Adding error message to validation summary from code behind

Question

I am doing something like below on a web forms application;

protected void button_transfer_search_Click(object sender, EventArgs e) {

    Page.Validate("val1");

    if (!Page.IsValid && int.Parse(txtArrivalDateTrf.Text) + 5 < 10) {
        return;
    }

also, I have following code on my aspx file;

<div class="search-engine-validation-summary">
    <asp:ValidationSummary ValidationGroup="transfer" runat="server" ShowMessageBox="false" />
</div>

my question is how to add an error message to the page before return so that validation summary can grab that and displays it. I know we can do this in mvc easily but I haven't figured out how to do that in web forms. thanks !

Answer 1

Whenever I find this situation this is what I do:

var val = new CustomValidator()
{
   ErrorMessage = "This is my error message.",
   Display = ValidatorDisplay.None,
   IsValid = false,
   ValidationGroup = vGroup
};
val.ServerValidate += (object source, ServerValidateEventArgs args) => 
   { args.IsValid = false; };
Page.Validators.Add(val);

And in my ASPX code I have a ValidationSummary control with a ValidationGroup set to the same value as vGroup.

Then, after I have loaded as many CustomValidators (or any other kind of validators) by codebehind as I want I simply call

Page.Validate()
if (Page.IsValid)
{
    //... set your valid code here
}

The call to Page.Validate() calls the lambda-linked method of all code-behind inserted validators and if any returns false the page is invalid and returned with no code executed. Otherwise, the page returns a valid value, and executes the valid code.