Check at run time if primary key exists - asp.net

Question

I am working in asp.net. I have a textbox named formidtxt and another textbox is colortxt. Now what I want is that when a user enters an Form ID in formidtxt then at the same time it should start checking whether there already exists a form id with same ID that has been entered and if Form ID already exists in database then the color of colortxt textbox should change to red else it should be green.
I have an idea that it can be done by using events in text boxes but can't understand the working. My database is in SQL Server 2008.

Answer 1

Try this C# code;
private void Page_Load(object sender, EventArgs e)
{
// formidtxt is the name of the textbox
this.formidtxt.TextChanged += FormIDTextBox_TextChanged;
formidtxt.AutoPostBack = true;
}
Note that this method was written inside the Page_Load method.
TextChanged is an event and it occurs when the text is modified in a TextBox.
In this case, when the formidtxt (textbox) text changes, it will call the FormIDTextBox_TextChanged method.
private void FormIDTextBox_TextChanged(object sender, EventArgs e)
{
int x = 0;
// convert textbox text (string) to int
Int32.TryParse(formidtxt.Text, out x);
// call IsIDAvailableDAO method
// x is the converted int value
if (IsIDAvailableDAO(x))
{
colortxt.BackColor = System.Drawing.Color.Red;
}
else
{
colortxt.BackColor = System.Drawing.Color.Green;
}
}
This method will get the text from the textbox (formidtxt) and send it to the IsIDAvailableDAO method as a parameter.
Using the IsIDAvailableDAO method, we can check whether the ID is available in the database or not. If it is available, then the method will return a TRUE boolean value. If not, it will return a False boolean value.
According to that boolean value, you can change the color of the colortxt textbox as you want or do something else.
private Boolean IsIDAvailableDAO(int id)
{
Boolean output;
using (SqlConnection myConnection = new SqlConnection("Data Source=localhost;Initial Catalog=Testing;Integrated Security=True"))
{
string query = #"SELECT CASE WHEN COUNT(ID) >= 1 THEN CAST( 1 as BIT ) ELSE CAST( 0 as BIT )
END As IsAvailable
FROM TableName
WHERE ID = #ID";
SqlCommand cmd = new SqlCommand(query, myConnection);
cmd.Parameters.AddWithValue("#ID", id);
myConnection.Open();
output = (Boolean)cmd.ExecuteScalar();
myConnection.Close();
}
return output;
}
In this method (IsIDAvailableDAO), Please change the query (TableName, ID, etc.) and connectionstring as appropriate.
You also has to add this namespace: using System.Data.SqlClient;
https://www.connectionstrings.com/sql-server-2008/
https://learn.microsoft.com/en-us/dotnet/csharp/programming-guide/namespaces/using-namespaces