Lock "classification" property in a document

Hi Yoann, thanks for you reply. I followed your procedure, but I have some problem. I created two methods related to Server Event of Document Itemtype. But when I save (update) the Document Form, I can't get any error message and the form is updated. It seems that two strings  (savedClassification and curClassification) are the identical and both updated to the current classification. What is wrong? I pasted two methods: OnBeforeUpdate:

RequestState.Add("classification", this.getProperty("classification")); return this;

  OnAfterUpdate:

string savedClassification = (string)RequestState["classification"]; string curClassification = this.getProperty("classification"); if (savedClassification != curClassification) { return this.getInnovator().newError("Classification is changed!"); } RequestState.Remove("classification"); RequestState.Clear(); return this;

Thanks Pierluigi  

Hi Yoann, thanks for you reply. I followed your procedure, but I have some problem. I created two methods related to Server Event of Document Itemtype. But when I save (update) the Document Form, I can't get any error message and the form is updated. It seems that two strings  (savedClassification and curClassification) are the identical and both updated to the current classification. What is wrong? I pasted two methods: OnBeforeUpdate:

RequestState.Add("classification", this.getProperty("classification")); return this;

  OnAfterUpdate:

string savedClassification = (string)RequestState["classification"]; string curClassification = this.getProperty("classification"); if (savedClassification != curClassification) { return this.getInnovator().newError("Classification is changed!"); } RequestState.Remove("classification"); RequestState.Clear(); return this;

Thanks Pierluigi