet below for Ordering my Linq queries dynamically and works great. I am not great at reflection or complex linq queries but I need a way that when ascendin

Answer 1

One approach is to pass an additional expression for testing for null into the method, and use it in an additional OrderBy/ThenBy clause.

Two OrderBy clauses would be produced - the first one will be on nullOrder, while the second one will be on the actual property.

private static IOrderedQueryable ApplyOrderBy(IQueryable collection, OrderByInfo orderByInfo, Expression> nullOrder) {
    ...
    if (!orderByInfo.Initial && collection is IOrderedQueryable) {
        if (orderByInfo.Direction == SortDirection.Ascending)
            methodName = "ThenBy";
        else
            methodName = "ThenByDescending";
    } else {
        if (orderByInfo.Direction == SortDirection.Ascending)
            methodName = "OrderBy";
        else
            methodName = "OrderByDescending";
    }
    if (nullOrder != null) {
        collection = (IQueryable)typeof(Queryable).GetMethods().Single(
                method => method.Name == methodName
                        && method.IsGenericMethodDefinition
                        && method.GetGenericArguments().Length == 2
                        && method.GetParameters().Length == 2)
                .MakeGenericMethod(typeof(T), type)
                .Invoke(null, new object[] { collection, nullOrder });
        // We've inserted the initial order by on nullOrder,
        // so OrderBy on the property becomes a "ThenBy"
        if (orderByInfo.Direction == SortDirection.Ascending)
            methodName = "ThenBy";
        else
            methodName = "ThenByDescending";
    }
    // The rest of the method remains the same
    return (IOrderedQueryable)typeof(Queryable).GetMethods().Single(
                method => method.Name == methodName
                        && method.IsGenericMethodDefinition
                        && method.GetGenericArguments().Length == 2
                        && method.GetParameters().Length == 2)
                .MakeGenericMethod(typeof(T), type)
                .Invoke(null, new object[] { collection, lambda });
}

The caller would need to pass a null checker explicitly. Passing null for non-nullable fields should work. You can construct them once, and pass as needed:

static readonly Expression> NullStringOrder = s => s == null ? 1 : 0;
static readonly Expression> NullIntOrder = i => !i.HasValue ? 1 : 0;
static readonly Expression> NullLongOrder = i => !i.HasValue ? 1 : 0;