Question

I have a search form that looks like this:

The code behind the form looks like this:

@using (Html.BeginForm())
{
    @Html.ValidationSummary()
    <div>
        @Html.DropDownList("SelectedType", Model.TypeOptions)
        @Html.DropDownList("SelectedSearch", Model.SearchOptions)
        @Html.TextBoxFor(x => x.SearchTerm)
        <input type="submit" value="Search" />
    </div>
}

What I want to do is dynamically construct a lambda where clause from the return options. E.g. if the user selects “Process No” and “Contains” then the lambda would look like

model.DataSource = _db.InstrumentLists.Where(x => x.Process_No.Contains(SearchTerm));

Or if the user selects “PLC No” and “Equals” then the lambda would look like

model.DataSource = _db.InstrumentLists.Where(x => x.PLC_No == SearchTerm);

I am trying to do this while avoiding a big case statement or an if stack, i.e. I don’t want the following:

if (SelectedType == "Process No" And SelectedSearch = "Contains")
   model.DataSource = _db.InstrumentLists.Where(x => x.Process_No.Contains(SearchTerm));
elseif (SelectedType == "Process No" And SelectedSearch = "Equals")
   model.DataSource = _db.InstrumentLists.Where(x => x.Process_No == SearchTerm);
...

Essentially I want to pass a reference to a class property, something to specify the test type (i.e contains, equals, starts with, etc) and a search term to a function, or something along those lines, and get back a predicate to put into my Where clause. I want this function to work dynamically so I shouldn’t have to modify it for every combination of property and test type.

Is this possible or is the only way to use Where with a string predicate parameter?

Edit: In case its important, I’m use EF for my data model so _db.InstrumentLists returns an ObjectSet<InstrumentList>.

Answer 1

you have 2 options here:

• make a “switch” based search method i.e. depending on the value of user selection executes a different Where and returns a DataSource

• use Dynamic Linq and construct the Where from a string

EDIT – example for Dynamic Linq:

model.DataSource = _db.InstrumentLists.Where(SelectedType + " == @0", SearchTerm);