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Winform copy button text to textbox using universal method

So this is a fairly straightforward thing, and I am just curious if there is a better way to do it to save lines of code. For class we are making a teletype machine. Basically there is a textbox, and a series of buttons A-Z and 0-9. When you click the button it adds the corresponding letter/number to the textbox. When you click send, it adds the contents of the textbox to a label and resets the textbox. Everything works and it only took a few minutes to build. However there is a mess of redundant lines and I was curious if there is a way to clean up the code with a method.
This is my current code.
private void btn_A_Click(object sender, EventArgs e)
{
box_UserInput.Text = box_UserInput.Text + "A";
}
As you can see, it is very simplistic and straight forward. Click A, and "A" gets added to the textbox. However the Text property of the button is also just "A" and I want to know if there is a way to just copy the text property of that button and add it to the textbox string.
Something like this, except with a universal approach where instead of having to specify btn_A it just inherits which button to copy based on the button clicked. That way I can use the same line of code on every button.
private void btn_A_Click(object sender, EventArgs e)
{
box_UserInput.Text = box_UserInput.Text + btn_A.Text;
}

You can use this which is more universal as the Control class contains the Text property. Also, using the best practice $"".
private void btn_A_Click(object sender, EventArgs e)
{
box_UserInput.Text = $"{box_UserInput.Text}{((Control)sender).Text}";
}
You can also assign the same event to each button. Create an event, say addControlTextOnClick and assign the same event to each button.
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
}
private void addControlTextOnClick(object sender, EventArgs e)
{
box_UserInput.Text = $"{box_UserInput.Text}{((Control)sender).Text}";
}
}
You can even shorten this more using this C# construct:
private void addControlTextOnClick(object sender, EventArgs e) =>
box_UserInput.Text = $"{box_UserInput.Text}{((Control)sender).Text}";

Windows Forms, Delete prewritten Text in textbox when starting to type (C#)

You all know those Login Textboxes, where inside it says "Username" and "Password".
So my problem is, that i can´t find anything in the Internet that explains to me how i can detect if anything is written in the textbox.
Then i could use "Username" as a background image, that turns white when i start typing. Something like:
if (Textbox_is_not_empty)
{
Change_Background_image_To_white_colour
}

You can erase the text inside the TextBox by handling the Enter event.
private void textBox1_Enter(object sender, EventArgs e)
{
textBox1.Clear();
}
And set the text to "Username" when you leave and the TextBox is empty.
private void textBox1_Leave(object sender, EventArgs e)
{
if(textBox1.Text == "")
textBox1.Text = "Username";
}

C# set focus on textbox

I'm trying to set the "txtMiles" textbox to focus after:
The form opens
When the "clear" button is clicked
I have tried using txtMiles.Focus(); but it doesn't seem to work for me.
CODE BEING USED ON THIS FORM
private void btnConvert_Click(object sender, EventArgs e)
{
//assigns variable in memory.
double txtMile = 0;
double Results;
try
{
// here is where the math happens.
txtMile = double.Parse(txtMiles.Text);
Results = txtMile * CONVERSION;
lblResults.Text = Results.ToString("n2");
txtMiles.Focus();
}
// if the user enters an incorrect value this test will alert them of such.
catch
{
//MessageBox.Show (ex.ToString());
MessageBox.Show("You entered an incorrect value");
txtMiles.Focus();
}
}
private void btnClear_Click(object sender, EventArgs e)
{
//This empties all the fields on the form.
txtMiles.Text = "";
txtMiles.Focus();
lblResults.Text = "";
}
private void btnExit_Click(object sender, EventArgs e)
{
// closes program
this.Close();
}
}
}
Thanks in advance for the help.

You should make sure your TabIndex is set and then instead of Focus(), try use Select(). See this MSDN link.
txtMiles.Select();
Also make sure there isn't a TabStop = true attribute set in the view file.

It's old, but someone could need this.
Control.Focus() is bugged. If it's not working, try workaround:
this.SelectNextControl(_controlname, true, true, true, true);
Change function parameters so they will work with your control, and remember about TabStop = true property of your control.

You already have your txtMiles focused after the clear-button click. As for the Startup, set txtMiles.Focus() in your load-method.
private void MilesToKilometers_Load(object sender, EventArgs e)
{
txtMiles.Focus();
}

using this solution worked perfectly...
txtMiles.Select();

Delete Selected Text in Richtextbox

I have a richtextbox and i want to delete not cut the selected when the user presses a button.
I have used
private void button1_Click(object sender, EventArgs e)
{
SendKeys.Send("DELETE");
}
This works but i want to know another way to do it.
I have tried
private void button1_Click(object sender, EventArgs e)
{
richTextBox1.SelectedText.Replace(richTextBox1.SelectedText, "");
}
This doesn't perform any action.
Pls what can i do?

Just do this:
private void button1_Click(object sender, EventArgs e)
{
richTextBox1.SelectedText = "";
}
Your code doesn't work because the string is immutable, you can't change the richTextBox1.SelectedText that way. All the methods (Replace, Insert, ...) performed on a string will create a new string. This new string will be used to initialize your string variable if you need.

The following line of code works for me:
SendKeys.Send("{DELETE}");
Click Link to visit the Official documentation on SendKeys methods.

Updating a textbox using no input controls

I have a windows form application which consists of a bunch of controls, but more specifically, two textBoxes. One of them is read only. The read only textBox value is supposed to be the same as the textBox that the user can type into.
So if the user types "Hello World" into textBox A, the value in textBox B should be automatically updated to "Hello World".
How do I go about doing this? I know I just need to set the text values, I'm just not sure where I place the code to get it done automatically rather than executed when a button is click or something along those lines.

TextChanged event:
private void textBox1_TextChanged(object sender, EventArgs e)
{
textBox2.Text = textBox1.Text;
}

It sounds like you want something like:
writableTextBox.TextChanged += delegate {
readonlyTextBox.Text = writableTextBox.Text;
};
In other words, whenever the text in one textbox changes, update the other. This uses the Control.TextChanged event.

If you want textBoxB to be updated as soon as the text of textBoxA is changed (i.e immediately after the user press a key in textBoxA) the event is TextChanged:
this.textBoxA.TextChanged += new System.EventHandler(this.textBoxA_TextChanged);
private void textBoxA_TextChanged(object sender, EventArgs e)
{
textBoxB.Text = textBoxA.Text;
}
If you prefer to update the text in textBoxB only after the user has finished to edit textBoxA, you should use the Leave event:
this.textBoxA.Leave += new System.EventHandler(this.textBoxA_Leave);
private void textBoxA_Leave(object sender, EventArgs e)
{
textBoxB.Text = textBoxA.Text;
}

This should do what you need:
private void textBox1_TextChanged(object sender, EventArgs e)
{
textBox2.Text = textBox1.Text;
}

Even shorter (better?) than the event approach is using winform's databinding. Just use this right after the InitializeComponents call:
readonlyTextBox.DataBindings.Add("Text", writableTextBox, "Text");