How can I autocomplete values in a TextBox from SQL Server table in C# with ASP.NET

You can either use stored procedure or LINQ with Entity Framework. Here is an example of stored procedure.

Stored procedure:

SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO

CREATE PROCEDURE [dbo].[AutoComplete_Search]
     @keyword NVARCHAR(50)
AS
BEGIN
    SELECT Name, Ascii_Name, Alternate_names
    FROM your_table
    WHERE Name LIKE '%' + @keyword + '%' OR Ascii_Name LIKE '%' + @keyword + '%' OR Alternate_names LIKE '%' + @keyword + '%'

END

.cshtml code is:

<script type="text/javascript">
        $(function () {
            $("#txtCustomer").autocomplete({
                source: function (request, response) {
                    $.ajax({
                        url: '/Home/AutoComplete/',
                        data: "{ 'keyword': '" + request.term + "'}",
                        dataType: "json",
                        type: "POST",

                        contentType: "application/json; charset=utf-8",
                        success: function (data) {
                            response($.map(data, function (item) {
                                return item;
                            }))
                        },
                        error: function (response) {
                            alert(response.responseText);
                        },
                        failure: function (response) {
                            alert(response.responseText);
                        }
                    });
                },
                select: function (e, i) {
                    $("#hfCustomer").val(i.item.val);
                },
                minLength: 1
            });
        });
    </script>
    @using (Html.BeginForm("Index", "Home", FormMethod.Post))
    {
        <input type="text" id="txtCustomer" name="CustomerName" />    
    }

This is controller.

[HttpPost]
public JsonResult AutoComplete(string keyword)
{
    using (DatabaseEntities entities = new DatabaseEntities())
    {
        var records = entities.AutoComplete_Search(keyword).ToList();
        return Json(records);
    }
}

You might use the Ajax Control Toolkit. Example:

<asp:ScriptManager ID="ScriptManager1" runat="server"
EnablePageMethods = "true">
</asp:ScriptManager>

<asp:TextBox ID="txtContactsSearch" runat="server"></asp:TextBox>
<cc1:AutoCompleteExtender ServiceMethod="SearchCustomers"
    MinimumPrefixLength="2"
    CompletionInterval="100" EnableCaching="false" CompletionSetCount="10"
    TargetControlID="txtContactsSearch"
    ID="AutoCompleteExtender1" runat="server" FirstRowSelected = "false">
</cc1:AutoCompleteExtender>

[System.Web.Script.Services.ScriptMethod()]
[System.Web.Services.WebMethod]
public static List<string> SearchCustomers(string prefixText, int count)
{
    using (SqlConnection conn = new SqlConnection())
    {
        conn.ConnectionString = ConfigurationManager
                .ConnectionStrings["constr"].ConnectionString;
        using (SqlCommand cmd = new SqlCommand())
        {
            cmd.CommandText = "select ContactName from Customers where " +
            "ContactName like @SearchText + '%'";
            cmd.Parameters.AddWithValue("@SearchText", prefixText);
            cmd.Connection = conn;
            conn.Open();
            List<string> customers = new List<string>();
            using (SqlDataReader sdr = cmd.ExecuteReader())
            {
                while (sdr.Read())
                {
                    customers.Add(sdr["ContactName"].ToString());
                }
            }
            conn.Close();
            return customers;
        }
    }
}

https://www.aspsnippets.com/Articles/AJAX-AutoCompleteExtender-Example-in-ASPNet.aspx

https://ajaxcontroltoolkit.devexpress.com/AutoComplete/AutoComplete.aspx