Sqlcommond cmd =new Sqlcommond();
textBox.Text=cmd.ExecuteScalar().ToString();

------解决方案--------------------
C# code

using System.Data.SqlClient;

using(SqlConnection con = new SqlConnection("Data Source=.;uid=sa;pwd=sa;Database=Northwind"))
{
    con.Open();
    string strsql6 = "select count(sno) from student where cno='" + class1 + "' and cet4>425";
    SqlCommand cmd = new SqlCommand(strsql6,con);
     this.TextBox1.Text = cmd.ExecuteScalar().ToString();
}

------解决方案--------------------
探讨

C# code


using System.Data.SqlClient;

using(SqlConnection con = new SqlConnection("Data Source=.;uid=sa;pwd=sa;Database=Northwind"))
{
    con.Open();
    string strsql6 = "select count(sno)……


------解决方案--------------------

楼上的只针对了楼主的例子情况，多字段的话，要参考ExecuteReader或者用SqlAdapter的fill方法，装到DataTable里面，然后写for循环，table.Rows[i]["字段名"].ToString()可以读取字段的值

------解决方案--------------------
探讨


C# code

using System.Data.SqlClient;

using(SqlConnection con = new SqlConnection("Data Source=.;uid=sa;pwd=sa;Database=Northwind"))
{
    con.Open();
    string strsql6 = "select count(sno) from s……


------解决方案--------------------
C# code

using System.Data.SqlClient;

using(SqlConnection con = new SqlConnection("Data Source=.;initial catalog=Northwind;user id=sa;password=sa"))
{
    string strsql6 = "select count(sno) from student where cno=@class1 and cet4>425";
    using(SqlCommand cmd = new SqlCommand(strsql6,con))
  { 
     con.Open();
     cmd.Parameters.AddWithValue("@class1",class1);     
     this.TextBox1.Text = cmd.ExecuteScalar().ToString();
  }
}