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Auto resizing items/controls in winforms c# showing error

I've been trying to make my windows form application auto resizing i.e. when the user resizes the application its controls also resize accordingly and after searching online I've come across the below code in the form of a .cs file :
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Windows.Forms;
public class clsResize
{
List<System.Drawing.Rectangle> _arr_control_storage = new List<System.Drawing.Rectangle>();
private bool showRowHeader = false;
public clsResize(Form _form_)
{
form = _form_; //the calling form
_formSize = _form_.ClientSize; //Save initial form size
_fontsize = _form_.Font.Size; //Font size
}
private float _fontsize { get; set; }
private System.Drawing.SizeF _formSize {get;set; }
private Form form { get; set; }
public void _get_initial_size() //get initial size//
{
var _controls = _get_all_controls(form);//call the enumerator
foreach (Control control in _controls) //Loop through the controls
{
_arr_control_storage.Add(control.Bounds); //saves control bounds/dimension
//If you have datagridview
if (control.GetType() == typeof(DataGridView))
_dgv_Column_Adjust(((DataGridView)control), showRowHeader);
}
}
public void _resize() //Set the resize
{
double _form_ratio_width = (double)form.ClientSize.Width /(double)_formSize.Width; //ratio could be greater or less than 1
double _form_ratio_height = (double)form.ClientSize.Height / (double)_formSize.Height; // this one too
var _controls = _get_all_controls(form); //reenumerate the control collection
int _pos = -1;//do not change this value unless you know what you are doing
foreach (Control control in _controls)
{
// do some math calc
_pos += 1;//increment by 1;
System.Drawing.Size _controlSize = new System.Drawing.Size((int)(_arr_control_storage[_pos].Width * _form_ratio_width),
(int)(_arr_control_storage[_pos].Height * _form_ratio_height)); //use for sizing
System.Drawing.Point _controlposition = new System.Drawing.Point((int)
(_arr_control_storage[_pos].X * _form_ratio_width),(int) (_arr_control_storage[_pos].Y * _form_ratio_height));//use for location
//set bounds
control.Bounds = new System.Drawing.Rectangle(_controlposition, _controlSize); //Put together
//Assuming you have a datagridview inside a form()
//if you want to show the row header, replace the false statement of
//showRowHeader on top/public declaration to true;
if (control.GetType() == typeof(DataGridView))
_dgv_Column_Adjust(((DataGridView)control), showRowHeader);
//Font AutoSize
control.Font = new System.Drawing.Font(form.Font.FontFamily,
(float)(((Convert.ToDouble(_fontsize) * _form_ratio_width) / 2) +
((Convert.ToDouble(_fontsize) * _form_ratio_height) / 2)));
}
}
private void _dgv_Column_Adjust(DataGridView dgv, bool _showRowHeader) //if you have Datagridview
//and want to resize the column base on its dimension.
{
int intRowHeader = 0;
const int Hscrollbarwidth = 5;
if (_showRowHeader)
intRowHeader = dgv.RowHeadersWidth;
else
dgv.RowHeadersVisible = false;
for (int i = 0; i < dgv.ColumnCount; i++)
{
if (dgv.Dock == DockStyle.Fill) //in case the datagridview is docked
dgv.Columns[i].Width = ((dgv.Width - intRowHeader) / dgv.ColumnCount);
else
dgv.Columns[i].Width = ((dgv.Width - intRowHeader - Hscrollbarwidth) / dgv.ColumnCount);
}
}
private static IEnumerable<Control> _get_all_controls(Control c)
{
return c.Controls.Cast<Control>().SelectMany(item =>
_get_all_controls(item)).Concat(c.Controls.Cast<Control>()).Where(control =>
control.Name != string.Empty);
}
}
I added this to my project used in code as below :
using System;
using System.Collections.Generic;
using System.Drawing;
using System.Windows.Forms;
namespace KryptonTest
{
/// <summary>
/// Description of MainForm.
/// </summary>
public partial class MainForm : KryptonForm
{
clsResize _form_resize;
public MainForm()
{
//
// The InitializeComponent() call is required for Windows Forms designer support.
//
InitializeComponent();
_form_resize = new clsResize(this);
this.Load += MainFormLoad;
this.Resize += MainFormResize;
}
void MainFormLoad(object sender, EventArgs e)
{
_form_resize._get_initial_size();
}
void MainFormResize(object sender, EventArgs e)
{
_form_resize._resize();
}
}
}
But when I run it I get the below error twice
How do I fix this?

