Can I create a generic GroupBy() in C# LINQ?-CodePudding

I am trying to modify the a generic IQueryable extension method to perform a GroupBy operation. The method I have performs a generic Where on a dynamically defined column:

        public static IQueryable<TEntity> WhereById<TEntity, TKey>(
                       this IQueryable<TEntity> query, TKey value, string colName)
            where TEntity : class
        {
            var param = Expression.Parameter(typeof(TEntity), "e");
            var propAccess = Expression.PropertyOrField(param, colName);
            var valExpr = Expression.Constant(value);
   
            BinaryExpression predicate;
            predicate = Expression.Equal(propAccess, valExpr);
            var predicateLambda = Expression.Lambda<Func<TEntity, bool>>(predicate, param);
            return query.Where(predicateLambda);
        }

This works perfectly as in:

IQueryable<TEntity> entities = _crudApiDbContext.Set<TEntity>()
                    .WhereById<TEntity, int>(id, selectField);

Now I need a generic GroupBy(). I am trying the following:

        public static IQueryable<TEntity> GroupBy<TEntity>(this IQueryable<TEntity> query, string colName)
            where TEntity : class
        {
            var param = Expression.Parameter(typeof(TEntity), "e");
            var propAccess = Expression.PropertyOrField(param, colName);

            BinaryExpression predicate;
            predicate = Expression.XXX(propAccess);  <=what should XXX be?
            var predicateLambda = Expression.Lambda<Func<TEntity, int>>(propAccess);
            return (IQueryable<TEntity>)query.GroupBy(predicateLambda);
        }

and I guess my question is what should XXX be? Or maybe because GroupBy() is an extension method the approach needs to be different?

Update There is a lot of interest as to why I need this - I am building a generic Blazor form for faceted browsing. So I have a generic method for filtered and sorted search, but I also need to know for certain columns with options, how many options remain after applying the search conditions. To that end I will perform a GroupBy on each of those columns with the IQueryable which has the search conditions applied to it. Example of faceted search:

CodePudding user response：

This is solution how to do grouping by dynamic field. Since we don't know type of grouping key, I've decided to make it as object.

public static class QueryableExtensions
{
    static Expression MakePropPath(Expression objExpression, string path)
    {
        return path.Split('.').Aggregate(objExpression, Expression.PropertyOrField);
    }

    public static IQueryable<IGrouping<object, TEntity>> GroupBy<TEntity>(this IQueryable<TEntity> query, string colName)
        where TEntity : class
    {
        var param      = Expression.Parameter(typeof(TEntity), "e");
        var propAccess = MakePropPath(param, colName);

        var keyLambda = Expression.Lambda(Expression.Convert(propAccess, typeof(object)), param);

        var groupCall = Expression.Call(typeof(Queryable), nameof(Queryable.GroupBy),
            new[] { typeof(TEntity), typeof(object) }, query.Expression,
            keyLambda);

        return query.Provider.CreateQuery<IGrouping<object, TEntity>>(groupCall);
    }
}

Usage is simple in your case:

var result = query.GroupBy("Genre")
    .Select(g => new 
    {
        g.Key,
        Count = g.Count()
    })
    .ToList();