Check Inserted Data In A SQL Database

Hi im having problems as im new to ASP.NET C#





i have created a button to add details into a SQL database but i want to check the details before i insert the new values from the textboxes





can anyone help....... this is what i have to insert into the database........i just want some help to compare the user name eg... if user name exists a message will appear telling the user to change a different user name








Thanks








private void Button1_Click(object sender, System.EventArgs e)


{


string connectionString = "server='(local)'; trusted_connection=true; database='tester'";





//System.Data.IDbConnection conn = new System.Data.SqlClient.SqlConnection(connectionString);


System.Data.IDbConnection conn = new System.Data.SqlClient.SqlConnection(connectionString);


conn.Open();





string commandString = "INSERT INTO Users (UserName, Password) " + "Values(@UserName, @Password)";





//SqlCommand dbCommand = new SqlCommand (commandString, dbconn);





System.Data.IDbCommand dbCommand = new System.Data.SqlClient.SqlCommand();





//System.Data.SqlClient.SqlCommand myCmd = new System.Data.SqlClient.SqlCommand(queryString, conn);





dbCommand.CommandText = commandString;





dbCommand.Connection = conn;





SqlParameter unParam = new SqlParameter ("@UserName", SqlDbType.NVarChar, 60);


unParam.Value = txtUser.Text;


dbCommand.Parameters.Add(unParam);


SqlParameter paParam = new SqlParameter ("@Password", SqlDbType.NVarChar, 60);


paParam.Value = txtPassword.Text;


dbCommand.Parameters.Add(paParam);





dbCommand.ExecuteNonQuery();





conn.Close();





Response.Redirect ("WebForm1.aspx");


}