this.dataGridView1.SelectionMode =DataGridViewSelectionMode.FullRowSelect;

dataGridView1即你的dataGridView名称。

转 ： https://www.cnblogs.com/net-sky/p/9302413.html

例子： 

C#winform中的 dataGridView 鼠标点行首时选中整行，点击单元格时 选中单元格

private void dgvReceive_CellClick(object sender, DataGridViewCellEventArgs e)
        {
            if (e.RowIndex!=-1 )
            {
                if (e.ColumnIndex==-1)
                {
                    dgvReceive.SelectionMode = DataGridViewSelectionMode.FullRowSelect;
                    dgvReceive.Rows[e.RowIndex].Selected = true;
                     
                }
                else
                {
                    dgvReceive.SelectionMode = DataGridViewSelectionMode.CellSelect;
                    dgvReceive.Rows[e.RowIndex].Cells[e.ColumnIndex].Selected = true;
                }
 
                 
 
            }
        }