When we resize a form we normally pick some controls to get bigger and others not - it doesn't really make much sense to double the width and height of a name TextBox, or a Checkbox because you don't type/see significantly more text in them. Other items like a logging list box, email body designer TextBox etc, it makes sense to have those things resize.
Arranging a layout that resizes is quite simple: every control has an Anchor property and when the anchor for a particular side is set then the control will move or resize so there is the same distance between the control and the given edge of the container. Obviously then if a control is anchored on opposing sides, then it will stretch when the container it is in gets bigger
Some examples:
TextBox is 100 wide and 20 high, anchored on Top Left. The form is made 200 pixels wider and 200 higher. The TextBox appears not to move or grow; it remains the same distance from the top left corner
TextBox is 100 wide and 20 high, anchored on Top Right. The form is made 200 pixels wider and 200 higher. The TextBox moves 200 pixels to the Right because it is Right anchored and keeps the same distance between the form right edge. It does not grow wider; it remains the same distance from the top left corner. It does not grow taller because it is Top anchored but not Bottom anchored
TextBox is 100 wide and 20 high, anchored on Top Left Bottom Right. It is a multi line TextBox. The form is made 200 pixels wider and 200 higher. The TextBox grows 200 pixels wider and 200 pixels higher because the form has grown by 200x200 - the anchors on all sides means the textbox edges stay the same distance from the form edges
You want 3 textboxes in a row, the middle one should get wider when the form gets wider; the other two textboxes should not grow
You anchor the left TextBox on the Top Left
You anchor the middle TextBox on Top Left Right
You anchor the right TextBox on the Top Right
When the form gets wider the left TextBox stays put, the right TextBox moves to the right by as much as the form got wider, the middle TextBox expands sideways by as much as the form got wider
When neither opposing side is anchored the control moves by half the distance of the resize in that direction
Don't forget you can put panels in that are anchored one way and then controls inside them that are anchored another way. For other kinds of layouts you might need a table or flow layout panel but for most UIs the anchoring system works quite well

