How to create an expression at runtime for use in GroupBy() with Entity Framework?

Question

I am building a new extension method which will dynamically be able to group query results from Entity Framework.

I have been able to build dynamic "where" expressions using LinqKit, but this seems to be a different animal.

Intended usage of new extension method:

var results = entities.GroupBy("someFieldName").ToList();

Extension method definition:

    public static IQueryable<IGrouping<object, TEntity>> GroupBy<TEntity>(
        this IQueryable<TEntity> source,
        string fieldName) 
        where TEntity : class, IDataEntity
    {
        if (string.IsNullOrEmpty(fieldName))
        {
            return new List<IGrouping<object, TEntity>>().AsQueryable();
        }

        var parameter = Expression.Parameter(typeof(TEntity), "x");
        var fieldXExpression = Expression.Property(parameter, fieldName);
        var lambda = Expression.Lambda<Func<TEntity, object>>(
            Expression.Convert(fieldXExpression, typeof(object)), // throws error when using EF
            parameter);
        return source.AsExpandable().GroupBy(lambda);
    }

I need to use Expression.Convert(...) because when I was testing using linqTOobjects, the code failed when the column was a int32. So I needed to manually convert the column to object and it worked great.

Now that I'm testing it with EF entities, I guess that EF is trying to translate the convert to equivalent SQL, which of course I know doesn't exist.

The error:

System.NotSupportedException: Unable to cast the type 'System.String' to type 'System.Object'. LINQ to Entities only supports casting EDM primitive or enumeration types.

Is there a way to generate an expression suitable for GroupBy at runtime that is also compatible with EF?

Thanks for any guidance.

Answer 1

I was able to accomplish what I needed by using Dynamic Linq. I created an extension method called ListGroupKeys:

    public static List<object> ListGroupKeys<TEntity>(this IQueryable<TEntity> source, string fieldName)
        where TEntity : class, IDataEntity
    {
        var results = source.GroupBy(fieldName, "it").Select("new (KEY as Group)");

        var list = new List<object>();
        foreach (var result in results)
        {
            var type = result.GetType();
            var prop = type.GetProperty("Group");

            list.Add(prop.GetValue(result));
        }

        return list;
    }

This allowed me to get the group key values for use in subsequent queries.