Passing an IComparer parameter to custom LINQ OrderBy extension method

问题

After a good dose of Googling and trying some things and not finding/getting the desired result I decided to post this question.

I have a custom made OrderBy extension method and now when performing the OrderBy operation I'd like to pass an AlphanumComparator like this:

return divergences.OrderBy(sort, new AlphanumComparator());

Here's the extension method:

public static IQueryable<T> OrderBy<T>(this IQueryable<T> collection,
    GridSortOptions sortOptions, AlphanumComparator comparer = null)
{
    if (string.IsNullOrEmpty(sortOptions.Column))
    {
        return collection;
    }

    Type collectionType = typeof(T);

    ParameterExpression parameterExpression = Expression.Parameter(collectionType, "p");

    Expression seedExpression = parameterExpression;

    Expression aggregateExpression = sortOptions.Column.Split('.').Aggregate(seedExpression, Expression.Property);

    MemberExpression memberExpression = aggregateExpression as MemberExpression;

    if (memberExpression == null)
    {
        throw new NullReferenceException(string.Format("Unable to cast Member Expression for given path: {0}.", sortOptions.Column));
    }

    LambdaExpression orderByExp = Expression.Lambda(memberExpression, parameterExpression);

    const string orderBy = "OrderBy";

    const string orderByDesc = "OrderByDescending";

    Type childPropertyType = ((PropertyInfo)(memberExpression.Member)).PropertyType;

    string methodToInvoke = sortOptions.Direction == MvcContrib.Sorting.SortDirection.Ascending ? orderBy : orderByDesc;

    MethodCallExpression orderByCall;

    orderByCall = Expression.Call(typeof(Queryable), methodToInvoke, new[] { collectionType, childPropertyType }, collection.Expression, Expression.Quote(orderByExp));

    if(comparer != null)
    {
       // How can I pass the comparator to the OrderBy MethodCallExpression?

       // Using the standard LINQ OrderBy, we can do this:
       // elements.OrderBy(e => e.Index, new AlphanumComparator())
    }

    return collection.Provider.CreateQuery<T>(orderByCall);
}

See the comment in the code where I think I should pass the IComparer... how could I approach this?

回答1:

I had to approach this differently.

I was trying to create a generic OrderBy to be used with MvcContrib Grid, but passing the IComparer to that custom OrderBy expression did not work as I imagined it would work.

So I created this helper that receives a string in dot notation like Element1.Standard.Chapter.Manual.Name and then returns an Expression<Func<T, string>>:

public static Func<T, string> CreateSelectorExpression<T>(string propertyName) where T : class
{
    ParameterExpression parameterExpression = Expression.Parameter(typeof(T));

    Expression aggregateExpression = propertyName.Split('.').
        Aggregate(parameterExpression as Expression, Expression.Property) as MemberExpression;

    LambdaExpression exp = Expression.Lambda(aggregateExpression, parameterExpression);

    return (Func<T, string>)exp.Compile();
}

This expression typed to T (in this case Divergence object type) can then be passed (see func.Invoke) to the standard LINQ OrderBy operator where I can also pass the custom IComparer AlphanumComparator like this:

if (sort.Column.Contains("Index"))
{
    var func = Helpers.ExtensionMethods.CreateSelectorExpression<Divergence>(sort.Column);

    if (sort.Direction == SortDirection.Ascending)
    {
        return divergences.OrderBy(func, new AlphanumComparator());
    }
    else
    {
        return divergences.OrderByDescending(func, new AlphanumComparator());
    }
}

This involved a little bit more work but solved the problem in a generic fashion the way I wanted it to be.

来源：https://stackoverflow.com/questions/17796800/passing-an-icomparer-parameter-to-custom-linq-orderby-extension-method

标签

linq

lambda

sql-order-by

icomparer