Append clicked Button Text to focused TextBox using C# and VB.Net in Windows Forms Application

Refer the below sample code for your reference. Created click event which will check the last textbox active by the static property which gets sets on Lostfocus of textbox control and on click on any button to which click event is assigned will add the number to the textbox.

FormDesign View

public partial class Form1 : Form
{
    // Static property to maintain to check the textbox was active or not
    public static bool isTextbox1Active = true;

    public Form1()
    {
        InitializeComponent();

        //Add event handler to txtNumber1 and txtNumber2 on LostFocus
        txtNumber1.LostFocus += new EventHandler(TextBox_LostFocus);
        txtNumber2.LostFocus += new EventHandler(TextBox_LostFocus);
        btn1.Click += new EventHandler(btnClick);
        btn2.Click += new EventHandler(btnClick);
        btn3.Click += new EventHandler(btnClick);
        btn4.Click += new EventHandler(btnClick);
        btn5.Click += new EventHandler(btnClick);
        btn6.Click += new EventHandler(btnClick);
        btn7.Click += new EventHandler(btnClick);
        btn8.Click += new EventHandler(btnClick);
        btn9.Click += new EventHandler(btnClick);
        btn0.Click += new EventHandler(btnClick);
    }

    // Click event to set the value in text box as per clicked button text and last active textbox
    private void btnClick(object sender, EventArgs e)
    {
        string number = (sender as Button).Text;
        if (isTextbox1Active)
        {
            txtNumber1.Text += number;
        }
        else
        {
            txtNumber2.Text += number;
        }
    }

    // Create The Event to handle on lost focus of textbox 
    // To find last Active textbox control
    private void TextBox_LostFocus(object sender, EventArgs e)
    {
        if ((sender as TextBox).Name == "txtNumber1")
        {
            isTextbox1Active = true;
        }
        else
        {
            isTextbox1Active = false;
        }
    }
}

VB.Net

Public Class Form1
    ' Static property to maintain to check the textbox was active or not
    Public Shared isTextbox1Active As Boolean = True

    Public Sub Form1()
        InitializeComponent()
    End Sub
    ' Click event to set the value in text box as per clicked button text and last active textbox
    Private Sub btnClick(ByVal sender As Object, ByVal e As EventArgs) Handles btn0.Click, btn1.Click, btn2.Click, btn3.Click, btn4.Click, btn5.Click, btn6.Click, btn7.Click, btn8.Click, btn9.Click
        Dim number As String = (TryCast(sender, Button)).Text
        If isTextbox1Active Then
            txtNumber1.Text += number
        Else
            txtNumber2.Text += number
        End If
    End Sub

    ' Create The Event to handle on lost focus of textbox 
    ' To find last Active textbox control
    Private Sub TextBox_LostFocus(ByVal sender As Object, ByVal e As EventArgs) Handles txtNumber1.LostFocus, txtNumber2.LostFocus
        If (TryCast(sender, TextBox)).Name = "txtNumber1" Then
            isTextbox1Active = True
        Else
            isTextbox1Active = False
        End If
    End Sub
End Class

Screenshot