A unique identifier for an Element in an Autodesk Revit project.
Namespace: Autodesk.Revit.DB
Assembly: RevitAPI (in RevitAPI.dll) Version: 2015.0.0.0 (2015.0.0.0)
Syntax
Remarks
Returns an object containing the project-wide unique identifier for this Element. The identifier with this id is only unique for the project, it is not unique across separate Autodesk Revit projects.
Examples

private void CheckElementId(Autodesk.Revit.DB.Document document, Autodesk.Revit.DB.Element element)
{
// Get the id of the element
Autodesk.Revit.DB.ElementId selectedId = element.Id;
int idInteger = selectedId.IntegerValue;
// create a new id and set the value
Autodesk.Revit.DB.ElementId id = new Autodesk.Revit.DB.ElementId(idInteger);
// Get the element
Autodesk.Revit.DB.Element first = document.GetElement(id);
// And get the element using the original Id (they should be the same)
Autodesk.Revit.DB.Element second = document.GetElement(selectedId);
String prompt = "Get an element twice. They should be the same element.";
// Use the Equal method to check two elements. It is not a good way.
bool isSame = first.Equals(second);
prompt += "\nUsing Equal method on elements, the result is: " + isSame;
isSame = first.Id.Equals(second.Id);
prompt += "\nUsing Equal method on element's Id properties, the result is: " + isSame;
TaskDialog.Show("Revit",prompt);
}

Private Sub CheckElementId(document As Autodesk.Revit.DB.Document, element As Autodesk.Revit.DB.Element)
' Get the id of the element
Dim selectedId As Autodesk.Revit.DB.ElementId = element.Id
Dim idInteger As Integer = selectedId.IntegerValue
' create a new id and set the value
Dim id As New Autodesk.Revit.DB.ElementId(idInteger)
' Get the element
Dim first As Autodesk.Revit.DB.Element = document.GetElement(id)
' And get the element using the original Id (they should be the same)
Dim second As Autodesk.Revit.DB.Element = document.GetElement(selectedId)
Dim prompt As [String] = "Get an element twice. They should be the same element."
' Use the Equal method to check two elements. It is not a good way.
Dim isSame As Boolean = first.Equals(second)
prompt += vbLf & "Using Equal method on elements, the result is: " & isSame
isSame = first.Id.Equals(second.Id)
prompt += vbLf & "Using Equal method on element's Id properties, the result is: " & isSame
TaskDialog.Show("Revit", prompt)
End Sub