Update als de naam bestaat, anders invoegen - in SQL Server

Je zou moeten

• gebruik parameters in uw zoekopdracht - ALTIJD! - geen uitzondering
• maak een enkele query die de IF EXISTS() . verwerkt deel op de server
• gebruik de algemeen aanvaarde best practices van ADO.NET om dingen in using() {....} te zetten blokken enz.

Probeer deze code:

public void SaveData(string filename, string jsonobject)
{
    // define connection string and query
    string connectionString = "Data Source=.;Initial Catalog=;Integrated Security=True";
    string query = @"IF EXISTS(SELECT * FROM dbo.T_Pages WHERE pagename = @pagename)
                        UPDATE dbo.T_Pages 
                        SET pageinfo = @PageInfo
                        WHERE pagename = @pagename
                    ELSE
                        INSERT INTO dbo.T_Pages(PageName, PageInfo) VALUES(@PageName, @PageInfo);";

    // create connection and command in "using" blocks
    using (SqlConnection conn = new SqlConnection(connectionString))
    using (SqlCommand cmd = new SqlCommand(query, conn))
    {
        // define the parameters - not sure just how large those 
        // string lengths need to be - use whatever is defined in the
        // database table here!
        cmd.Parameters.Add("@PageName", SqlDbType.VarChar, 100).Value = filename;
        cmd.Parameters.Add("@PageInfo", SqlDbType.VarChar, 200).Value = jsonobject;

        // open connection, execute query, close connection
        conn.Open();
        int rowsAffected = cmd.ExecuteNonQuery();
        conn.Close();
    }
}