How do I combine multiple linq queries into one results set?

Question

I have a multi-select checkbox. Depending on which one is checked, I want to combine the results into a single query. Sort of like:

if (Checkbox1.Checked)
{
    var query1 = from t in table1 ...
}

if (Checkbox2.Checked)
{
    var query2 = from t in table2 ...
}

DataGridView1.DataSource = query1.Union(query2); // obviously doesnt
      // work since query1 and query2 are not defined in this scope.

Any idea how to combine these selectively?

Answer 1

If you make sure that they return the same anonymous type, you can do

query1.Concat(query2)
or query1.Union(query2)

depending on the behavior you want.

If the two results have the exact same property names/types then the compiler will generate one type for both and these will work.

Answer 2

Assuming the queries are of the same type, you could define the queries outside of the conditional statements.

First, a helper method that creates an empty enumerable of the same type as the parameter:

static IEnumerable<T> CreateEmptyEnumerable<T>(IEnumerable<T> templateQuery)
{
    return Enumerable.Empty<T>();
}

Then, the new code:

var query1 = from t in table1 ...
var query2 = from t in table2 ...
var finalQuery = CreateEmptyEnumerable(query1);

if (Checkbox1.Checked)
{
    finalQuery = query1;
}

if (Checkbox2.Checked)
{
    finalQuery = finalQuery.Union(query2);
}

DataGridView1.DataSource = finalQuery.ToList(); // to avoid re-enumeration

This performs just fine because the queries aren't actually executed until they're enumerated over, as in the call to ToList().

Answer 3

If you would not use var but a known type, then you could initialize query1 and query2 via

Enumerable.Empty<T> in case of the checkbox not being checked.

Answer 4

I just tried this in a console application, as I was curious, and it just worked....

class Program
{
    private class Data1
    {
        public int Code1 { get; set; }
        public string Value1 { get; set; }
    }

    private class Data2
    {
        public int Code2 { get; set; }
        public string Value2 { get; set; }
    }

    static void Main(string[] args)
    {
        var data1 = new[] { new Data1 { Code1 = 1, Value1 = "one" }, new Data1 { Code1 = 2, Value1 = "two" }, new Data1 { Code1 = 3, Value1 = "three" } };
        var data2 = new[] { new Data2 { Code2 = 101, Value2 = "aaa" }, new Data2 { Code2 = 102, Value2 = "bbb" }, new Data2 { Code2 = 103, Value2 = "ccc" } };

        var query1 = from d1 in data1 select new { code = d1.Code1, text = d1.Value1 };
        var query2 = from d2 in data2 select new { code = d2.Code2, text = d2.Value2 };

        var datasource = query1.Union(query2);

        foreach (var d in datasource)
        {
            Console.WriteLine(d);
        }
    }
}

However, if I change the fieldnames in the anonymous types to be different I get a compiler error, so it looks like the key is to have the names the same in your anonymous types. Or just have the result in a defined type :)

Answer 5

Using Rx you can do something like this:

public partial class _Default : System.Web.UI.Page
{
    IEnumerable<int> table1;
    IEnumerable<int> table2;

    protected void Page_Load(object sender, EventArgs e)
    {
        table1 = Enumerable.Range(0, 10);
        table2 = Enumerable.Range(10, 10);
    }

    protected void Button1_Click(object sender, EventArgs e)
    {
        var query = 
            Observable.If(() => CheckBox1.Checked,
                (from t in table1 select t).ToObservable(), Observable.Empty<int>())
            .Concat(
                Observable.If(() => CheckBox2.Checked,
                (from t in table2 select t).ToObservable(), Observable.Empty<int>())
            );

        query.Subscribe(i => Response.Write(i));
    }
}