Are you sure the enum is working properly? The error message is telling you the list has less items than you're trying to call.
Index was out of range. Must be non-negative and less than the size
of the collection. Parameter name: index
Edit:
I just copied and pasted your code into a new WinForms project in VS 2022 and it worked with no issues though I did have to drop the inheritance reference to KryptonForm since I didn't have it. If it helps, I ran it using .Net 6.0 (LTS)
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Windows.Forms;
namespace WinFormsApp1
{
/// <summary>
/// Description of MainForm.
/// </summary>
public partial class MainForm : Form
{
clsResize _form_resize;
public MainForm()
{
InitializeComponent();
_form_resize = new clsResize(this);
this.Load += MainFormLoad;
this.Resize += MainFormResize;
}
void MainFormLoad(object sender, EventArgs e)
{
_form_resize._get_initial_size();
}
void MainFormResize(object sender, EventArgs e)
{
_form_resize._resize();
}
}
public class clsResize
{
List<System.Drawing.Rectangle> _arr_control_storage = new List<System.Drawing.Rectangle>();
private bool showRowHeader = false;
public clsResize(Form _form_)
{
form = _form_; //the calling form
_formSize = _form_.ClientSize; //Save initial form size
_fontsize = _form_.Font.Size; //Font size
}
private float _fontsize { get; set; }
private System.Drawing.SizeF _formSize { get; set; }
private Form form { get; set; }
public void _get_initial_size() //get initial size//
{
var _controls = _get_all_controls(form);//call the enumerator
foreach (Control control in _controls) //Loop through the controls
{
_arr_control_storage.Add(control.Bounds); //saves control bounds/dimension
//If you have datagridview
if (control.GetType() == typeof(DataGridView))
_dgv_Column_Adjust(((DataGridView)control), showRowHeader);
}
}
public void _resize() //Set the resize
{
double _form_ratio_width = (double)form.ClientSize.Width / (double)_formSize.Width; //ratio could be greater or less than 1
double _form_ratio_height = (double)form.ClientSize.Height / (double)_formSize.Height; // this one too
var _controls = _get_all_controls(form); //reenumerate the control collection
int _pos = -1;//do not change this value unless you know what you are doing
foreach (Control control in _controls)
{
// do some math calc
_pos += 1;//increment by 1;
System.Drawing.Size _controlSize = new System.Drawing.Size((int)(_arr_control_storage[_pos].Width * _form_ratio_width),
(int)(_arr_control_storage[_pos].Height * _form_ratio_height)); //use for sizing
System.Drawing.Point _controlposition = new System.Drawing.Point((int)
(_arr_control_storage[_pos].X * _form_ratio_width), (int)(_arr_control_storage[_pos].Y * _form_ratio_height));//use for location
//set bounds
control.Bounds = new System.Drawing.Rectangle(_controlposition, _controlSize); //Put together
//Assuming you have a datagridview inside a form()
//if you want to show the row header, replace the false statement of
//showRowHeader on top/public declaration to true;
if (control.GetType() == typeof(DataGridView))
_dgv_Column_Adjust(((DataGridView)control), showRowHeader);
//Font AutoSize
control.Font = new System.Drawing.Font(form.Font.FontFamily,
(float)(((Convert.ToDouble(_fontsize) * _form_ratio_width) / 2) +
((Convert.ToDouble(_fontsize) * _form_ratio_height) / 2)));
}
}
private void _dgv_Column_Adjust(DataGridView dgv, bool _showRowHeader) //if you have Datagridview
//and want to resize the column base on its dimension.
{
int intRowHeader = 0;
const int Hscrollbarwidth = 5;
if (_showRowHeader)
intRowHeader = dgv.RowHeadersWidth;
else
dgv.RowHeadersVisible = false;
for (int i = 0; i < dgv.ColumnCount; i++)
{
if (dgv.Dock == DockStyle.Fill) //in case the datagridview is docked
dgv.Columns[i].Width = ((dgv.Width - intRowHeader) / dgv.ColumnCount);
else
dgv.Columns[i].Width = ((dgv.Width - intRowHeader - Hscrollbarwidth) / dgv.ColumnCount);
}
}
private static IEnumerable<Control> _get_all_controls(Control c)
{
return c.Controls.Cast<Control>().SelectMany(item =>
_get_all_controls(item)).Concat(c.Controls.Cast<Control>()).Where(control =>
control.Name != string.Empty);
}
}
}

Grow windows form with button click

I want to grow a form when I click the button and it should be in the center of the screen .so I wrote following code snippet.
private void ord_Click(object sender, EventArgs e)
{
this.StartPosition = FormStartPosition.CenterScreen;
this.Size = new Size(1308,599);
this.Show();
}
But when I click the button window grows but half of the window can not see.Here is the picture of that.
GUI after growing
How can I get rid of this problem.?
Whats wrong with my code?

You have to compute both Size and Location:
private void ord_Click(object sender, EventArgs e) {
// Ensure that suggested form size doesn't exceed the screen width and height
this.Size = new System.Drawing.Size(
Screen.GetWorkingArea(this).Width >= 1308 ? 1308 : Screen.GetWorkingArea(this).Width,
Screen.GetWorkingArea(this).Height >= 599 ? 599 : Screen.GetWorkingArea(this).Height);
// locate the form in the center of the working area
this.Location = new System.Drawing.Point(
(Screen.GetWorkingArea(this).Width - Width) / 2,
(Screen.GetWorkingArea(this).Height - Height) / 2);
}

You can use the PrimaryScreen property of the Screen class.
//this.StartPosition = FormStartPosition.CenterScreen;
//this.Show();
Omit these lines you've written, except setting the Size property of the form:
private void ord_Click(object sender, EventArgs e)
{
this.Size = new Size(1308,599);
CenterForm();
}
Create a method named CenterForm() which will set a new location of the form. You can achieve this by calling this method in your button click event.
private void CenterForm()
{
int getWidth = Screen.PrimaryScreen.Bounds.Width;
int getHeight = Screen.PrimaryScreen.Bounds.Height;
int X = getWidth - this.Width;
int Y = getHeight - this.Height;
this.Location = new Point(X / 2, Y / 2);
}
Note: Always remember to anchor your controls when the size of the form has changed.

