at the usercontrol code top

int myindex = 0;
MainForm mainForm;

the constructor

public HistogramaDesenat(MainForm mf)
        {
            mainForm = mf;
            UP();
        }

the UP method

public void UP()
        {
            myindex = mainForm.index;
        }

in the main form

public int index = 0;
        private void listView1_SelectedIndexChanged(object sender, EventArgs e)
        {
            if (listView1.SelectedItems.Count > 0)
            {
                index = listView1.SelectedIndices[0];
                histogram.UP();
                Histogram.Invalidate();
            }
        }

in the usercontrol class in the UP method when using a breakpoint i see that the myindex value is getting the value from the selected index in the main form.

i selected in the listView the item in index 10 and i see that index in the usercontrol

but then when i'm using the variable myindex in the usercontrol paint event for some reason the value of myindex is 0 and not 10 :

private void HistogramaDesenat_Paint(object sender, PaintEventArgs e)
        {
            if (myIsDrawing)
            {

                Graphics g = e.Graphics;
                Pen myPen1 = new Pen(new SolidBrush(Color.Red), myXUnit);
                Pen myPen = new Pen(new SolidBrush(myColor),myXUnit);
                //The width of the pen is given by the XUnit for the control.
                for (int i=0;i<myValues.Length;i  )
                {
                    //We draw each line
                    g.DrawLine(myPen,
                        new PointF(myOffset   (i*myXUnit), this.Height - myOffset), 
                        new PointF(myOffset   (i*myXUnit), this.Height - myOffset - myValues[i] * myYUnit));

                    //We plot the coresponding index for the maximum value.
                    if (myValues[i]==myMaxValue)
                    {
                        SizeF mySize = g.MeasureString(i.ToString(),myFont);

                        g.DrawString(i.ToString(),myFont,new SolidBrush(myColor),
                            new PointF(myOffset   (i*myXUnit) - (mySize.Width/2), this.Height - myFont.Height ),
                            System.Drawing.StringFormat.GenericDefault);
                    }
                }

                
                    g.DrawLine(myPen1,
                        new PointF(myOffset   (myindex * myXUnit), this.Height - myOffset),
                        new PointF(myOffset   (myindex * myXUnit), this.Height - myOffset - myValues[myindex] * myYUnit));

CodePudding user response：

C# is case sensitive. I see this in the listView1_SelectedIndexChanged() method:

histogram.UP();
Histogram.Invalidate();

Notice the difference between the lower and upper-case H on the two lines. Those are two different variables, likely referring to different instances of the control.