1.新建一个form，设为父窗体，在属性ismdicontainer中设置
2. 新建子窗口，设置其的父窗体
Form fm2 = new Form2();
fm2.MdiParent = this;
fm2.Show();
Form fm3 = new Form3();
fm3.MdiParent = this;
fm3.Show();
Form fm4 = new Form4();
fm4.MdiParent = this;
fm4.Show();
4.对齐 layoutmdi
private void 纵向ToolStripMenuItem_Click(object sender, EventArgs e)
{
LayoutMdi(MdiLayout.TileHorizontal);
}
private void 横向ToolStripMenuItem_Click(object sender, EventArgs e)
{
LayoutMdi(MdiLayout.TileVertical);
}
原文：https://www.cnblogs.com/yaoyue68/p/14645203.html