Why am I having trouble writing a pixel to a picturebox in Visual C#?

I am writing an emulator program, and the virtual display is supposed to be able to take in 3 bytes of color data and display the correct color pixel, similar to how a real screen works. But when I set up some scroll bars to test the generation of pixels nothing happens. Here is my code and a screenshot of the form:
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
namespace TSC_Multi_System_Emulator
{
public partial class Form1 : Form
{
private PictureBox Display = new PictureBox();
string #emulationfolderpath = Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments);
Bitmap screen = new Bitmap(#Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments) + #"\Resource_Folder\" + #"FirstFrame.bmp");
int x = 0;
int y = 0;
public Form1()
{
InitializeComponent();
}
private void Form1_Load(object sender, System.EventArgs e) {
// Dock the PictureBox to the form and set its background to black.
Display.BackColor = Color.Black;
// Connect the Paint event of the PictureBox to the event handler method.
// Add the PictureBox control to the Form.
this.Controls.Add(Display);
}
public void DigitalGraphicsDisplay(int red, int green, int blue) {
Graphics g = Display.CreateGraphics();
screen.SetPixel(x, y, Color.FromArgb(red, green, blue));
g.DrawImage(screen, 0, 0, screen.Width, screen.Height);
g.Save();
if (x < screen.Width)
{
x = x + 1;
}
else if (x == screen.Width)
{
x = 0;
if (y < screen.Height)
{
y = y + 1;
}
else if (y == screen.Height)
{
y = 0;
}
}
}
private void button1_Click(object sender, EventArgs e){
int rchannel = redControl.Value;
int gchannel = greenControl.Value;
int bchannel = blueControl.Value;
DigitalGraphicsDisplay(rchannel, gchannel, bchannel);
}
}
}
UPDATE:
The code is now working somewhat, but I can't test the code using just a test button. I had to use the exact code given to me in the first answer, which only displayed a gradient, I wonder what I am doing wrong... :(
public partial class Form1 : Form
{
string #emulationfolderpath = Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments);
Bitmap screen = new Bitmap(#Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments) + #"\Resource_Folder\" + #"FirstFrame.bmp");
int x = 0;
int y = 0;
public Form1()
{
InitializeComponent();
}
private void Form1_Load(object sender, System.EventArgs e) {
// Dock the PictureBox to the form and set its background to black.
Display.BackColor = Color.Black;
// Connect the Paint event of the PictureBox to the event handler method.
// Add the PictureBox control to the Form.
this.Controls.Add(Display);
}
public void DigitalGraphicsDisplay(int red, int green, int blue)
{
if (Display.Image == null)
{
Bitmap NewBMP = new Bitmap(Display.ClientRectangle.Width, Display.ClientRectangle.Height);
using (Graphics g = Graphics.FromImage(NewBMP))
{
g.Clear(Color.White);
}
Display.Image = NewBMP;
}
(Display.Image as Bitmap).SetPixel(x, y, Color.FromArgb(red, green, blue));
Display.Invalidate();
x++;
if (x >= Display.Image.Width)
{
x = 0;
y++;
if (y >= Display.Image.Height)
{
y = 0;
}
}
}
private void button1_Click(object sender, EventArgs e){
Boolean a = false;
int b = 0;
do
{
DigitalGraphicsDisplay(51, 153, 102);
if (b == 10000)
{
a = true;
}
b = b + 1;
} while (a);
}
}
}
All I am getting is a white picturebox with nothing else in it...
(The gradient code did work though)

