Restrict user to enter value in TextBox based on DropDownList using C# and VB.Net in ASP.Net

Hi ryutenkan,

Check this sample now take its reference and correct your code.

HTML

<asp:ScriptManager ID="ScriptManager1" runat="server">
</asp:ScriptManager>
Select Test :
<asp:ComboBox ID="cbTest" runat="server" AutoCompleteMode="SuggestAppend">
    <asp:ListItem Text="1" Value="1"></asp:ListItem>
    <asp:ListItem Text="2" Value="2"></asp:ListItem>
    <asp:ListItem Text="3" Value="3"></asp:ListItem>
</asp:ComboBox><br /><br />
Enter Value :
<asp:TextBox ID="txtValue" runat="server" AutoPostBack="true" OnTextChanged="OnCheckValue" />
<asp:Label ID="lblMessage" Text="" runat="server" />

Namespaces

C#

using System.Configuration;
using System.Data;
using System.Data.SqlClient;
using System.Drawing;

VB.Net

Imports System.Configuration
Imports System.Data
Imports System.Data.SqlClient
Imports System.Drawing

Code

C#

protected void OnCheckValue(object sender, EventArgs e)
{
    string value = txtValue.Text.Trim();
    string cbxValue = cbTest.SelectedValue;
    using (SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["constr"].ConnectionString))
    {
        using (SqlCommand cmd = new SqlCommand("SELECT Start FROM tblComboBox WHERE Start=@Start AND Test=@Test", con))
        {
            cmd.CommandType = CommandType.Text;
            cmd.Parameters.AddWithValue("@Start", value);
            cmd.Parameters.AddWithValue("@Test", cbxValue);
            con.Open();
            int start = Convert.ToInt32(cmd.ExecuteScalar());
            con.Close();
            if (start != 0)
            {
                lblMessage.Text = "Already Exists";
                lblMessage.ForeColor = Color.Red;
            }
            else
            {
                lblMessage.Text = "Available";
                lblMessage.ForeColor = Color.Green;
            }
        }
    }
}

VB.Net

Protected Sub OnCheckValue(ByVal sender As Object, ByVal e As EventArgs)
    Dim value As String = txtValue.Text.Trim()
    Dim cbxValue As String = cbTest.SelectedValue
    Using con As SqlConnection = New SqlConnection(ConfigurationManager.ConnectionStrings("constr").ConnectionString)
        Using cmd As SqlCommand = New SqlCommand("SELECT Start FROM tblComboBox WHERE Start=@Start AND Test=@Test", con)
            cmd.CommandType = CommandType.Text
            cmd.Parameters.AddWithValue("@Start", value)
            cmd.Parameters.AddWithValue("@Test", cbxValue)
            con.Open()
            Dim start As Integer = Convert.ToInt32(cmd.ExecuteScalar())
            con.Close()
            If start <> 0 Then
                lblMessage.Text = "Already Exists"
                lblMessage.ForeColor = Color.Red
            Else
                lblMessage.Text = "Available"
                lblMessage.ForeColor = Color.Green
            End If
        End Using
    End Using
End Sub

Screenshot