No generic method 'ThenBy' on type 'System.Linq.Queryable'

Question

I\'m trying to create a generic OrderBy(fields collection) method to use in a new EF .Net Core 2.1 and managed to write the below code:

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Answer 1

I feel you are overcomplicating everything:

// Or IEnumerable<SortColumn> columns
public static Func<IQueryable<TEntity>, IOrderedQueryable<TEntity>> GetOrderByFunction<TEntity>(params SortColumn[] columns)
{
    Type typeQueryable = typeof(IQueryable<TEntity>);
    ParameterExpression argQueryable = Expression.Parameter(typeQueryable, "p");
    Type entityType = typeof(TEntity);

    ParameterExpression arg = Expression.Parameter(entityType, "x");

    Expression resultExp = argQueryable;

    bool first = true;

    foreach (SortColumn sc in columns)
    {
        PropertyInfo pi = entityType.GetProperty(sc.FieldName, BindingFlags.IgnoreCase | BindingFlags.Public | BindingFlags.Instance);
        Expression propertyExpr = Expression.Property(arg, pi);
        Type propertyType = pi.PropertyType;
        LambdaExpression lambdaExp = Expression.Lambda(propertyExpr, arg);

        string methodName;

        if (first)
        {
            first = false;

            methodName = sc.Descending ? "OrderBy" : "OrderByDescending";
        }
        else
        {
            methodName = sc.Descending ? "ThenBy" : "ThenByDescending";
        }

        resultExp = Expression.Call(typeof(Queryable), methodName, new Type[] { entityType, propertyType }, resultExp, Expression.Quote(lambdaExp));
    }

    // Case empty columns: simply append a .OrderBy(x => true)
    if (first)
    {
        LambdaExpression lambdaExp = Expression.Lambda(Expression.Constant(true), arg);
        resultExp = Expression.Call(typeof(Queryable), "OrderBy", new Type[] { entityType, typeof(bool) }, resultExp, Expression.Quote(lambdaExp));
    }

    LambdaExpression orderedLambda = Expression.Lambda(resultExp, argQueryable);
    return (Func<IQueryable<TEntity>, IOrderedQueryable<TEntity>>)orderedLambda.Compile();
}

I'll add that calling FieldName what is really a PropertyName is a little... misleading :-)

Note that I've added the case for when columns is empty: this because you return a IOrderedQueryable, so an ordering must always be present.