Question

I just updated the StudentID and StudentName into the database.

I use these following codes to see value in the textbox but when i want to select the row it should be ID row not StudentID.

I mean i want to see value with StudentID, not ID row.

I don’t want to use the ID row to see the StudentName.

I want to see StudentName when i enter StudentID.

sql = new SqlConnection(@"Data Source=PC-PC\PC;Initial Catalog=Test;Integrated Security=True");
adapter = new SqlDataAdapter("select * from Entry",sql);
dt = new DataTable();
adapter.Fill(dt);
textBox1.Text = dt.Rows[3]["StudentName"].ToString();

Answer 1

If studentID is primary key of table, then use:

DataRow row = dt.Rows.Find(studentID);
if (row != null)
    textBox1.Text = row["StudentName"].ToString();

Otherwise use dt.Select method.
Btw mixing data access code with UI code is not very good idea

UPDATE: also you can use LINQ

string name = (from row in dt.AsEnumerable()
              where row.Field<int>("StudentID") == studentID
              select row.Field<string>("StudenName"))
              .Single();

UPDATE: If you are entering student id and want to get student name, then you can just retrieve student name from database, passing parameter to sql command:

private string GetStudentName(int studentID)
{
    string connString = @"Data Source=PC-PC\PC;Initial Catalog=Test;Integrated Security=True";
    using (SqlConnection conn = new SqlConnection(connString))
    {
        string query = "SELECT StudentName FROM Entry WHERE StudentID = @studentID";
        SqlCommand cmd = new SqlCommand(query, conn);
        cmd.Parameters.Add("@studentID", SqlDbType.Int).Value = studentID;
        conn.Open();
        return (string)cmd.ExecuteScalar();
    }
}

Consider also returning only first entry (if StudentID is not PK) and verifying for DbNull.

UPDATE: If you need to retrieve several attributes of student, then I’d created class Student:

public class Student
{
    public int Id { get; set; }
    public string Name { get; set; }
    public string Grade { get; set; }
}

And filled its properties from data reader:

private Student GetStudent(int studentID)
{
    string connString = @"Data Source=PC-PC\PC;Initial Catalog=Test;Integrated Security=True";
    using (SqlConnection conn = new SqlConnection(connString))
    {
        string query = "SELECT * FROM Entry WHERE StudentID = @studentID";
        SqlCommand cmd = new SqlCommand(query, conn);
        cmd.Parameters.Add("@studentID", SqlDbType.Int).Value = studentID;
        conn.Open();
        SqlDataReader reader = cmd.ExecuteReader();

        if (!reader.Read())
             throw new Exception("Student not found");

        return new Student()
        {
            Id = (int)reader["StudentID"],
            Name = (string)reader["StudentName"],
            Grade = (string)reader["Grade"]
        };
    }
}

Then when you enter student id in text box, retrieve student from database and show its properties in controls:

int studentID = Int32.Parse(idTextBox.Text);
Student student = GetStudent(studentID);
nameTextBox.Text = student.Name;
gradeTextBox.Text = student.Grade;