C#中 一开始点击“下一行”就溢出，求解？

C#中 一开始点击“上一行”就溢出，求解？！
左图是我的代码 
中图是我的表 
右图是我一开始运行我的表时候 点击“last”就出错了，但是如果我先按一下“next”再按一下“last”是没有问题的。这是为什么呢？ 
帮一下忙，我改怎么修改我的代码呢 

如下是左图代码 
i--; 
if ((i < db.Rows.Count)==false ) 
{ i++; MessageBox.Show("最后一条"); } 

账号栏.Text = db.Rows[i][0].ToString(); 
密码栏.Text = db.Rows[i][1].ToString();

[解决办法]

C# code

        private int _rowIndex;        public int RowIndex        {            get            {                return this._rowIndex;            }            set            {                if (value < 0)                    this._rowIndex = 0;                else if (value >= db.Rows.Count)                    this._rowIndex = db.Rows.Count - 1;                else                    this._rowIndex = value;            }        }        private void next_Click(object sender, EventArgs e)        {            this.RowIndex++;        }        private void last_Click(object sender, EventArgs e)        {            this.RowIndex--;            if (this.RowIndex == db.Rows.Count)            {                MessageBox.Show("最后一条");            }            帐号栏.Text = db.Rows[this.RowIndex][0].ToString();            密码栏.Text = db.Rows[this.RowIndex][1].ToString();        }
[解决办法]
C# code
        private int _rowIndex;        public int RowIndex        {            get            {                return this._rowIndex;            }            set            {                if (value < 0)                    this._rowIndex = 0;                else if (value >= db.Rows.Count)                    this._rowIndex = db.Rows.Count - 1;                else                    this._rowIndex = value;            }        }        //第一条        private void firlt_Click(object sender, EventArgs e)        {            this.RowIndex = 0;        }        //前一条        private void pre_Click(object sender, EventArgs e)        {            this.RowIndex--;        }        //下一条        private void next_Click(object sender, EventArgs e)        {            this.RowIndex++;        }        //最后一条        private void last_Click(object sender, EventArgs e)        {            this.RowIndex = db.Rows.Count - 1;            帐号栏.Text = db.Rows[this.RowIndex][0].ToString();            密码栏.Text = db.Rows[this.RowIndex][1].ToString();        }
[解决办法]

  private void last_Click(object sender, EventArgs e)
       {
if(i==0)
return;
i--; 
if ((i < db.Rows.Count)==false ) 
{ i++; MessageBox.Show("最后一条"); } 

账号栏.Text = db.Rows[i][0].ToString(); 
密码栏.Text = db.Rows[i][1].ToString();
}