It looks like you are trying to draw directly on the PictureBox control itself.
Instead you should have an Image assigned to the PictureBox and then draw on the image.
Try changing your code as shown below. (Including the click event for testing.)
Note, the PictureBox keeps the reference to the image directly so you don't need a separate screen image in your class, unless you have a different purpose for it.
Also, this uses Bitmap.SetPixel() which is an extremely slow way to set pixels. There is a much faster but slightly more complex way, in these other links:
SetPixel is too slow. Is there a faster way to draw to bitmap?
Work with bitmaps faster in C#
Remember your button click will only draw one pixel at a time.
So be sure to look carefully:
Running my test code within the click event will yield this:
int x = 0;
int y = 0;
public void DigitalGraphicsDisplay(int red, int green, int blue)
{
if (Display.Image == null)
{
Bitmap NewBMP = new Bitmap(Display.ClientRectangle.Width, Display.ClientRectangle.Height);
using (Graphics g = Graphics.FromImage(NewBMP))
{
g.Clear(Color.White);
}
Display.Image = NewBMP;
}
(Display.Image as Bitmap).SetPixel(x, y, Color.FromArgb(red, green, blue));
Display.Invalidate();
x++;
if (x >= Display.Image.Width)
{
x = 0;
y++;
if (y >= Display.Image.Height)
{
y = 0;
}
}
}
private void button1_Click(object sender, EventArgs e)
{
// Temporary code to show that it works (Due to Bitmap.SetPixel() it will be slow)
for (int I = 1; I < Display.ClientRectangle.Width * Display.ClientRectangle.Height; I++)
DigitalGraphicsDisplay((I/255)%255, (I % Display.ClientRectangle.Width) % 255, 127);
}
UPDATE: Per your comment, try this sample code:
private void button1_Click(object sender, EventArgs e)
{
Boolean a = true;
int b = 0;
do
{
DigitalGraphicsDisplay(51, 153, 102);
if (b == 10000)
{
a = false;
}
b = b + 1;
} while (a);
}

public void DigitalGraphicsDisplay(int red, int green, int blue) {
Graphics g = Display.CreateGraphics();
screen.SetPixel(x, y, Color.FromArgb(red, green, blue));
g.DrawImage(screen, 0, 0, screen.Width, screen.Height);
g.Save();
All possible mistakes in one go..
Never use CreateGraphics to draw persistent Graphics! Always either go for the Paint event or draw into the Image.
Graphics.Save does not save any drawn pixels. It saves the state of the Graphics object, which does not contain graphics but is a tool to write into a related bitmap. The state includes scale, rotation, smoothing-mode and then some..
You already write into the Bitmap so you can simply make it your new PictureBox.Image..
Or the PictureBox.BackgroundImage.
And, as I said, you can instead write on top of both that is onto the PBox's surface. For this use the Paint event, Invalidate to trigger it and class level variables to hold the necessary data..
The latter is for graphics that will change a lot, the two former ones are for changes that accumulate.
Control.CreateGraphics is for transient graphics only, like a rubber-band line or a cursor cross..

Track cusor location and add image from cusor location into PDF with iTextSharp

I'm looking for the most appropriate way to allow the user to import an image to a PDF and allow them to drag the picture around the PDF/winform and specify where the image is placed.
I'm thinking the best way to go about doing this is pulling the location from the cusor.
Something like:
Rectangle rect = new Rectangle(400, 772, 545, 792);
Instead of pre-defined coordinates, have the output be the selected cursor location of the user.
Any help would be very much appreciated.
Thank you in advance!

