dynamic LINQ string... c# part 2

Table<Search> is the table. The items should be of type Search.

ParameterExpression o = Expression.Parameter(typeof(Search));
Func<Search, string> getField = Expression.Lambda < Func<Search, string>(Expression.Property(o, fieldName), o);

Select allOpen in new window

What is a "dynamic LINQ string"?

louisfr,

I got the following two errors:

Error      1      'System.Func<Search,string>' is a 'type' but is used like a 'variable'       at line 2:
Func<Search, string> getField = Expression.Lambda < Func<Search, string>(Expression.Property(o, fieldName), o);

Error      2      'System.Web.UI.WebControls.ListItem' does not contain a definition for 'ToArray' and no extension method 'ToArray' accepting a first argument of type 'System.Web.UI.WebControls.ListItem' could be found (are you missing a using directive or an assembly reference?) at line 7:
select new ListItem(getField(theGroup.FirstOrDefault())).ToArray());

           ParameterExpression o = Expression.Parameter(typeof(Search));
           Func<Search, string> getField = Expression.Lambda < Func<Search, string>(Expression.Property(o, fieldName), o);

ListItem[] outputs = (from output in wscgsContext.Searches
                      orderby getField(output)
                      group output by getField(output) into theGroup
                      select new ListItem(getField(theGroup.FirstOrDefault())).ToArray());
           lb.Items.AddRange(outputs);

Select allOpen in new window

-------------------------
Ste5an,
The following worked to handle field named City. This is what I started with but it has been evolved the code I have in my original question.

           ListItem[] outputs4 = (from output in wscgsContext.Searches
                                  orderby output.City
                                  group output by output.City into theGroup
                                  select new ListItem(theGroup.FirstOrDefault().City)).ToArray();
           lb.Items.AddRange(outputs4);

Select allOpen in new window

re:> What is a "dynamic LINQ string"?
Take this code (which handles specifically field City) and make it work for any field name created by:
 string fieldName = lb.ID.Substring(3);

There's a missing > and a call to compile to create the delegate from the expression.

ParameterExpression o = Expression.Parameter(typeof(Search));
Func<Search, string> getField = Expression.Lambda<Func<Search, string>>(Expression.Property(o, fieldName), o).Compile();

Select allOpen in new window

louisfr,

This fixed one error and revised the second error stating:
Error      1      'System.Web.UI.WebControls.ListItem' does not contain a definition for 'ToArray' and no extension method 'ToArray' accepting a first argument of type 'System.Web.UI.WebControls.ListItem' could be found (are you missing a using directive or an assembly reference?)

at:

select  new ListItem(getField(theGroup.FirstOrDefault())).ToArray());

Check the parentheses. There is one too much after ToArray() that should be before it.

select  new ListItem(getField(theGroup.FirstOrDefault()))).ToArray();

Select allOpen in new window

runtime error at the end of this line:

select  new ListItem(getField(theGroup.FirstOrDefault()))).ToArray();

Method 'System.Object DynamicInvoke(System.Object[])' has no supported translation to SQL.

FYI, I am reading this: http://tomasp.net/blog/dynamic-linq-queries.aspx/

That means we can't use delegates. And we cannot use reflection and GetProperty. We have to use expressions without compiling them. Easy for the orderby and group. Let's see...

ParameterExpression o = Expression.Parameter(typeof(Table<Search>));
           var getField = Expression.Lambda<Func<Search, string>>(Expression.Property(o, fieldName), o);
ListItem[] outputs = (wscgsContext.Searches
                     .OrderBy(getField)
                     .GroupBy(getField)
                     .Select(theGroup=>theGroup.FirstOrDefault())
                     .Select(getField)
                     .Select(field => new ListItem(field))).ToArray();

Select allOpen in new window

1. I had to add ) in the line below:
.Select(field => new ListItem(field))).ToArray();

2. Runtime error: Instance property 'First_Name' is not defined for type 'System.Data.Linq.Table`1[Search]' at the end of line.
 var getField = Expression.Lambda<Func<Search, string>>(Expression.Property(o, fieldName), o);

You tried to use the expression with a field name which isn't in the Search table.

louisfr,

Basically, I am looking for a solution that woks. The link I gave you (http://tomasp.net/blog/dynamic-linq-queries.aspx/), seems to suggest this is possible.

I am starting to do its exercises to eventually make this happen. I will post the result here later.

Wish me luck,

Mike

Is there a "First_Name" property in the "Search" table?

Yes, it is the first one.

        Dictionary<string, bool> listBoxes =   new Dictionary<string, bool>();
        listBoxes.Add("First_Name", true);
        listBoxes.Add("Last_Name", true);
        listBoxes.Add("Address", true);
        listBoxes.Add("City", true);
        listBoxes.Add("State", true);
        listBoxes.Add("Zip_Code", true);
        listBoxes.Add("Phone", true);
        listBoxes.Add("Email", true);
        listBoxes.Add("Order_Date", true);
        listBoxes.Add("Title", true);
        listBoxes.Add("Unit_Price", true);
        listBoxes.Add("Quantity", true);

BTW, any help at https://www.experts-exchange.com/questions/28562832/Store-meta-data-from-dbml-table-in-a-dictionary-variable.html will be also appreciated.

Mike

Amazing solution. Thank you very much.

Mike