How to display sql query results into textbox on c#?

 private void button1_Click(object sender, EventArgs e)
 {
     string idcan = textBox3.Text;
     string score = textBox4.Text;
     Connection con = new Connection();
     SqlConnection sqlcon = con.Sambung();

     sqlcon.Open();
     string cek = "select count (ID_Candidate) as Score from DataVote where ID_Candidate = @idcan";

     using (sqlcon)
     {
         SqlCommand com = new SqlCommand(cek, sqlcon);
         com.Parameters.Add("idcan", SqlDbType.VarChar, 5).Value = score;

     }
     try
     {
         sqlcon.Open();
         textBox4.Text=Convert.ToString(com.ExecuteScalar()); 
     }
     catch (Exception ex)
     {
         MessageBox.Show(ex.Message);
     }

 }

You can actually do this code:

private void button1_Click(object sender, EventArgs e)
 {

 string idcan = textBox3.Text;
 string score = textBox4.Text;
 Datatable dt = new DataTable();
 Connection con = new Connection();
 SqlConnection sqlcon = con.Sambung();
 SqlCommand com ;

 sqlcon.Open();
 string cek = "select count (ID_Candidate) as Score from DataVote where ID_Candidate = @idcan";  

using(sqlcon)
{
   using(com = new SqlCommand(cek, sqlcon))
   {
     sqlcon.Open();
    com.Parameter.Add("@idcan",score );
    SqlDataAdapter da = new SqlDataAdapter(com);
    da.File(dt);
    if(dt.Rows.Count > 0)
    {
        textBox4.Text = dt.Rows[0]["Score"].ToString();
    }
  }
}
 }

Since your SELECT statement returns one row with one column, you can use ExecuteScalar to get it.

textBox4.Text = com.ExecuteScalar().ToString();

And your code needs a little bit refactoring like;

using(SqlConnection sqlcon = con.Sambung())
using(SqlCommand com = sqlcon.CreateCommand())
{
    com.CommandText = "select count (ID_Candidate) as Score from DataVote where ID_Candidate = @idcan";
    com.Parameters.Add("@idcan", SqlDbType.VarChar, 5).Value = score;
    sqlcon.Open();
    textBox4.Text = com.ExecuteScalar().ToString();
}

By the way, I strongly suspect your ID_Candidate column should be some numeric type instead of VarChar based on it's name.

Expand your question: If your query return as a table (many columns, many rows), you can use this code below. Similar for case return single value.

//Create class to store result value
public class ClassName
{
    public string Col1 { get; set; }
    public int Col2 { get; set; }
}
// In query code
ClassName[] allRecords = null;
SqlCommand command = ...; // your code
using (var reader = command.ExecuteReader())
{
    var list = new List<ClassName>();
    while (reader.Read())
        list.Add(new ClassName {Col1 = reader.GetString(0), Col2 = reader.GetInt32(1)});
    allRecords = list.ToArray();
}