Selecting distinct records in a declarative linq data source?

How can I select distinct records when using a declarative data source?

<asp:LinqDataSource ID="dsColors" runat="server" 
    ContextTypeName="Context" 
 开发者_C百科   OrderBy="Id, Name" 
    Select='new (Id, String.Concat(Id + ": " + Name) as ColorName)'
    TableName="Colors">
</asp:LinqDataSource>

Hack it with a Group By.

<asp:LinqDataSource ID="dsColors" runat="server" 
    ContextTypeName="Context" 
    OrderBy="Id, Name" 
    Select='new (Id, String.Concat(Id + ": " + Name) as ColorName)'
    TableName="Colors"
    GroupBy='new(Id,Name)'
>
</asp:LinqDataSource>

It's better to do it via on selecting

<asp:LinqDataSource ID="dsColors" runat="server" 
    ContextTypeName="Context" 
    OnSelecting="dsColors_Selecting"
    TableName="Colors">
</asp:LinqDataSource>

then in the code behind

private Context ctx;

protected void Page_Init(object sender, EventArgs e)
{
    ctx = new Context();
}

protected void dsColors_Selecting(object sender, LinqDataSourceSelectEventArgs e)
{
    var results = (from c in Colors
                   select c.ID).Distinct();
    e.Result = results;
}

something to this extent should work.

Using group by creates an object called key.

Group by single value:

GroupBy="Name"
Select="new (key as Name)"

Group by multiple values:

GroupBy="new(ID, Name)"
Select="new (key.ID, key.Name)"

To access other properties not in the group by use aggregate queries:

GroupBy="new(ID, Name)"
Select="new (key.ID, key.Name, min(OtherValue) as OtherValue)"