Selecting columns from a LINQ query

Question

I want to be able to select the columns that are displayed in my DataGridView. I.e., I have a linq query that returns an IEnumerable but I don't want to display all the properties of Policy - I want to allow the user to choose what to display. So I thought something like this might work to create a "narrower" object with only the columns I want.

IEnumerable<Policy> policies = repository.GetPolicies();
var results = from p in policies select new {
    if (userPicked("PropertyOne")) PropertyOne = p.PropertyOne,
    if (userPicked("PropertyTwo")) PropertyTwo = p.PropertyTwo,
    ...
};
dataGridView1.DataSource = results;

Is there an elegant way to do this?

Part 2: Why does this work:

IEnumerable<Policy> policies = repository.GetPolicies();
dataGridView1.DataSource = policies;

but this doesn't:

IEnumerable<Policy> policies = repository.GetPolicies();
var results = from p in policies select p;
dataGridView1.DataSource = results;

In the second case nothing is displayed in the grid.

Robert Harvey · Accepted Answer · 2009-10-21 18:25:08Z

3

Use Dynamic Linq if you need to select fields dynamically. It contains a Select extension method that accepts a string of fields, so you can decide which fields to select at runtime:

public static IQueryable Select(this IQueryable source,    
       string selector, params object[] values);

Example of usage:

var products = db.Products
    .Where(condition)
    .OrderBy("Name")
    .Select(" new (ItemId,Name,Model,onsale)");

Part 2: Your grid is expecting a bindable interface such as IList, but the second code example returns an IQueryable. You can convert it to an IList by calling ToList().

var results = (from p in policies select p).ToList();

edited Oct 21, 2009 at 18:25

answered Oct 20, 2009 at 21:58

Robert Harvey

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