Align controls to center in a FlowLayout

I'd go with TableLayoutPanel instead:

• Put TableLayoutPanel on your form
• Set dock style Fill to panel
• Leave only one column inside panel
• Create row for every button (and put buttons to table cells)
• Set row size type Autosize
• Set dock style Fill to every button, except last one
• Set dock style Top to last button

BTW in your solution you should iterate over flowLayoutPanel controls instead of form controls. Also consider subtracting horizontal margin and padding from width:

foreach (Control control in flowLayoutPanel.Controls)
{
    control.Size = new Size(flowLayoutPanel.Width - control.Margin.Horizontal,
                            control.Height); 
}

But I advise you to use TableLayoutPanel instead.

You can do it two ways but with some limitation of each one.

1. Using Anchor property
2. Using the layout control with help of Docking and Anchor properties.

Method 1: Anchor Property

Controls are anchored by default to the top left of the form which means when the form size will be changed, their distance from the top left side of the form will remain constant. If you change the control anchor to bottom left, then the control will keep the same distance from the bottom and left sides of the form when the form if resized.

Turning off the anchor in a direction will keep the control centred in that direction when resizing.

Example :

public TestForm12()
{
   InitializeComponent();

   Button btn = new Button();
   btn.Width = this.Width - 10;
   btn.Height = 20;
   btn.Left = (this.ClientSize.Width - btn.Width) / 2;
   btn.Top = (this.ClientSize.Height - btn.Height) / 2;
   btn.Text = "click me";
   this.Controls.Add(btn);
   btn.Anchor = AnchorStyles.None;               

}

2. Using the layout control

1. Add TableLayout Control, Set it’s Dock property to Fill.
2. Add 1 Row with Size Type style Percent 100%
3. Add 3 Columns Column1(Size Type – Percent(100%)), Column2(Size Type – Absolute(200px)), Column3(Size Type – Percent(100%)).
4. Now Add Panel Control to Column2 and Set it’s Dock property to Fill
5. Add Buttons to this control and set their Size as you want and Set Their Anchor Property to AnchorStyles.Left | AnchorStyles.Right | AnchorStyles.Top

Example - Designer.cs code snippet of the form.

private void InitializeComponent()
 {
     this.tableLayoutPanel1 = new System.Windows.Forms.TableLayoutPanel();
     this.panel1 = new System.Windows.Forms.Panel();
     this.button1 = new System.Windows.Forms.Button();
     this.button2 = new System.Windows.Forms.Button();
     this.tableLayoutPanel1.SuspendLayout();
     this.panel1.SuspendLayout();
     this.SuspendLayout();
     // 
     // tableLayoutPanel1
     // 
     this.tableLayoutPanel1.ColumnCount = 3;
     this.tableLayoutPanel1.ColumnStyles.Add(new System.Windows.Forms.ColumnStyle(System.Windows.Forms.SizeType.Percent, 50F));
     this.tableLayoutPanel1.ColumnStyles.Add(new System.Windows.Forms.ColumnStyle(System.Windows.Forms.SizeType.Absolute, 200F));
     this.tableLayoutPanel1.ColumnStyles.Add(new System.Windows.Forms.ColumnStyle(System.Windows.Forms.SizeType.Percent, 50F));
      this.tableLayoutPanel1.Controls.Add(this.panel1, 1, 0);
      this.tableLayoutPanel1.Dock = System.Windows.Forms.DockStyle.Fill;
      this.tableLayoutPanel1.Location = new System.Drawing.Point(0, 0);
      this.tableLayoutPanel1.Name = "tableLayoutPanel1";
      this.tableLayoutPanel1.RowCount = 1;
      this.tableLayoutPanel1.RowStyles.Add(new System.Windows.Forms.RowStyle(System.Windows.Forms.SizeType.Percent, 100F));
      this.tableLayoutPanel1.Size = new System.Drawing.Size(284, 262);
      this.tableLayoutPanel1.TabIndex = 0;
      // 
      // panel1
      // 
      this.panel1.Controls.Add(this.button2);
      this.panel1.Controls.Add(this.button1);
      this.panel1.Dock = System.Windows.Forms.DockStyle.Fill;
      this.panel1.Location = new System.Drawing.Point(45, 3);
      this.panel1.Name = "panel1";
      this.panel1.Size = new System.Drawing.Size(194, 256);
      this.panel1.TabIndex = 0;
      // 
      // button1
      // 
      this.button1.Anchor = ((System.Windows.Forms.AnchorStyles)(((System.Windows.Forms.AnchorStyles.Top | System.Windows.Forms.AnchorStyles.Left) 
    | System.Windows.Forms.AnchorStyles.Right)));
    this.button1.Location = new System.Drawing.Point(3, 9);
    this.button1.Name = "button1";
    this.button1.Size = new System.Drawing.Size(188, 23);
    this.button1.TabIndex = 0;
    this.button1.Text = "button1";
    this.button1.UseVisualStyleBackColor = true;
    // 
    // button2
    // 
    this.button2.Anchor = ((System.Windows.Forms.AnchorStyles)(((System.Windows.Forms.AnchorStyles.Top | System.Windows.Forms.AnchorStyles.Left) 
    | System.Windows.Forms.AnchorStyles.Right)));
    this.button2.Location = new System.Drawing.Point(3, 38);
    this.button2.Name = "button2";
    this.button2.Size = new System.Drawing.Size(188, 23);
    this.button2.TabIndex = 0;
    this.button2.Text = "button1";
    this.button2.UseVisualStyleBackColor = true;
    // 
    // TestForm11
    // 
    this.AutoScaleDimensions = new System.Drawing.SizeF(6F, 13F);
    this.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font;
    this.ClientSize = new System.Drawing.Size(284, 262);
    this.Controls.Add(this.tableLayoutPanel1);
    this.Name = "TestForm11";
    this.Text = "TestForm11";
    this.tableLayoutPanel1.ResumeLayout(false);
    this.panel1.ResumeLayout(false);
    this.ResumeLayout(false);

}

 #endregion

private System.Windows.Forms.TableLayoutPanel tableLayoutPanel1;
private System.Windows.Forms.Panel panel1;
private System.Windows.Forms.Button button2;
private System.Windows.Forms.Button button1;

Hope this help..