C# .Net 4.5 PropertyGrid: how to hide Properties

The easiest way to hide a property in PropertGrid and in a Custom Control for me is this:

public class Question
{
   ...
  
  [Browsable(false)]
  public int Element
  {
    get; set;
  }
}

To do it dynamically you can use this code, where Question is your class and your property is Element, so you can show or hide it without remove element from collection:

PropertyDescriptorCollection propCollection = TypeDescriptor.GetProperties(Question.GetType());
PropertyDescriptor descriptor = propCollection["Element"];

BrowsableAttribute attrib = (BrowsableAttribute)descriptor.Attributes[typeof(BrowsableAttribute)];
FieldInfo isBrow = attrib.GetType().GetField("browsable", BindingFlags.NonPublic | BindingFlags.Instance);
//Condition to Show or Hide set here:
isBrow.SetValue(attrib, true);
propertyGrid1.Refresh(); //Remember to refresh PropertyGrid to reflect your changes

So to refine the answer:

public class Question
{
   ...
   private int element;
   [Browsable(false)]
   public int Element
   {
      get { return element; }
      set { 
            element = value; 
            PropertyDescriptorCollection propCollection = TypeDescriptor.GetProperties(Question.GetType());
            PropertyDescriptor descriptor = propCollection["Element"];
    
            BrowsableAttribute attrib = (BrowsableAttribute)descriptor.Attributes[typeof(BrowsableAttribute)];
            FieldInfo isBrow = attrib.GetType().GetField("browsable", BindingFlags.NonPublic | BindingFlags.Instance);
            if(element==0)
            {
              isBrow.SetValue(attrib, false);
            }
            else
            {
              isBrow.SetValue(attrib, true);
            }
          }
   }
}

What you could do is reuse the DynamicTypeDescriptor class described in my answer to this question here on SO: PropertyGrid Browsable not found for entity framework created property, how to find it?

like this for example:

public Form1()
{
    InitializeComponent();

    DynamicTypeDescriptor dt = new DynamicTypeDescriptor(typeof(Question));

    Question q = new Question(); // initialize question the way you want    
    if (q.Element == 0)
    {
        dt.RemoveProperty("Element");
    }
    propertyGrid1.SelectedObject = dt.FromComponent(q);
}