One group of searches that consistently brings traffic here is variations on the error: Invalid JSON primitive. Unfortunately, the post that Google sends that traffic to doesn’t address the issue until somewhere within its 150+ comments.

Today, the topic gets its own post.

If you’ve worked with ASMX ScriptServices or Page Methods without ASP.NET AJAX’s client-side proxy (e.g. using jQuery or pure XMLHttpRequest code), you’ve may have seen this cryptic error yourself. Or, perhaps you’ve just arrived here due to seeing it for the first time.

Either way, you may be surprised to learn that the most common reason for this error is that you’ve lied to ASP.NET during your AJAX request.

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Continuing my series of posts about ASMX services and JSON, in this post I’m going to cover two common mistakes that plague the process of getting a project’s first ASMX ScriptService working: Installing System.Web.Extensions into the GAC and configuring your web.config.

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While we were recording episode 5 of Mastering jQuery, I found myself running down a lengthy list of misconceptions and potential pitfalls when it comes to using ASMX services for AJAX callbacks. After years of fielding questions revolving around that topic, I suppose I’ve developed a decent handle on the issues most often encountered.

To preemptively surface some of that commonly requested information, I’m going to publish a series of relatively short posts, each describing one mistake or misconception that I’ve seen come up frequently.

To get started, I want to cover one of the most fundamental of these misconceptions: That ASMX services can’t return JSON.

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