using System;
using System.Collections.Generic;
using
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Data.SqlClient;
using System.Windows.Forms;

namespace WindowsFormsApplication5
{
    public partial class Form1 : Form
    {
       public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            if (textBox1.Text == "")
            {
                MessageBox.Show("请输入要连接的数据库名称");
            }

            else
            {
                try
                {
                    string ConStr = "sever=.;database=" + textBox1.Text.Trim() + ";uid=sa;pwd=sasa";
                    SqlConnection conn = new SqlConnection(ConStr);
                  
                    Conn.Open();
                    if (conn.State == ConnectionState.Open)
                    {
                        label1.Text = "数据库[" + textBox1.Text.Trim() + "]已经连接并打开";

                    }

                }
                catch
                {
                    MessageBox.Show("连接数据库失败");
                }

            }

        }

        private void textBox1_TextChanged(object sender, EventArgs e)
        {

        }

    }
}

因为你应该写conn，不Conn。

你头字母错了，同意二楼的。以后写代码时注意好好读读。不要范这样子的错误了

C#是区分大小写的。

你实例化连接对像为conn,你后面用的是Conn,字母大小写问题，C#对大小写敏感