C#
// create and open connection
SqlConnection connection = new SqlConnection(@"Data Source=.\sqlexpress;Initial Catalog=basics;Integrated Security=True");
connection.Open();

// create and execute sql command
SqlCommand command = new SqlCommand();
command.CommandText = "SELECT count(*) FROM MY_SCHEMA.Persons";
command.Connection = connection;
int i = (int)command.ExecuteScalar();

connection.Close();

C#
command.CommandText = "INSERT INTO Persons (FirstName, LastName) VALUES ('John','First');SELECT SCOPE_IDENTITY();";
int i = Convert.ToInt32(command.ExecuteScalar());

C#
// create and open connection
SqlConnection connection = new SqlConnection(@"Data Source=.\sqlexpress;Initial Catalog=basics;Integrated Security=True");

// create SQLCommand
SqlCommand command = new SqlCommand();
command.CommandText = "SELECT * FROM MY_SCHEMA.Persons";
command.Connection = connection;

// create DataTable with SqlAdapter and load data
DataTable dt = new DataTable();
SqlDataAdapter da = new SqlDataAdapter(command);
try {
    connection.Open();
    int i = da.Fill(dt); 
}
finally { connection.Close(); }

C#
DataTable dt = new DataTable();
using (SqlConnection connnection = new SqlConnection(@"Data Source=.\sqlexpress;Initial Catalog=basics;Integrated Security=True"))
{
    SqlCommand command = new SqlCommand("SELECT * FROM MY_SCHEMA.Persons", connnection);
    connnection.Open();
    SqlDataReader dr = command.ExecuteReader(CommandBehavior.CloseConnection);
    if (dr.HasRows)
    {
        dt.Load(dr);
    }
    return dt;
}

C#
SqlConnection connection = new SqlConnection(@"Data Source=.\sqlexpress;Initial Catalog=basics;Integrated Security=True");

SqlCommand command = new SqlCommand();
command.CommandText = "SELECT * FROM MY_SCHEMA.Persons";
command.Connection = connection;

DataTable dt = new DataTable();

try
{
    connection.Open();

    SqlDataReader reader = command.ExecuteReader();
    bool tableCreated = false;
    while (reader.Read())
    {
        if (!columnsCreated)
        {
            // create table columns
            for (int i = 0; i < reader.FieldCount; i++)
            {
                string lsName = reader.GetName(i);
                Type t = reader[i].GetType();

                DataColumn column;
                if (t.Name != "DBNull")
                {
                    column = new DataColumn(lsName, t);
                }
                else
                {
                    column = new DataColumn(lsName, System.Type.GetType("System.Byte[]"));
                }
                
                dt.Columns.Add(column);
            }
            columnsCreated = true;
        }

        DataRow dr = dt.NewRow();
        for (int i = 0; i < reader.FieldCount; i++)
        {
            dr[i] = reader.GetValue(i);
        }
        dt.Rows.Add(dr);
    }
}
finally { connection.Close(); }

C#
// create and open connection
SqlConnection connection = new SqlConnection(@"Data Source=.\sqlexpress;Initial Catalog=basics;Integrated Security=True");
connection.Open();

// create and execute sql command
SqlCommand command = new SqlCommand();
command.CommandText = "UPDATE MY_SCHEMA.Persons SET Height = 185 WHERE firstName='John'";
command.Connection = connection;

int i = (int)command.ExecuteNonQuery();

connection.Close();

C#
// create and open connection
SqlConnection connection = new SqlConnection(@"Data Source=.\sqlexpress;Initial Catalog=basics;Integrated Security=True");

// create SQLCommand
SqlCommand command = new SqlCommand();
command.CommandText = "SELECT * FROM MY_SCHEMA.Persons";
command.Connection = connection;

// create DataTable with SqlAdapter and load data
DataTable dt = new DataTable();
SqlDataAdapter da = new SqlDataAdapter(command);
try
{
    connection.Open();
    int i = da.Fill(dt);

    dt.Rows[1]["Height"] = 187;  // This line simulates user interaction. Imagine user changed height of the person in the first row. 

    SqlCommand updateCommand = new SqlCommand();
    updateCommand.CommandText = "UPDATE MY_SCHEMA.Persons SET Height = @height WHERE id = @id";
    updateCommand.Connection = connection;
    updateCommand.Parameters.Add(new SqlParameter("@id", SqlDbType.Int, 5, "id"));  // this parameter identifies the record being updated (5 is max length))
    updateCommand.Parameters.Add(new SqlParameter("@height", SqlDbType.Int, 5, "Height"));  // this parameter updates actual value
    
    da.UpdateCommand = updateCommand;
    da.Update(dt);
}
finally { connection.Close(); }