Users might have a hard time picking an image location just by pressing the mouse cursor on a form. Instead I recommend allowing them to drag a relatively sized rectangle around a grid. You'll have to translate coordinates appropriately, including fixing the Y value since iTextSharp starts at the bottom left instead of the top left, but that shouldn't be too hard.
Below is a full working C# 2010 WinForms app that allows you to drag a red rectangle around a black square. The comments in the code should pretty much explain everything. It has one big problem that need to be addressed, it stores the X/Y coordinates are screen-based and not form based, so if you drag once, move the entire form and drag again it will get "funky". This can be solved by calculating the x/y relative to the form instead which I'll leave up to you.
Hopefully this gets you down a path that works for you!
using System;
using System.ComponentModel;
using System.Drawing;
using System.Windows.Forms;
namespace WindowsFormsApplication1
{
public partial class Form1 : Form
{
//Basic dimensions of our "border" and our "image". Assumes 72 dots per inch which isn't technically correct but gives us something to work with
private int BORDER_WIDTH = (int)11 * 72;
private int BORDER_HEIGHT = (int)8.5 * 72;
private int IMAGE_WIDTH = (int)2 * 72;
private int IMAGE_HEIGHT = (int)3 * 72;
private int IMAGE_OFFSET = 5;
//These will store the x/y when we press our mouse down so that we can calculate the drag later
private int offsetX;
private int offsetY;
//Our main "image" that we'll move around
PictureBox pb;
//The "border" to move the image around in
PictureBox border;
public Form1()
{
InitializeComponent();
}
private void Form1_Load(object sender, EventArgs e)
{
//Resize the form and make it not user-resizable
this.Size = new Size(BORDER_WIDTH + 30, BORDER_HEIGHT + 50);
this.FormBorderStyle = System.Windows.Forms.FormBorderStyle.FixedSingle;
//Create our "image"
pb = new PictureBox();
pb.Size = new Size(IMAGE_WIDTH, IMAGE_HEIGHT);
pb.Location = new Point(IMAGE_OFFSET, IMAGE_OFFSET);
pb.BackColor = Color.Red;
//Bind a handler to the mouse down event
pb.MouseDown += new MouseEventHandler(this.pbMouseDown);
this.Controls.Add(pb);
//Create our "border"
border = new PictureBox();
border.Size = new Size(BORDER_WIDTH, BORDER_HEIGHT);
border.Location = new Point(IMAGE_OFFSET, IMAGE_OFFSET);
border.BackColor = Color.Black;
this.Controls.Add(border);
}
private void pbMouseDown(object sender, MouseEventArgs e)
{
//Store the current x/y so that we can use them in calculations later
offsetX = e.X;
offsetY = e.Y;
//When the mouse is down we want to remove the original mouse down handler
pb.MouseDown -= new MouseEventHandler(this.pbMouseDown);
//Add to more handler looking for mouse up and mouse movement
pb.MouseUp += new MouseEventHandler(this.pbMouseUp);
pb.MouseMove += new MouseEventHandler(this.pbMouseMove);
}
private void pbMouseUp(object sender, MouseEventArgs e)
{
//When the mouse button is released, remove old handlers and add back the down event
pb.MouseMove -= new MouseEventHandler(this.pbMouseMove);
pb.MouseUp -= new MouseEventHandler(this.pbMouseUp);
pb.MouseDown += new MouseEventHandler(this.pbMouseDown);
//Pop up a message, this is where something with iTextSharp would be done
MessageBox.Show(String.Format("The top left of the image is at {0}x{1}", pb.Top - IMAGE_OFFSET, pb.Left - IMAGE_OFFSET));
}
private void pbMouseMove(object sender, MouseEventArgs e)
{
//Calculate where to draw the "image" at next
//First, calculate based on the current image's location, the offset that we stored ealier and the current mouse position
int newLeft = e.X + pb.Left - offsetX;
int newTop = e.Y + pb.Top - offsetY;
//Next, make sure that we haven't gone over one of the boundaries of the "border"
if (newLeft < border.Left) newLeft = border.Left;
if (newTop < border.Top) newTop = border.Top;
if (newLeft + pb.Width > border.Right) newLeft = border.Right - pb.Width;
if (newTop + pb.Height > border.Bottom) newTop = border.Bottom - pb.Height;
//Finally, assign the new value
pb.Left = newLeft;
pb.Top = newTop;
}
}
}

