Attaching a Button_Click event occurs when you press enter in a ASP.NET TextBox

<asp:Panel ID="PanelSearch" runat="server" DefaultButton="ButtonSearch">
	<asp:TextBox ID="TextBoxSearch" runat="server" />
	<asp:Button ID="ButtonSearch" runat="server" Text="Search" OnClick="ButtonSearch_Click" />
</asp:Panel>
<asp:Panel ID="PanelSearchResults" runat="server">
	<asp:Literal ID="LiteralSearchResults" runat="server"></asp:Literal>
</asp:Panel>

Code-behind:

    /// <summary>
    /// Event that occurs when you click ButtonSearch,
    /// or when you press enter on TextBoxSearch
    /// </summary>
    /// <param name="sender"></param>
    /// <param name="e"></param>
    protected void ButtonSearch_Click(object sender, EventArgs e)
    {
        String searchResults = String.Empty;
        if(TextBoxSearch.Text!="")
        {
            String searchItem = TextBoxSearch.Text;
            int numbersOfResult = GetNumbersOfResult(searchItem);

            if (0 == numbersOfResult)
                searchResults = "No items were found";
            else
                searchResults = String.Format("{0} items found", numbersOfResult);
            LiteralSearchResults.Text = searchResults;

            // Other code ...
        }
    }

1. Wrap the TextBoxSearch and ButtonSearch in an <asp:Panel ID="PanelSearch
2. Set the DefaultButton property of the PanelSearch to the ID of button, in the case of the example ButtonSearch.
3. Write the necessary code, inside the method ButtonSearch_Click( ... )