How to show error message on the detail page of Salesforce sobject through before delete trigger?

Hello Shariq,

Try this once...

1. Go to Setup | Customize | <Standard Object Name> | Buttons, Links & Actions and then Click 'New Button or Link' or if it is a Custom Object then go to Setup | Create | Objects and then click on the Custom Object name. Scroll down to find the section 'Buttons, Links, and Actions' and then click 'New Button or Link'.
2. Give it the Label: <A Valid Name>.
3. Display Type: Detail Page Button.
4. Behavior: Execute JavaScript.
5. Content Source: OnClick JavaScript.

Here is the JS:

{!REQUIRESCRIPT('//ajax.googleapis.com/ajax/libs/jquery/1.10.2/jquery.min.js')}
{!REQUIRESCRIPT('//ajax.googleapis.com/ajax/libs/jqueryui/1.10.3/jquery-ui.min.js')}
try{
jQuery(function() {
/*Append the jQuery CSS CDN Link to the Head tag.*/
jQuery('head').append('<link rel="stylesheet" href="//ajax.googleapis.com/ajax/libs/jqueryui/1.10.3/themes/start/jquery-ui.css" type="text/css" />');

/*Create the HTML(DIV Tag) for the Dialog.*/
var html =
'<div id="dialog" title="Go Home"><p>Do you want to go to the Home tab ?</p></div>';

/*Check if the Dialog(DIV Tag) already exists if not then Append the same to the Body tag.*/
if(!jQuery('[id=dialog]').size()){
jQuery('body').append(html);
}

/*Open the jQuery Dialog.*/
jQuery( "#dialog" ).dialog({
autoOpen: true,
modal: true,
show: {
effect: "bounce",
duration: 1000
},
hide: {
effect: "bounce",
duration: 1000
},
buttons: {
"Continue": function() {
location.replace('/home/home.jsp');
jQuery( this ).dialog( "close" );
},
Cancel: function() {
jQuery( this ).dialog( "close" );
}
}
});
});
}
catch(e){
alert('An Error has Occured. Error: ' + e);
}

Save the same!

7. Now, just open any Record, click Edit Layout at top-right..
9. From the Page Host, select Buttons list and drag the new button onto the Page Layout.
10. Save and you are done!

Using standard page we can have these kind of messages only when we save info , but we cannot have this for delete actions as the page will be redirected to some predefined page. The only way is to customize the detail page using visual force and there we can have own custom validation.

Workaround :

You can override Delete button with your javascript custom delete.
Check you condition in JAVASCRIPT and if satisfied then display alert message. else delete that record.