How to handle mouse event

I want to click inside a square and then an "X" should appear, but I'm not sure what to put inside the Form1_MouseDown, Form1_Paint and Form1_MouseUp events. How can I implement this is C#?
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
namespace VTest
{
public partial class Form1 : Form
{
Rectangle rect; // single rect
int sqsize, n;
int margin;
public Form1()
{
n = 3;
margin = 25;
sqsize = 50;
rect = new Rectangle(10, 10, 150, 150);
InitializeComponent();
}
private void Form1_MouseDown(object sender, MouseEventArgs e)
{
// what goes here?
}
private void Form1_Paint(object sender, PaintEventArgs e)
{
// what goes here?
}
private void Form1_MouseUp(object sender, MouseEventArgs e)
{
// what goes here?
}
// ...

In your MouseDown event, determining whether the click has occurred within your rectangle is easy:
if (rect.Contains(e.Location))
{
// the user has clicked inside your rectangle
}
Drawing the "X" on the form is also easy:
Graphics g = this.CreateGraphics();
g.DrawString("X", this.Font, SystemBrushes.WindowText,
(float)e.X, (float)e.Y);
However, the "X" in this case will not be persistent, meaning that if you drag another form over your form and then move it away, the "X" will not be there anymore. To draw a persistent "X", create a form-level Point variable like this:
private Point? _Xlocation = null;
Use your MouseDown event to set this variable if the user clicks in your Rectangle:
if (rect.Contains(e.Location))
{
_Xlocation = e.Location;
this.Invalidate(); // this will fire the Paint event
}
Then, in your form's Paint event, draw the "X":
if (_Xlocation != null)
{
e.Graphics.DrawString("X", this.Font, SystemBrushes.WindowText,
(float)e.X, (float)e.Y);
}
else
{
e.Graphics.Clear(this.BackColor);
}
If you want the "X" to then disappear when the user lets go of the mouse button, just put this code in the MouseUp event:
_Xlocation = null;
this.Invalidate();
You can make this as much more complicated as you like. With this code, the "X" will be drawn just below and to the right of wherever you click on the form. If you want the "X" to be centered on the click location, you can use the Graphics object's MeasureString method to determine how high and how wide the "X" will be, and offset the DrawString location accordingly.

You don't need both a mousedown and mouseup event handler.
Pick one to react to, I tend to react to the MouseDown event instead.
But, when you will want to look at the MouseEventArgs properties and you should be able to determine if you are inside the square.
You will probably want to call:
System.Diagnostics.Debug.WriteLine(...)
using the x and y properties in MouseEventArgs, so you can see where the mouse clicks are, and determine when you are in the square.
Once you are there, then you can draw the X.
You may want to write a function to draw an X and test it by having it draw an X at 300,300 so that you can ensure it looks as you want, while you are experimenting with MouseDown.

Update: I like the Rectangle.contains(location) method demonstrated by MusiGenesis.
public partial class formDemo : Form
{
Rectangle rec;
public formDemo() => InitializeComponent();
private void formDemo_Load(object sender, EventArgs e) =>
rec = new Rectangle(150,100,100,100);
private void frmDemo_Paint(object sender, PaintEventArgs e)
{
var p = new Pen(Color.Blue);
var g = e.Graphics;
g.DrawRectangle(p, rec);
}
private void formDemo_MouseMove(object sender, MouseEventArgs e) =>
Cursor = rec.Contains(e.Location) ? Cursors.Cross : Cursors.Default;
private void formDemo_MouseDown(object sender, MouseEventArgs e)
{
if (rec.Contains(e.Location))
{
// Mouse position adjust for window postion and border size.
// You may have to adjust the borders depending your
// Windows theme
int x = MousePosition.X - this.Left - 4;
int y = MousePosition.Y - this.Top - 29;
var g = this.CreateGraphics();
var p = new Pen(Color.Black);
var p1 = new Point(x - 10, y - 10);
var p2 = new Point(x + 10, y + 10);
var p3 = new Point(x - 10, y + 10);
var p4 = new Point(x + 10, y - 10);
g.DrawLines(p, new Point[] { p1, p2 });
g.DrawLines(p, new Point[] { p3, p4 });
}
}
}