I developed a Pre-Update Plugin for the Case entity. In this plugin I want to set a string field to a new value. It works smoothly if the new value is not null. But if the new value si null, it is just ignored.

This works:

Incident caseTarget = ((Entity) localContext.PluginExecutionContext.InputParameters["Target"]).ToEntity<Incident>();
caseTarget.ProductSerialNumber = "new value";

After the execution of the plugin, the ProductSerialNumber field has value "new value".

This doesn't work:

Incident caseTarget = ((Entity) localContext.PluginExecutionContext.InputParameters["Target"]).ToEntity<Incident>();
caseTarget.ProductSerialNumber = null;

After the execution of the plugin, the ProductSerialNumber field has still its old value.

How can I set the target's field to null?