This is a rough sample but basically what you can do is instead of creating a control parameter you can create a session parameter or something similar:
So when you click enter it will use the textbox value or when you change the dropdownlist it will use the dropdownlist's value.
You can also have radio button's giving the user the option to specify from where he want's the value.
<asp:DropDownList ID="ddl" runat="server" AutoPostBack="true"
onselectedindexchanged="ddl_SelectedIndexChanged"></asp:DropDownList>
<asp:TextBox ID="txt" runat="server"></asp:TextBox>
<asp:Button runat="server" Text="ClickMe" ID="btnOne" OnClick="btnOne_Click"/>
<asp:ObjectDataSource ID="ObjectDataSource1" runat="server">
<SelectParameters>
<asp:SessionParameter SessionField="ObjectParameterName" />
</SelectParameters>
</asp:ObjectDataSource>
Code Behind:
protected void ddl_SelectedIndexChanged(object sender, EventArgs e)
{
var ddl = (DropDownList)sender;
Session["ObjectParameterName"] = ddl.SelectedValue;
ObjectDataSource1.Select();
}
protected void btnOne_Click(object sender, EventArgs e)
{
var ddl = (DropDownList)sender;
Session["ObjectParameterName"] = txt.Text;
ObjectDataSource1.Select();
}
EDIT AFTERTHOUGHT
You can also instead of assigning the parameter to a session field, just set the objectdatasource's parameter directly (Barring Exception handling).
protected void ddl_SelectedIndexChanged(object sender, EventArgs e)
{
var ddl = (DropDownList)sender;
ObjectDataSource1.SelectParameters.Add(new Parameter() {Name="Name",DefaultValue=ddl.SelectedValue });
}