Question

Is it safe to blindly overwrite the ModelMetaData of a binding context to set the type of the model? I’m worried the binder may be used to update pre-built models in edit scenarios and I will lose data if I just overwrite the ModelMetaData.

Sample code:

Public Class CustomModelBinder
    Inherits DefaultModelBinder

    Public Overrides Function BindModel(ByVal controllerContext As ControllerContext, ByVal bindingContext As ModelBindingContext) As Object
        bindingContext.ModelMetadata = ModelMetadataProviders.Current.GetMetadataForType(Nothing, GetSpecificModelTypeBasedOnBindingData(bindingContext))

        Return MyBase.BindModel(controllerContext, bindingContext)
    End Function
End Class

Answer 1

Yes, it is possible that the model will be already specified. This occurs, for example, when someone tries to update an existing model using TryUpdateModel/UpdateModel, as shown here.

public ActionResult Update(int id)
{
   var modelToUpdate = GetExistingModel(id);

   if (TryUpdateModel(modelToUpdate)) // replacing the Model or ModelBinderContext.Metadata in the model binder could have unexpected and unwanted results.
   {
      // etc.
   }

   // etc.
}