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)
        {

        }

    }
}

肯定会不存在啊，你把C打成了大写！

能编译通过么？

SqlConnection conn = new SqlConnection(ConStr);
                  
                    Conn.Open();//二楼说的正确 你定义的是小写 你使用的是大写  C#对大小写很严格的