Dynamic Order (SQL ORDERBY) in LINQ CompiledQuery

问题

how can I create a dynamic ORDERBY in my LINQ CompiledQuery (e.g. supply Order Field and Direction as parameters for the compiled query)?

回答1:

I think I found it:

Check out this link. It will point you to the VS2008 code samples which contains a Dynamic Linq Query Library which contains the extension method below. This will allow you to go:

Object.OrderBy("ColumnName");

Here is the extension methods, but you may want the whole library.

public static IQueryable<T> OrderBy<T>(this IQueryable<T> source, string ordering, params object[] values) {
    return (IQueryable<T>)OrderBy((IQueryable)source, ordering, values);
}

public static IQueryable OrderBy(this IQueryable source, string ordering, params object[] values) {
    if (source == null) throw new ArgumentNullException("source");
    if (ordering == null) throw new ArgumentNullException("ordering");
    ParameterExpression[] parameters = new ParameterExpression[] {
        Expression.Parameter(source.ElementType, "") };
    ExpressionParser parser = new ExpressionParser(parameters, ordering, values);
    IEnumerable<DynamicOrdering> orderings = parser.ParseOrdering();
    Expression queryExpr = source.Expression;
    string methodAsc = "OrderBy";
    string methodDesc = "OrderByDescending";
    foreach (DynamicOrdering o in orderings) {
        queryExpr = Expression.Call(
            typeof(Queryable), o.Ascending ? methodAsc : methodDesc,
            new Type[] { source.ElementType, o.Selector.Type },
            queryExpr, Expression.Quote(Expression.Lambda(o.Selector, parameters)));
        methodAsc = "ThenBy";
        methodDesc = "ThenByDescending";
    }
    return source.Provider.CreateQuery(queryExpr);
}

回答2:

I would do it this way, first all you really need is a way to access the property value by string on an object. You could use reflection, but its slow. So use this helper class approach which is based on the tests of http://stefan.rusek.org/Posts/LINQ-Expressions-as-Fast-Reflection-Invoke/3/

    public static class LINQHelper
    {
        public static IComparable OrderByProperty<TClass>(TClass item, 
                                                          string propertyName)
        {
            var t = Expression.Parameter(typeof(TClass), "t");
            var prop = Expression.Property(t, propertyName);
            var exp = Expression.Lambda(prop, t).Compile();
            return (IComparable)exp.DynamicInvoke(item);
        }

     }

The in your code where you want your order by string of property name, in this example col1, you just do the following.

     var myQuery = from i in Items
                   select i;

     myQuery.OrderBy(i=>LINQHelper.OrderByProperty(i,"col1"));

Hope this helps.

来源：https://stackoverflow.com/questions/1425110/dynamic-order-sql-orderby-in-linq-compiledquery

标签

linq

compiled-query