Set focus on textbox /combobox in winforms if it is empty

If you're talking about data validation, you can use ErrorProvider Class (System.Windows.Forms) | Microsoft Docs[^]

For further details, please see: How to: Display Error Icons for Form Validation with the Windows Forms ErrorProvider Component | Microsoft Docs[^]

Note: if there's a set of different rules for each validation, i'd create a dictionary of ErrorProviders for each control you want to validate. For example, if you need to check if entered value is a number, you need to create a "NumberErrorProvider", if you want to check if phone no. is valid, create "PhoneNoErrorProvider", etc...
Finally, you can create custom method (for example: IsFormValid) which steps through the collection ErrorProviders and returns true/false.

public partial class Form1 : Form
{
    private Dictionary<Control, ErrorProvider> validators = new Dictionary<Control, ErrorProvider>();

    public Form1()
    {
        InitializeComponent();

        validators.Add(textBox1, new ErrorProvider(this){//other properties here});
        validators.Add(comboBox1, new ErrorProvider(this){//...});
    }


    private void textBox1_Validating(object sender, CancelEventArgs e)
    {
        if (IsValidNumber(textBox1.Text)) //to do: add IsValidNumber method
            validators[textBox1].SetError(textBox1, "Field 'Number' has to be a valid number!");
        else
            validators[textBox1].Clear();
    }

    //other code here....

    private bool IsFormValid()
    {
        foreach (Control c in validators.Keys)
        {
            if(validators[c].GetError(c) !="")
                return false;
        }
        return true;
    }

}

This is a basic concept for WinForms data validatation. You can implement your own.

Good luck!