How to prevent PostBack while clicking button in ASP.NET - asp.net

Question

The following picture is view of one of my WebForms in project. I am currently facing following problem: I am loading CSV file, the program analyzes it and displays it as a Table. Then, using DropDownLists and Textboxes below, user chooses day, project and hours in order to decrease hours, which have been assigned to the project (imagine project x, which has budget of 100.000$ and has 150h to be burned by workers. Each worker burns his own hours and web admin can see how many hours can be burned and how does the budget look like). When I decrease hours for some day, for example project x, Monday, 8h- the page reloads and all the things, including table, dropdownlists and textboxes are closed and user add project hours for another days.
Please see the code that is responsible for changes in the database and is executed during Submit button click:
private void decreaseProjHours()
{
if (IsPostBack)
{
string nameOfProject = DropDownList1.SelectedValue;
int dayID = DropDownList2.SelectedIndex;
int hours = Convert.ToInt32(TextBox3.Text);
if ((nameOfProject == "") || (dayID > 7 || dayID < 0) || hours < 0)
{
Response.Write("You have entered invalid data in one of the fields!");
return;
}
//decreasing from total hours assigned to project
SqlConnection conn = new SqlConnection(ConfigurationManager.ConnectionStrings["WorkTimeDBConnectionString"].ConnectionString);
conn.Open();
string getTotalProjectH = "select hours from projects where project_name='" + nameOfProject + "'";
SqlCommand com = new SqlCommand(getTotalProjectH, conn);
int check = Convert.ToInt32(com.ExecuteScalar().ToString());
if (check - hours < 0)
{
Response.Write("No more hours can be reported for project" + nameOfProject);
conn.Close();
return;
}
else
{
try
{
string tryUpdate = "update projects set hours='" + (check - hours) + "'" + " where project_name='" + nameOfProject + "'";
SqlCommand com1 = new SqlCommand(tryUpdate, conn);
com1.ExecuteNonQuery();
}
catch (Exception ex)
{
Response.Write(ex.ToString());
conn.Close();
return;
}
}
}
}
Is is the problem with PostBack? I already tried making OnClientClick return false and Button3.Attributes.Add("onclick", "return false;")- the operation of modifying the database fails then and nothing is submitted.
What should I do? Is it really problem with PostBack or something else refreshes my webpage?
EDIT:
Button3.Attributes.Add("onclick", "return false;");
Button3.OnClientClick = "decreaseProjHours(); return false;";
Button3.UseSubmitBehavior = false;
Thanks in advance,
Kokos

Answer 1

You have more or less the correct code in the line
Button3.OnClientClick = "decreaseProjHours(); return false;";
However the problem is that it looks for a javascript function decreaseProjHours(), which I'm assuming you don't have since you have that method on the server side in your code behind. OnClientClick is by definition a client-side handler and so the method needs to be defined on the client side. Since it can't find the javascript function, it fails to execute the next line return false;.
If you need that code to execute first and then return false to prevent postback, you need to put the code in a javascript function instead of in the code-behind. You can then have that fuction either return true or false if you wish, instead of having the additional return statement in the OnClientClick definition.
Note that if you have a server-side click handler defined as well, this won't execute if there is no postback; i.e. the server-side OnClick handler only executes if the client side returns true.