c#窗体和数据库的相关问题

c#的一个注册登录界面，根据图中代码希望添加注册的账号不能重复的功能。

这2句代码改为下面的

SqlCommand cmd = <span class="hljs-keyword">new</span> <span class="hljs-constructor">SqlCommand(<span class="hljs-string">"select count(1) from login where username='"</span>+<span class="hljs-params">tbUserName</span>.Text+<span class="hljs-string">"'"</span>, <span class="hljs-params">conn</span>)</span>;
<span class="hljs-keyword">if</span> ((<span class="hljs-built_in">int</span>)cmd.<span class="hljs-constructor">ExecuteScalar()</span> > <span class="hljs-number">0</span>)<span class="hljs-comment">//唯一性判断</span>
{
    MessageBox.<span class="hljs-constructor">Show(<span class="hljs-string">"用户名“"</span> + <span class="hljs-params">tbUserName</span>.Text + <span class="hljs-string">"”已经存在，请确认！"</span>)</span>;
    return;
}
cmd.CommandText = sql;<span class="hljs-comment">///通过唯一验证更换sql语句</span>
<span class="hljs-built_in">int</span> returnvalue = cmd.<span class="hljs-constructor">ExecuteNonQuery()</span>;