ASP.Net Error: Violation of PRIMARY KEY constraint Cannot insert duplicate key in object

Hi NoorAhmed2,

Please refer below sample.

HTML

Name :
<asp:TextBox runat="server" ID="txtName" />
<br />
Country:
<asp:TextBox runat="server" ID="txtCountry" />
<asp:Button Text="Insert" runat="server" OnClick="Insert" />
<br />
<asp:GridView ID="gvCustomers" runat="server" AutoGenerateColumns="false">
    <Columns>
        <asp:BoundField DataField="CustomerId" HeaderText="CustomerId" />
        <asp:BoundField DataField="Name" HeaderText="Name" />
        <asp:BoundField DataField="Country" HeaderText="Country" />
    </Columns>
</asp:GridView>

Namespaces

C#

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

VB.Net

Imports System.Data.SqlClient
Imports System.Data

Code

C#

protected void Page_Load(object sender, EventArgs e)
{
    if (!this.IsPostBack)
    {
        BindGrid();
    }
}

private void BindGrid()
{
    string strConnString = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
    SqlConnection con = new SqlConnection(strConnString);
    using (SqlCommand cmd = new SqlCommand("SELECT CustomerId, Name, Country FROM Customers", con))
    {
        using (SqlDataAdapter da = new SqlDataAdapter(cmd))
        {
            DataTable dt = new DataTable();
            da.Fill(dt);
            this.gvCustomers.DataSource = dt;
            this.gvCustomers.DataBind();
        }
    }
}

protected void Insert(object sender, EventArgs e)
{
    Insert();
}

public void Insert()
{
    bool UserExist;
    UserExist = this.UserExit();
    if (UserExist == false)
    {
        string constr = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
        string sql = "INSERT INTO Customers(Name,Country) VALUES(@Name,@Country)";
        SqlConnection con = new SqlConnection(constr);
        SqlCommand cmd = new SqlCommand(sql, con);
        cmd.Parameters.AddWithValue("@Name", txtName.Text);
        cmd.Parameters.AddWithValue("@Country", txtCountry.Text);
        con.Open();
        cmd.ExecuteNonQuery();
        con.Close();
    }
    else
    {
        ScriptManager.RegisterStartupScript(this, this.GetType(), "CropImage", "alert('Name Already exists');", true);
    }
    BindGrid();
}

public bool UserExit()
{
    string strConnString = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
    SqlConnection con = new SqlConnection(strConnString);
    SqlCommand cmd = new SqlCommand("Select * From Customers Where Name =@Name ");
    cmd.Parameters.AddWithValue("@Name", txtName.Text);
    cmd.CommandType = CommandType.Text;
    cmd.Connection = con;
    con.Open();
    SqlDataReader rdr;
    rdr = cmd.ExecuteReader();
    if (rdr.HasRows)
    {
        return true;
    }
    else
    {
        return false;
    }
    con.Close();
}

VB.Net

Protected Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs) Handles Me.Load
    If Not Me.IsPostBack Then
        BindGrid()
    End If
End Sub

Private Sub BindGrid()
    Dim strConnString As String = ConfigurationManager.ConnectionStrings("constr").ConnectionString
    Dim con As SqlConnection = New SqlConnection(strConnString)
    Using cmd As SqlCommand = New SqlCommand("SELECT CustomerId, Name, Country FROM Customers", con)
        Using da As SqlDataAdapter = New SqlDataAdapter(cmd)
            Dim dt As DataTable = New DataTable()
            da.Fill(dt)
            Me.gvCustomers.DataSource = dt
            Me.gvCustomers.DataBind()
        End Using
    End Using
End Sub

Protected Sub Insert(ByVal sender As Object, ByVal e As EventArgs)
    Insert()
End Sub

Public Sub Insert()
    Dim UserExist As Boolean
    UserExist = Me.UserExit()

    If UserExist = False Then
        Dim constr As String = ConfigurationManager.ConnectionStrings("constr").ConnectionString
        Dim sql As String = "INSERT INTO Customers(Name,Country) VALUES(@Name,@Country)"
        Dim con As SqlConnection = New SqlConnection(constr)
        Dim cmd As SqlCommand = New SqlCommand(sql, con)
        cmd.Parameters.AddWithValue("@Name", txtName.Text)
        cmd.Parameters.AddWithValue("@Country", txtCountry.Text)
        con.Open()
        cmd.ExecuteNonQuery()
        con.Close()
    Else
        ScriptManager.RegisterStartupScript(Me, Me.[GetType](), "CropImage", "alert('Name Already exists');", True)
    End If

    BindGrid()
End Sub

Public Function UserExit() As Boolean
    Dim strConnString As String = ConfigurationManager.ConnectionStrings("constr").ConnectionString
    Dim con As SqlConnection = New SqlConnection(strConnString)
    Dim cmd As SqlCommand = New SqlCommand("Select * From Customers Where Name =@Name ")
    cmd.Parameters.AddWithValue("@Name", txtName.Text)
    cmd.CommandType = CommandType.Text
    cmd.Connection = con
    con.Open()
    Dim rdr As SqlDataReader
    rdr = cmd.ExecuteReader()

    If rdr.HasRows Then
        Return True
    Else
        Return False
    End If

    con.Close()
End Function

Screenshot