IQueryable does not contain a definition for Count or OrderBy .. Can't figure out why

Brand new to C#, ASP.NET Core and web development in general. I'm trying to implement a generic method that I can reuse with the JQuery DataGrid. The method is as follows:

protected String ItemsToJson(IQueryable items, List<String> columnNames, String sort, String order, Int32 limit, Int32 offset)
        {
            try
            {
                Int32 count = items.Count();
                String sortExpression = "";

                if (sort != null && sort.Length > 0)
                    sortExpression += String.Format("{0} {1}", sort, order);

                if (limit == 0)
                    limit = count;

                var result = new
                {
                    total = count,
                    rows = items.OrderBy(sortExpression).Skip(offset).Take(limit).Select("new (" + String.Join(",", columnNames) + ")")
                };

                return JsonConvert.SerializeObject(result, Formatting.None, new JsonSerializerSettings() { MetadataPropertyHandling = MetadataPropertyHandling.Ignore });
            }
            catch (Exception ex)
            {
                Console.WriteLine(ex.Message);
                return null;
            }
        }

The line reading Int32 count = items.Count(); returns the following error:

'IQueryable' does not contain a definition for 'Count' and no accessible extension method 'Count' accepting a first argument of type 'IQueryable' could be found (are you missing a using directive or an assembly reference?)

The line reading rows = Items.OrderBy(sortExpression)..... returns the following error:

'IQueryable' does not contain a definition for 'OrderBy' and no accessible extension method 'Count' accepting a first argument of type 'IQueryable' could be found (are you missing a using directive or an assembly reference?)

I have a using for System.Linq. Any thoughts? It's driving me crazy!

Thanks bunch.

--- Val

What is the type of item provided by the IQueryable items?

The Count method is available against IQueryable<T>, meaning if you change the incoming parameter to IQueryable<SomeType> items, you should be able to access it.

https://learn.microsoft.com/en-us/dotnet/api/system.linq.iqueryable-1

The IQueryable interface is intended for implementation by query providers. It is only supposed to be implemented by providers that also implement IQueryable<T>. If the provider does not also implement IQueryable<T>, the standard query operators cannot be used on the provider's data source.