新建一个网站，在web.config中添加代码：
<connectionStrings>
<add name="SqlConn" connectionString="server=.;database=Test;uid=sa;pwd=123"/>
</connectionStrings>
然后Default.aspx代码：
<body>
<form id="form1" runat="server">
<div>
用户名：
<asp:TextBox ID="txtUserName" runat="server" ontextchanged="txtUserName_TextChanged" AutoPostBack="true" ></asp:TextBox>
<asp:Label ID="lblMessage" runat="server" Text="用户名不能为空！" Font-Size="Small" ForeColor="Red"></asp:Label>
<br />
密码：
<asp:TextBox ID="TextBox2" runat="server"></asp:TextBox>
<asp:Label ID="Label2" runat="server" Text="密码长度6，由字符、数字、组成" Font-Size="Small" ForeColor="Red"></asp:Label>
</div>
</form>
</body>
接着是Default.aspx.cs
public partial class _Default : System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{
}
protected void txtUserName_TextChanged(object sender, EventArgs e)
{
bool result = CheckUserName(txtUserName.Text.Trim());
if (result == true)
{
lblMessage.Text = "恭喜您，此用户名可以使用！";
}
if (result == false)
{
lblMessage.Text = "该用户名已存在，请更换用户名！";
}
}
public static bool CheckUserName(string username)
{
string connectionString = ConfigurationManager.ConnectionStrings["SqlConn"].ConnectionString;
SqlConnection conn = new SqlConnection(connectionString);
conn.Open();
string sql = "select * from T_User where UserName = '" + username +"'";
SqlCommand cmd = new SqlCommand(sql, conn);
try
{
int count = Convert.ToInt32(cmd.ExecuteScalar());
conn.Close();
if (count > 0)
{
return false;
}
else
{
return true;
}
}
catch (Exception ex)
{
Console.WriteLine(ex.Message);
return false;
}
}
}
最后上图：
