C#

public class ValidTexbox : TextBox 
{
    protected override void OnValidating(CancelEventArgs e)
    {
        base.OnValidating (e);
        if(this.Visible == true && this.Text.Length == 0) 
        {
            e.Cancel = true;
            MessageBox.Show("Input invalid"); //notify the user that it can't be empty
        }
    }
}

public class Form1 : System.Windows.Forms.Form
{
    private System.Windows.Forms.TextBox textBox1;
    private System.ComponentModel.Container components = null;
    public Form1()
    {
        InitializeComponent();
    }
	
    private void InitializeComponent()
    {
        this.textBox1 = new System.Windows.Forms.TextBox();
        this.SuspendLayout();
        // 
        // textBox1
        // 
        this.textBox1.Location = new System.Drawing.Point(8, 16);
        this.textBox1.Name = "textBox1";
        this.textBox1.TabIndex = 0;
        this.textBox1.Text = "textBox1";
        this.textBox1.Validating += new System.ComponentModel.CancelEventHandlerthis.textBox1_Validating);
        // 
        // Form1
        // 
        this.AutoScaleBaseSize = new System.Drawing.Size(5, 13);
        this.ClientSize = new System.Drawing.Size(292, 266);
        this.Controls.Add(this.textBox1);
        this.Name = "Form1";
        this.Text = "Form1";
        this.ResumeLayout(false);
    }

    private void textBox1_Validating(object sender, System.ComponentModel.CancelEventArgs e)
    {
        if(textBox1.Visible == true && textBox1.Text.Length == 0) 
        {
            MessageBox.Show("Input invalid"); //notify the user that it can't be empty
        }
    }
}

if(this.Focused==true)
{
    //Do Something
}

SetWindowPos(this.Handle, HWND_TOPMOST,0,0,0,0,
             SWP_NOSIZE | SWP_NOMOVE | SWP_NOACTIVATE);

bool IsTopWindow=(GetWindow((IntPtr)0,GW_HWNDFIRST)==this.Handle);

[DllImport("user32.dll")]
private static extern IntPtr GetWindow(IntPtr hwnd,
                                    int flags);