how to retrive the username of the current user after login in asp.net

If you're using the ASP.NET membership:

string userName = Membership.GetUser().UserName

However, obviously you are not using it (i strongly recommend). Why do you redirect to logout.aspx when the user successfully provided his username and password?

Apart from that you're not using the provided informatiuons at all in your query.

SqlCommand cmdr = new SqlCommand("Select name,password From registration", con);

So you should use parameters to filter for the correct record:

using(var cmdr = new SqlCommand("Select name,password From registration where name=@name and password=@password", con))
{
    cmdr.Parameters.AddWithValue("@name", userName);
    cmdr.Parameters.AddWithValue("@password", password);
    if(cmdr.ExecuteReader().HasRows)
    {
        // user + password correct
    }
    else
    {
        // user or password incorrect
    }
}

Membership.GetUser().UserName in a membership provider. Under no circumstances should you run the code you have there. It is asking for a hack. You should not be loading the passwords into memory and you are going to have performance issues as you gain more users because of your loop through all users!

protected void Button1_Click(object sender, EventArgs e)
{
    SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["con1"].ConnectionString);
    con.Open();
    SqlCommand cmdr = new SqlCommand("Select name,password From registration", con);
    SqlDataReader dr = cmdr.ExecuteReader();
    while (dr.Read())
    {
        if (txt_name.Text == dr[0].ToString() && txt_pass.Text == dr[1].ToString())
        {
            Session["UserName"]=dr[0].ToString(); // OR Session["UserName"]=txt_name.Text;
            Response.Redirect("logout.aspx");
        }
        else
        {
            label4.Text ="Invalid Username/Password";
        }
    }

}

You can fetch the session values in HTML(if your using it in your application)

<div>
Welcome <%=HttpContext.Current.Session["UserName"]%>
</div>

You should be able to get the identity of the user from the User property in code-behind.

myLabel.Text = User.Identity.Name;

The full namespace etc. for this is HttpContext.Current.User.Identity.Name.

Reference for HttpContext.User property: http://msdn.microsoft.com/en-us/library/system.web.httpcontext.user.aspx

put the username in a session

Session["username"] = dr[0].ToString();

then on the other page

if Session["username"] != null
{
  String username = Session["username"].ToString();
}
else
{
  Page.Redirect("login.aspx");
}

You can check each different page

You should ideally use built-in ASP.NET Forms Authentication - see this article for quick start. Coming to your question, on successful login, you should use line such as

FormsAuthentication.RedirectFromLoginPage(txt_name.Text, true);

This would store the user name into authentication cookie and which can be subsequently retrieved using code such as User.Identity.Name that you can user anywhere in your pages.

You can store the username in session object and get username anywhere in the application if session is present

But you have to update your query also, The way you are calling is not good approach

protected void Button1_Click(object sender, EventArgs e)
    {
        SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["con1"].ConnectionString);
        con.Open();
        SqlCommand cmdr = new SqlCommand("Select name From registration where name = @username and password = @password", con);
        cmdr.Parameters.Add("@username", txt_name.Text.trim());
        cmdr.Parameters.Add("@password", txt_pass.Text.trim());

        SqlDataReader dr = cmdr.ExecuteReader();

        if(cmdr.ExecuteReader().HasRows)
        {
             Response.Redirect("logout.aspx");
             Session["userName"] = txt_name.Text.trim();
        }
        else{
        //Error page path
        }               
        //DOn't forget to close the connection when you Open it
      con.Close();
    }

You should learn more from some tutorials:

1. http://csharp-station.com/Tutorial/AdoDotNet/Lesson01
2. MSDN

..and if you're just wanting to use the User object:

var username = User.Identity.Name;