Can't set SelectedIndex property for ComboBox in Windows Forms designer

Problem

While designing a form one might need to add a ComboBox control and select a default item for it. Howerver, ensuring the Items collection is pre-filled, there seems to be no way to set the default value for ComboBox using the SelectedIndex property. This property is missing in the list of properties available in designer.

Cause

This is true, the SelectedIndex property is hidden from the list and can't be configured in designer. The SelectedIndex depends entirely on the Items collection. Since the designer doesn't know exactly when or how you'll be populating that list (especially if you're binding to a database later), Microsoft chose to exclude it from the visual property grid to prevent errors. If you try to set an index of 0 in a designer before any items are added, the code would technically fail or reset immediately.

Resolution

To pre-set the value for the ComboBox, do it from the code behind, for e.g. in Form() constructor or in the Form_Load() event handler:

public Form1()
{
    InitializeComponent();
    
    // This ensures the first item is selected as soon as the form is created
    if (comboBox1.Items.Count > 0)
    {
        comboBox1.SelectedIndex = 0;
    }
}

// or

private void Form1_Load(object sender, EventArgs e)
{
    comboBox1.SelectedIndex = 0;
}