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private int NumOfPicBoxen = 10;
private string OrdnerPfad;
List<string> Bildnamen = new List<string>();

private void AddPictureBoxen(int cNumber)
{
    picBoxArray = new PictureBox[cNumber];
    for (int i = 0; i < cNumber; i++)
    {
        picBoxArray[i] = new PictureBox();
    }
}

private void Form1_Load(object sender, EventArgs e)
{
    if (panel1 != null)
    {        
        AddPictureBoxen(NumOfPicBoxen);

        for (int i = 0; i < NumOfPicBoxen; i++)
        {
            picBoxArray[i].BackColor = Color.Transparent;
            picBoxArray[i].BorderStyle = BorderStyle.FixedSingle;
            picBoxArray[i].Size = new Size(198, 147);
            picBoxArray[i].Visible = true;
            picBoxArray[i].SizeMode = PictureBoxSizeMode.Zoom;
            picBoxArray[i].Name = i.ToString();
            picBoxArray[0].Location = new Point(75, 50);
            picBoxArray[1].Location = new Point(293, 50);
            picBoxArray[2].Location = new Point(510, 50);
            picBoxArray[3].Location = new Point(728, 50);
            picBoxArray[4].Location = new Point(75, 267);
            picBoxArray[5].Location = new Point(293, 267);
            picBoxArray[6].Location = new Point(510, 267);
            picBoxArray[7].Location = new Point(728, 267);
            picBoxArray[8].Location = new Point(293, 485);
            picBoxArray[9].Location = new Point(510, 485);
            picBoxArray[i].MouseClick  += new MouseEventHandler(picBox_MouseClick);

            this.panel1.Controls.Add(picBoxArray[i]);
            Application.DoEvents();
        }
    }
}


private void öffnenToolStripMenuItem1_Click(object sender, EventArgs e)
{
    FolderBrowserDialog openBrowserDialog = new FolderBrowserDialog();
    int fileCount = 0;
    int fileMax = 0;

    if (openBrowserDialog.ShowDialog() == DialogResult.OK)
    {
        OrdnerPfad = openBrowserDialog.SelectedPath;
        DirectoryInfo dir = new DirectoryInfo(OrdnerPfad);
        fileCount = Bildnamen.Count;

        if (dir.Exists)
        {
            FileInfo[] str_gif = dir.GetFiles("*.gif", SearchOption.AllDirectories);
            FileInfo[] str_jpg = dir.GetFiles("*.jpg", SearchOption.AllDirectories);
            FileInfo[] str_bmp = dir.GetFiles("*.gif", SearchOption.AllDirectories);
            FileInfo[] str_png = dir.GetFiles("*.png", SearchOption.AllDirectories);

            foreach (FileInfo gifBilder in str_gif)
            {
                if (gifBilder.Exists)
                    Bildnamen.Add(gifBilder.FullName);
                else
                    return;
            }
            foreach (FileInfo jpgBilder in str_jpg)
            {
                if (jpgBilder.Exists)
                    Bildnamen.Add(jpgBilder.FullName);
                else
                    return;
            } 
            foreach (FileInfo bmpBilder in str_bmp)
            {
                if (bmpBilder.Exists)
                    Bildnamen.Add(bmpBilder.FullName);
                else
                    return;
            } 
            foreach (FileInfo pngBilder in str_png)
            {
                if (pngBilder.Exists)
                    Bildnamen.Add(pngBilder.FullName);
                else
                    return;
            }

            foreach (string str in Bildnamen)
            {
                if (str.Length > 0 && str.Length <=10)
                {
                    picBoxArray[fileMax].ImageLocation = str;
                    fileCount++;
                    fileMax++;
                }
                else
                    MessageBox.Show("In diesem Ordner befinden sich keine Bilder.\nFormate: jpg,gif,png,bmp");
            }
        }
    }
}

private void btn_vor_Click(object sender, EventArgs e)
{
    if (picBoxArray != null)
    {
        if (Bildnamen.Count <= 10 && Bildnamen.Count > 0) return;
        if (Bildnamen.Count == 0) return;

        if (Bildnamen.Count > 10 && Bildnamen.Count <= 20)
        { 
                    
        }
    }
}					

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List<PictureBox> pbList=new List<PictureBox>()
for(int i=0;i<picboxcount;i++)
{
PictureBox neu=new PictureBox();
pbList.Add(neu);
}
zuweisung dann
foreach(PictureBox pb in pbList)
{
pb.BackColor=....;
}