Step 1: Click New Project, then select Visual C# on the left, then Windows and then select Windows Forms Application. Name your project "LinkComboboxWithDatabaseValue" and then click OK
Step 2: Design your form as below

Step 3: Create an Entity Framework Model, then select Category and Product table from Northwind database

Step 4: Add code to handle your form as below
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
namespace LinkComboboxWithDatabaseValue
{
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
}
private void cboCategory_SelectionChangeCommitted(object sender, EventArgs e)
{
Category obj = cboCategory.SelectedItem as Category;
if (obj != null)
{
using (NorthwindEntities db = new NorthwindEntities())
{
//Get products by categoryid
productBindingSource.DataSource = db.Products.Where(p => p.CategoryID == obj.CategoryID).ToList();
}
}
}
private void Form1_Load(object sender, EventArgs e)
{
//Init data
using (NorthwindEntities db = new NorthwindEntities())
{
categoryBindingSource.DataSource = db.Categories.ToList();
Category obj = cboCategory.SelectedItem as Category;
if (obj != null)
productBindingSource.DataSource = db.Products.Where(p => p.CategoryID == obj.CategoryID).ToList();
}
}
}
}
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